global: introduce `USE_THE_REPOSITORY_VARIABLE` macro
Use of the `the_repository` variable is deprecated nowadays, and we
slowly but steadily convert the codebase to not use it anymore. Instead,
callers should be passing down the repository to work on via parameters.
It is hard though to prove that a given code unit does not use this
variable anymore. The most trivial case, merely demonstrating that there
is no direct use of `the_repository`, is already a bit of a pain during
code reviews as the reviewer needs to manually verify claims made by the
patch author. The bigger problem though is that we have many interfaces
that implicitly rely on `the_repository`.
Introduce a new `USE_THE_REPOSITORY_VARIABLE` macro that allows code
units to opt into usage of `the_repository`. The intent of this macro is
to demonstrate that a certain code unit does not use this variable
anymore, and to keep it from new dependencies on it in future changes,
be it explicit or implicit
For now, the macro only guards `the_repository` itself as well as
`the_hash_algo`. There are many more known interfaces where we have an
implicit dependency on `the_repository`, but those are not guarded at
the current point in time. Over time though, we should start to add
guards as required (or even better, just remove them).
Define the macro as required in our code units. As expected, most of our
code still relies on the global variable. Nearly all of our builtins
rely on the variable as there is no way yet to pass `the_repository` to
their entry point. For now, declare the macro in "biultin.h" to keep the
required changes at least a little bit more contained.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2024-06-14 14:50:23 +08:00
|
|
|
#define USE_THE_REPOSITORY_VARIABLE
|
|
|
|
|
|
2023-04-11 15:41:54 +08:00
|
|
|
#include "git-compat-util.h"
|
2023-03-21 14:25:54 +08:00
|
|
|
#include "gettext.h"
|
2008-02-28 13:25:17 +08:00
|
|
|
#include "pack-revindex.h"
|
2023-04-11 15:41:53 +08:00
|
|
|
#include "object-file.h"
|
2023-05-16 14:34:06 +08:00
|
|
|
#include "object-store-ll.h"
|
2019-04-06 02:04:24 +08:00
|
|
|
#include "packfile.h"
|
2023-05-16 14:34:06 +08:00
|
|
|
#include "strbuf.h"
|
2023-04-11 11:00:38 +08:00
|
|
|
#include "trace2.h"
|
2023-09-30 05:20:51 +08:00
|
|
|
#include "parse.h"
|
pack-revindex: read multi-pack reverse indexes
Implement reading for multi-pack reverse indexes, as described in the
previous patch.
Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.
There are three new functions exposed by the revindex API:
- load_midx_revindex(): loads the reverse index corresponding to the
given multi-pack index.
- midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
multi-pack index and pseudo-pack order.
load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.
load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.
pack_pos_to_midx() simply reads the corresponding entry out of the
table.
midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.
So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-03-30 23:04:26 +08:00
|
|
|
#include "midx.h"
|
2023-04-18 00:21:39 +08:00
|
|
|
#include "csum-file.h"
|
2008-02-28 13:25:17 +08:00
|
|
|
|
2021-01-14 06:25:06 +08:00
|
|
|
struct revindex_entry {
|
|
|
|
|
off_t offset;
|
|
|
|
|
unsigned int nr;
|
|
|
|
|
};
|
|
|
|
|
|
2008-02-28 13:25:17 +08:00
|
|
|
/*
|
|
|
|
|
* Pack index for existing packs give us easy access to the offsets into
|
|
|
|
|
* corresponding pack file where each object's data starts, but the entries
|
|
|
|
|
* do not store the size of the compressed representation (uncompressed
|
|
|
|
|
* size is easily available by examining the pack entry header). It is
|
|
|
|
|
* also rather expensive to find the sha1 for an object given its offset.
|
|
|
|
|
*
|
pack-revindex: drop hash table
The main entry point to the pack-revindex code is
find_pack_revindex(). This calls revindex_for_pack(), which
lazily computes and caches the revindex for the pack.
We store the cache in a very simple hash table. It's created
by init_pack_revindex(), which inserts an entry for every
packfile we know about, and we never grow or shrink the
hash. If we ever need the revindex for a pack that isn't in
the hash, we die() with an internal error.
This can lead to a race, because we may load more packs
after having called init_pack_revindex(). For example,
imagine we have one process which needs to look at the
revindex for a variety of objects (e.g., cat-file's
"%(objectsize:disk)" format). Simultaneously, git-gc is
running, which is doing a `git repack -ad`. We might hit a
sequence like:
1. We need the revidx for some packed object. We call
find_pack_revindex() and end up in init_pack_revindex()
to create the hash table for all packs we know about.
2. We look up another object and can't find it, because
the repack has removed the pack it's in. We re-scan the
pack directory and find a new pack containing the
object. It gets added to our packed_git list.
3. We call find_pack_revindex() for the new object, which
hits revindex_for_pack() for our new pack. It can't
find the packed_git in the revindex hash, and dies.
You could also replace the `repack` above with a push or
fetch to create a new pack, though these are less likely
(you would have to somehow learn about the new objects to
look them up).
Prior to 1a6d8b9 (do not discard revindex when re-preparing
packfiles, 2014-01-15), this was safe, as we threw away the
revindex whenever we re-scanned the pack directory (and thus
re-created the revindex hash on the fly). However, we don't
want to simply revert that commit, as it was solving a
different race.
So we have a few options:
- We can fix the race in 1a6d8b9 differently, by having
the bitmap code look in the revindex hash instead of
caching the pointer. But this would introduce a lot of
extra hash lookups for common bitmap operations.
- We could teach the revindex to dynamically add new packs
to the hash table. This would perform the same, but
would mean adding extra code to the revindex hash (which
currently cannot be resized at all).
- We can get rid of the hash table entirely. There is
exactly one revindex per pack, so we can just store it
in the packed_git struct. Since it's initialized lazily,
it does not add to the startup cost.
This is the best of both worlds: less code and fewer
hash table lookups. The original code likely avoided
this in the name of encapsulation. But the packed_git
and reverse_index code are fairly intimate already, so
it's not much of a loss.
This patch implements the final option. It's a minimal
conversion that retains the pack_revindex struct. No callers
need to change, and we can do further cleanup in a follow-on
patch.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2015-12-21 14:19:49 +08:00
|
|
|
* The pack index file is sorted by object name mapping to offset;
|
|
|
|
|
* this revindex array is a list of offset/index_nr pairs
|
2008-02-28 13:25:17 +08:00
|
|
|
* ordered by offset, so if you know the offset of an object, next offset
|
|
|
|
|
* is where its packed representation ends and the index_nr can be used to
|
|
|
|
|
* get the object sha1 from the main index.
|
|
|
|
|
*/
|
|
|
|
|
|
pack-revindex: radix-sort the revindex
The pack revindex stores the offsets of the objects in the
pack in sorted order, allowing us to easily find the on-disk
size of each object. To compute it, we populate an array
with the offsets from the sha1-sorted idx file, and then use
qsort to order it by offsets.
That does O(n log n) offset comparisons, and profiling shows
that we spend most of our time in cmp_offset. However, since
we are sorting on a simple off_t, we can use numeric sorts
that perform better. A radix sort can run in O(k*n), where k
is the number of "digits" in our number. For a 64-bit off_t,
using 16-bit "digits" gives us k=4.
On the linux.git repo, with about 3M objects to sort, this
yields a 400% speedup. Here are the best-of-five numbers for
running
echo HEAD | git cat-file --batch-check="%(objectsize:disk)
on a fully packed repository, which is dominated by time
spent building the pack revindex:
before after
real 0m0.834s 0m0.204s
user 0m0.788s 0m0.164s
sys 0m0.040s 0m0.036s
This matches our algorithmic expectations. log(3M) is ~21.5,
so a traditional sort is ~21.5n. Our radix sort runs in k*n,
where k is the number of radix digits. In the worst case,
this is k=4 for a 64-bit off_t, but we can quit early when
the largest value to be sorted is smaller. For any
repository under 4G, k=2. Our algorithm makes two passes
over the list per radix digit, so we end up with 4n. That
should yield ~5.3x speedup. We see 4x here; the difference
is probably due to the extra bucket book-keeping the radix
sort has to do.
On a smaller repo, the difference is less impressive, as
log(n) is smaller. For git.git, with 173K objects (but still
k=2), we see a 2.7x improvement:
before after
real 0m0.046s 0m0.017s
user 0m0.036s 0m0.012s
sys 0m0.008s 0m0.000s
On even tinier repos (e.g., a few hundred objects), the
speedup goes away entirely, as the small advantage of the
radix sort gets erased by the book-keeping costs (and at
those sizes, the cost to generate the the rev-index gets
lost in the noise anyway).
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-07-11 20:16:00 +08:00
|
|
|
/*
|
|
|
|
|
* This is a least-significant-digit radix sort.
|
|
|
|
|
*
|
|
|
|
|
* It sorts each of the "n" items in "entries" by its offset field. The "max"
|
|
|
|
|
* parameter must be at least as large as the largest offset in the array,
|
|
|
|
|
* and lets us quit the sort early.
|
|
|
|
|
*/
|
|
|
|
|
static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
|
2008-02-28 13:25:17 +08:00
|
|
|
{
|
pack-revindex: radix-sort the revindex
The pack revindex stores the offsets of the objects in the
pack in sorted order, allowing us to easily find the on-disk
size of each object. To compute it, we populate an array
with the offsets from the sha1-sorted idx file, and then use
qsort to order it by offsets.
That does O(n log n) offset comparisons, and profiling shows
that we spend most of our time in cmp_offset. However, since
we are sorting on a simple off_t, we can use numeric sorts
that perform better. A radix sort can run in O(k*n), where k
is the number of "digits" in our number. For a 64-bit off_t,
using 16-bit "digits" gives us k=4.
On the linux.git repo, with about 3M objects to sort, this
yields a 400% speedup. Here are the best-of-five numbers for
running
echo HEAD | git cat-file --batch-check="%(objectsize:disk)
on a fully packed repository, which is dominated by time
spent building the pack revindex:
before after
real 0m0.834s 0m0.204s
user 0m0.788s 0m0.164s
sys 0m0.040s 0m0.036s
This matches our algorithmic expectations. log(3M) is ~21.5,
so a traditional sort is ~21.5n. Our radix sort runs in k*n,
where k is the number of radix digits. In the worst case,
this is k=4 for a 64-bit off_t, but we can quit early when
the largest value to be sorted is smaller. For any
repository under 4G, k=2. Our algorithm makes two passes
over the list per radix digit, so we end up with 4n. That
should yield ~5.3x speedup. We see 4x here; the difference
is probably due to the extra bucket book-keeping the radix
sort has to do.
On a smaller repo, the difference is less impressive, as
log(n) is smaller. For git.git, with 173K objects (but still
k=2), we see a 2.7x improvement:
before after
real 0m0.046s 0m0.017s
user 0m0.036s 0m0.012s
sys 0m0.008s 0m0.000s
On even tinier repos (e.g., a few hundred objects), the
speedup goes away entirely, as the small advantage of the
radix sort gets erased by the book-keeping costs (and at
those sizes, the cost to generate the the rev-index gets
lost in the noise anyway).
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-07-11 20:16:00 +08:00
|
|
|
/*
|
|
|
|
|
* We use a "digit" size of 16 bits. That keeps our memory
|
|
|
|
|
* usage reasonable, and we can generally (for a 4G or smaller
|
|
|
|
|
* packfile) quit after two rounds of radix-sorting.
|
|
|
|
|
*/
|
|
|
|
|
#define DIGIT_SIZE (16)
|
|
|
|
|
#define BUCKETS (1 << DIGIT_SIZE)
|
|
|
|
|
/*
|
|
|
|
|
* We want to know the bucket that a[i] will go into when we are using
|
|
|
|
|
* the digit that is N bits from the (least significant) end.
|
|
|
|
|
*/
|
|
|
|
|
#define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* We need O(n) temporary storage. Rather than do an extra copy of the
|
|
|
|
|
* partial results into "entries", we sort back and forth between the
|
|
|
|
|
* real array and temporary storage. In each iteration of the loop, we
|
|
|
|
|
* keep track of them with alias pointers, always sorting from "from"
|
|
|
|
|
* to "to".
|
|
|
|
|
*/
|
2016-02-23 06:44:25 +08:00
|
|
|
struct revindex_entry *tmp, *from, *to;
|
pack-revindex: radix-sort the revindex
The pack revindex stores the offsets of the objects in the
pack in sorted order, allowing us to easily find the on-disk
size of each object. To compute it, we populate an array
with the offsets from the sha1-sorted idx file, and then use
qsort to order it by offsets.
That does O(n log n) offset comparisons, and profiling shows
that we spend most of our time in cmp_offset. However, since
we are sorting on a simple off_t, we can use numeric sorts
that perform better. A radix sort can run in O(k*n), where k
is the number of "digits" in our number. For a 64-bit off_t,
using 16-bit "digits" gives us k=4.
On the linux.git repo, with about 3M objects to sort, this
yields a 400% speedup. Here are the best-of-five numbers for
running
echo HEAD | git cat-file --batch-check="%(objectsize:disk)
on a fully packed repository, which is dominated by time
spent building the pack revindex:
before after
real 0m0.834s 0m0.204s
user 0m0.788s 0m0.164s
sys 0m0.040s 0m0.036s
This matches our algorithmic expectations. log(3M) is ~21.5,
so a traditional sort is ~21.5n. Our radix sort runs in k*n,
where k is the number of radix digits. In the worst case,
this is k=4 for a 64-bit off_t, but we can quit early when
the largest value to be sorted is smaller. For any
repository under 4G, k=2. Our algorithm makes two passes
over the list per radix digit, so we end up with 4n. That
should yield ~5.3x speedup. We see 4x here; the difference
is probably due to the extra bucket book-keeping the radix
sort has to do.
On a smaller repo, the difference is less impressive, as
log(n) is smaller. For git.git, with 173K objects (but still
k=2), we see a 2.7x improvement:
before after
real 0m0.046s 0m0.017s
user 0m0.036s 0m0.012s
sys 0m0.008s 0m0.000s
On even tinier repos (e.g., a few hundred objects), the
speedup goes away entirely, as the small advantage of the
radix sort gets erased by the book-keeping costs (and at
those sizes, the cost to generate the the rev-index gets
lost in the noise anyway).
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-07-11 20:16:00 +08:00
|
|
|
int bits;
|
2016-02-23 06:44:25 +08:00
|
|
|
unsigned *pos;
|
|
|
|
|
|
|
|
|
|
ALLOC_ARRAY(pos, BUCKETS);
|
|
|
|
|
ALLOC_ARRAY(tmp, n);
|
|
|
|
|
from = entries;
|
|
|
|
|
to = tmp;
|
pack-revindex: radix-sort the revindex
The pack revindex stores the offsets of the objects in the
pack in sorted order, allowing us to easily find the on-disk
size of each object. To compute it, we populate an array
with the offsets from the sha1-sorted idx file, and then use
qsort to order it by offsets.
That does O(n log n) offset comparisons, and profiling shows
that we spend most of our time in cmp_offset. However, since
we are sorting on a simple off_t, we can use numeric sorts
that perform better. A radix sort can run in O(k*n), where k
is the number of "digits" in our number. For a 64-bit off_t,
using 16-bit "digits" gives us k=4.
On the linux.git repo, with about 3M objects to sort, this
yields a 400% speedup. Here are the best-of-five numbers for
running
echo HEAD | git cat-file --batch-check="%(objectsize:disk)
on a fully packed repository, which is dominated by time
spent building the pack revindex:
before after
real 0m0.834s 0m0.204s
user 0m0.788s 0m0.164s
sys 0m0.040s 0m0.036s
This matches our algorithmic expectations. log(3M) is ~21.5,
so a traditional sort is ~21.5n. Our radix sort runs in k*n,
where k is the number of radix digits. In the worst case,
this is k=4 for a 64-bit off_t, but we can quit early when
the largest value to be sorted is smaller. For any
repository under 4G, k=2. Our algorithm makes two passes
over the list per radix digit, so we end up with 4n. That
should yield ~5.3x speedup. We see 4x here; the difference
is probably due to the extra bucket book-keeping the radix
sort has to do.
On a smaller repo, the difference is less impressive, as
log(n) is smaller. For git.git, with 173K objects (but still
k=2), we see a 2.7x improvement:
before after
real 0m0.046s 0m0.017s
user 0m0.036s 0m0.012s
sys 0m0.008s 0m0.000s
On even tinier repos (e.g., a few hundred objects), the
speedup goes away entirely, as the small advantage of the
radix sort gets erased by the book-keeping costs (and at
those sizes, the cost to generate the the rev-index gets
lost in the noise anyway).
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-07-11 20:16:00 +08:00
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* If (max >> bits) is zero, then we know that the radix digit we are
|
|
|
|
|
* on (and any higher) will be zero for all entries, and our loop will
|
|
|
|
|
* be a no-op, as everybody lands in the same zero-th bucket.
|
|
|
|
|
*/
|
|
|
|
|
for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
|
|
|
|
|
unsigned i;
|
|
|
|
|
|
|
|
|
|
memset(pos, 0, BUCKETS * sizeof(*pos));
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* We want pos[i] to store the index of the last element that
|
|
|
|
|
* will go in bucket "i" (actually one past the last element).
|
|
|
|
|
* To do this, we first count the items that will go in each
|
|
|
|
|
* bucket, which gives us a relative offset from the last
|
|
|
|
|
* bucket. We can then cumulatively add the index from the
|
|
|
|
|
* previous bucket to get the true index.
|
|
|
|
|
*/
|
|
|
|
|
for (i = 0; i < n; i++)
|
|
|
|
|
pos[BUCKET_FOR(from, i, bits)]++;
|
|
|
|
|
for (i = 1; i < BUCKETS; i++)
|
|
|
|
|
pos[i] += pos[i-1];
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* Now we can drop the elements into their correct buckets (in
|
|
|
|
|
* our temporary array). We iterate the pos counter backwards
|
|
|
|
|
* to avoid using an extra index to count up. And since we are
|
|
|
|
|
* going backwards there, we must also go backwards through the
|
|
|
|
|
* array itself, to keep the sort stable.
|
|
|
|
|
*
|
|
|
|
|
* Note that we use an unsigned iterator to make sure we can
|
|
|
|
|
* handle 2^32-1 objects, even on a 32-bit system. But this
|
|
|
|
|
* means we cannot use the more obvious "i >= 0" loop condition
|
|
|
|
|
* for counting backwards, and must instead check for
|
|
|
|
|
* wrap-around with UINT_MAX.
|
|
|
|
|
*/
|
|
|
|
|
for (i = n - 1; i != UINT_MAX; i--)
|
|
|
|
|
to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* Now "to" contains the most sorted list, so we swap "from" and
|
|
|
|
|
* "to" for the next iteration.
|
|
|
|
|
*/
|
2017-01-29 05:40:58 +08:00
|
|
|
SWAP(from, to);
|
pack-revindex: radix-sort the revindex
The pack revindex stores the offsets of the objects in the
pack in sorted order, allowing us to easily find the on-disk
size of each object. To compute it, we populate an array
with the offsets from the sha1-sorted idx file, and then use
qsort to order it by offsets.
That does O(n log n) offset comparisons, and profiling shows
that we spend most of our time in cmp_offset. However, since
we are sorting on a simple off_t, we can use numeric sorts
that perform better. A radix sort can run in O(k*n), where k
is the number of "digits" in our number. For a 64-bit off_t,
using 16-bit "digits" gives us k=4.
On the linux.git repo, with about 3M objects to sort, this
yields a 400% speedup. Here are the best-of-five numbers for
running
echo HEAD | git cat-file --batch-check="%(objectsize:disk)
on a fully packed repository, which is dominated by time
spent building the pack revindex:
before after
real 0m0.834s 0m0.204s
user 0m0.788s 0m0.164s
sys 0m0.040s 0m0.036s
This matches our algorithmic expectations. log(3M) is ~21.5,
so a traditional sort is ~21.5n. Our radix sort runs in k*n,
where k is the number of radix digits. In the worst case,
this is k=4 for a 64-bit off_t, but we can quit early when
the largest value to be sorted is smaller. For any
repository under 4G, k=2. Our algorithm makes two passes
over the list per radix digit, so we end up with 4n. That
should yield ~5.3x speedup. We see 4x here; the difference
is probably due to the extra bucket book-keeping the radix
sort has to do.
On a smaller repo, the difference is less impressive, as
log(n) is smaller. For git.git, with 173K objects (but still
k=2), we see a 2.7x improvement:
before after
real 0m0.046s 0m0.017s
user 0m0.036s 0m0.012s
sys 0m0.008s 0m0.000s
On even tinier repos (e.g., a few hundred objects), the
speedup goes away entirely, as the small advantage of the
radix sort gets erased by the book-keeping costs (and at
those sizes, the cost to generate the the rev-index gets
lost in the noise anyway).
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-07-11 20:16:00 +08:00
|
|
|
}
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* If we ended with our data in the original array, great. If not,
|
|
|
|
|
* we have to move it back from the temporary storage.
|
|
|
|
|
*/
|
|
|
|
|
if (from != entries)
|
2016-09-25 15:24:03 +08:00
|
|
|
COPY_ARRAY(entries, tmp, n);
|
pack-revindex: radix-sort the revindex
The pack revindex stores the offsets of the objects in the
pack in sorted order, allowing us to easily find the on-disk
size of each object. To compute it, we populate an array
with the offsets from the sha1-sorted idx file, and then use
qsort to order it by offsets.
That does O(n log n) offset comparisons, and profiling shows
that we spend most of our time in cmp_offset. However, since
we are sorting on a simple off_t, we can use numeric sorts
that perform better. A radix sort can run in O(k*n), where k
is the number of "digits" in our number. For a 64-bit off_t,
using 16-bit "digits" gives us k=4.
On the linux.git repo, with about 3M objects to sort, this
yields a 400% speedup. Here are the best-of-five numbers for
running
echo HEAD | git cat-file --batch-check="%(objectsize:disk)
on a fully packed repository, which is dominated by time
spent building the pack revindex:
before after
real 0m0.834s 0m0.204s
user 0m0.788s 0m0.164s
sys 0m0.040s 0m0.036s
This matches our algorithmic expectations. log(3M) is ~21.5,
so a traditional sort is ~21.5n. Our radix sort runs in k*n,
where k is the number of radix digits. In the worst case,
this is k=4 for a 64-bit off_t, but we can quit early when
the largest value to be sorted is smaller. For any
repository under 4G, k=2. Our algorithm makes two passes
over the list per radix digit, so we end up with 4n. That
should yield ~5.3x speedup. We see 4x here; the difference
is probably due to the extra bucket book-keeping the radix
sort has to do.
On a smaller repo, the difference is less impressive, as
log(n) is smaller. For git.git, with 173K objects (but still
k=2), we see a 2.7x improvement:
before after
real 0m0.046s 0m0.017s
user 0m0.036s 0m0.012s
sys 0m0.008s 0m0.000s
On even tinier repos (e.g., a few hundred objects), the
speedup goes away entirely, as the small advantage of the
radix sort gets erased by the book-keeping costs (and at
those sizes, the cost to generate the the rev-index gets
lost in the noise anyway).
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2013-07-11 20:16:00 +08:00
|
|
|
free(tmp);
|
|
|
|
|
free(pos);
|
|
|
|
|
|
|
|
|
|
#undef BUCKET_FOR
|
|
|
|
|
#undef BUCKETS
|
|
|
|
|
#undef DIGIT_SIZE
|
2008-02-28 13:25:17 +08:00
|
|
|
}
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* Ordered list of offsets of objects in the pack.
|
|
|
|
|
*/
|
2015-12-21 14:20:33 +08:00
|
|
|
static void create_pack_revindex(struct packed_git *p)
|
2008-02-28 13:25:17 +08:00
|
|
|
{
|
2019-02-03 07:16:45 +08:00
|
|
|
const unsigned num_ent = p->num_objects;
|
2013-07-10 19:50:26 +08:00
|
|
|
unsigned i;
|
2008-02-28 13:25:17 +08:00
|
|
|
const char *index = p->index_data;
|
2018-10-15 08:01:53 +08:00
|
|
|
const unsigned hashsz = the_hash_algo->rawsz;
|
2008-02-28 13:25:17 +08:00
|
|
|
|
2016-02-27 05:37:16 +08:00
|
|
|
ALLOC_ARRAY(p->revindex, num_ent + 1);
|
2008-02-28 13:25:17 +08:00
|
|
|
index += 4 * 256;
|
|
|
|
|
|
|
|
|
|
if (p->index_version > 1) {
|
|
|
|
|
const uint32_t *off_32 =
|
compute pack .idx byte offsets using size_t
A pack and its matching .idx file are limited to 2^32 objects, because
the pack format contains a 32-bit field to store the number of objects.
Hence we use uint32_t in the code.
But the byte count of even a .idx file can be much larger than that,
because it stores at least a hash and an offset for each object. So
using SHA-1, a v2 .idx file will cross the 4GB boundary at 153,391,650
objects. This confuses load_idx(), which computes the minimum size like
this:
unsigned long min_size = 8 + 4*256 + nr*(hashsz + 4 + 4) + hashsz + hashsz;
Even though min_size will be big enough on most 64-bit platforms, the
actual arithmetic is done as a uint32_t, resulting in a truncation. We
actually exceed that min_size, but then we do:
unsigned long max_size = min_size;
if (nr)
max_size += (nr - 1)*8;
to account for the variable-sized table. That computation doesn't
overflow quite so low, but with the truncation for min_size, we end up
with a max_size that is much smaller than our actual size. So we
complain that the idx is invalid, and can't find any of its objects.
We can fix this case by casting "nr" to a size_t, which will do the
multiplication in 64-bits (assuming you're on a 64-bit platform; this
will never work on a 32-bit system since we couldn't map the whole .idx
anyway). Likewise, we don't have to worry about further additions,
because adding a smaller number to a size_t will convert the other side
to a size_t.
A few notes:
- obviously we could just declare "nr" as a size_t in the first place
(and likewise, packed_git.num_objects). But it's conceptually a
uint32_t because of the on-disk format, and we correctly treat it
that way in other contexts that don't need to compute byte offsets
(e.g., iterating over the set of objects should and generally does
use a uint32_t). Switching to size_t would make all of those other
cases look wrong.
- it could be argued that the proper type is off_t to represent the
file offset. But in practice the .idx file must fit within memory,
because we mmap the whole thing. And the rest of the code (including
the idx_size variable we're comparing against) uses size_t.
- we'll add the same cast to the max_size arithmetic line. Even though
we're adding to a larger type, which will convert our result, the
multiplication is still done as a 32-bit value and can itself
overflow. I didn't check this with my test case, since it would need
an even larger pack (~530M objects), but looking at compiler output
shows that it works this way. The standard should agree, but I
couldn't find anything explicit in 6.3.1.8 ("usual arithmetic
conversions").
The case in load_idx() was the most immediate one that I was able to
trigger. After fixing it, looking up actual objects (including the very
last one in sha1 order) works in a test repo with 153,725,110 objects.
That's because bsearch_hash() works with uint32_t entry indices, and the
actual byte access:
int cmp = hashcmp(table + mi * stride, sha1);
is done with "stride" as a size_t, causing the uint32_t "mi" to be
promoted to a size_t. This is the way most code will access the index
data.
However, I audited all of the other byte-wise accesses of
packed_git.index_data, and many of the others are suspect (they are
similar to the max_size one, where we are adding to a properly sized
offset or directly to a pointer, but the multiplication in the
sub-expression can overflow). I didn't trigger any of these in practice,
but I believe they're potential problems, and certainly adding in the
cast is not going to hurt anything here.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2020-11-13 13:06:48 +08:00
|
|
|
(uint32_t *)(index + 8 + (size_t)p->num_objects * (hashsz + 4));
|
2008-02-28 13:25:17 +08:00
|
|
|
const uint32_t *off_64 = off_32 + p->num_objects;
|
|
|
|
|
for (i = 0; i < num_ent; i++) {
|
2019-02-03 07:16:45 +08:00
|
|
|
const uint32_t off = ntohl(*off_32++);
|
2008-02-28 13:25:17 +08:00
|
|
|
if (!(off & 0x80000000)) {
|
2015-12-21 14:20:33 +08:00
|
|
|
p->revindex[i].offset = off;
|
2008-02-28 13:25:17 +08:00
|
|
|
} else {
|
2018-01-18 03:08:23 +08:00
|
|
|
p->revindex[i].offset = get_be64(off_64);
|
|
|
|
|
off_64 += 2;
|
2008-02-28 13:25:17 +08:00
|
|
|
}
|
2015-12-21 14:20:33 +08:00
|
|
|
p->revindex[i].nr = i;
|
2008-02-28 13:25:17 +08:00
|
|
|
}
|
|
|
|
|
} else {
|
|
|
|
|
for (i = 0; i < num_ent; i++) {
|
2019-02-03 07:16:45 +08:00
|
|
|
const uint32_t hl = *((uint32_t *)(index + (hashsz + 4) * i));
|
2015-12-21 14:20:33 +08:00
|
|
|
p->revindex[i].offset = ntohl(hl);
|
|
|
|
|
p->revindex[i].nr = i;
|
2008-02-28 13:25:17 +08:00
|
|
|
}
|
|
|
|
|
}
|
|
|
|
|
|
2018-10-15 08:01:53 +08:00
|
|
|
/*
|
|
|
|
|
* This knows the pack format -- the hash trailer
|
2008-02-28 13:25:17 +08:00
|
|
|
* follows immediately after the last object data.
|
|
|
|
|
*/
|
2018-10-15 08:01:53 +08:00
|
|
|
p->revindex[num_ent].offset = p->pack_size - hashsz;
|
2015-12-21 14:20:33 +08:00
|
|
|
p->revindex[num_ent].nr = -1;
|
|
|
|
|
sort_revindex(p->revindex, num_ent, p->pack_size);
|
2008-02-28 13:25:17 +08:00
|
|
|
}
|
|
|
|
|
|
packfile: prepare for the existence of '*.rev' files
Specify the format of the on-disk reverse index 'pack-*.rev' file, as
well as prepare the code for the existence of such files.
The reverse index maps from pack relative positions (i.e., an index into
the array of object which is sorted by their offsets within the
packfile) to their position within the 'pack-*.idx' file. Today, this is
done by building up a list of (off_t, uint32_t) tuples for each object
(the off_t corresponding to that object's offset, and the uint32_t
corresponding to its position in the index). To convert between pack and
index position quickly, this array of tuples is radix sorted based on
its offset.
This has two major drawbacks:
First, the in-memory cost scales linearly with the number of objects in
a pack. Each 'struct revindex_entry' is sizeof(off_t) +
sizeof(uint32_t) + padding bytes for a total of 16.
To observe this, force Git to load the reverse index by, for e.g.,
running 'git cat-file --batch-check="%(objectsize:disk)"'. When asking
for a single object in a fresh clone of the kernel, Git needs to
allocate 120+ MB of memory in order to hold the reverse index in memory.
Second, the cost to sort also scales with the size of the pack.
Luckily, this is a linear function since 'load_pack_revindex()' uses a
radix sort, but this cost still must be paid once per pack per process.
As an example, it takes ~60x longer to print the _size_ of an object as
it does to print that entire object's _contents_:
Benchmark #1: git.compile cat-file --batch <obj
Time (mean ± σ): 3.4 ms ± 0.1 ms [User: 3.3 ms, System: 2.1 ms]
Range (min … max): 3.2 ms … 3.7 ms 726 runs
Benchmark #2: git.compile cat-file --batch-check="%(objectsize:disk)" <obj
Time (mean ± σ): 210.3 ms ± 8.9 ms [User: 188.2 ms, System: 23.2 ms]
Range (min … max): 193.7 ms … 224.4 ms 13 runs
Instead, avoid computing and sorting the revindex once per process by
writing it to a file when the pack itself is generated.
The format is relatively straightforward. It contains an array of
uint32_t's, the length of which is equal to the number of objects in the
pack. The ith entry in this table contains the index position of the
ith object in the pack, where "ith object in the pack" is determined by
pack offset.
One thing that the on-disk format does _not_ contain is the full (up to)
eight-byte offset corresponding to each object. This is something that
the in-memory revindex contains (it stores an off_t in 'struct
revindex_entry' along with the same uint32_t that the on-disk format
has). Omit it in the on-disk format, since knowing the index position
for some object is sufficient to get a constant-time lookup in the
pack-*.idx file to ask for an object's offset within the pack.
This trades off between the on-disk size of the 'pack-*.rev' file for
runtime to chase down the offset for some object. Even though the lookup
is constant time, the constant is heavier, since it can potentially
involve two pointer walks in v2 indexes (one to access the 4-byte offset
table, and potentially a second to access the double wide offset table).
Consider trying to map an object's pack offset to a relative position
within that pack. In a cold-cache scenario, more page faults occur while
switching between binary searching through the reverse index and
searching through the *.idx file for an object's offset. Sure enough,
with a cold cache (writing '3' into '/proc/sys/vm/drop_caches' after
'sync'ing), printing out the entire object's contents is still
marginally faster than printing its size:
Benchmark #1: git.compile cat-file --batch-check="%(objectsize:disk)" <obj >/dev/null
Time (mean ± σ): 22.6 ms ± 0.5 ms [User: 2.4 ms, System: 7.9 ms]
Range (min … max): 21.4 ms … 23.5 ms 41 runs
Benchmark #2: git.compile cat-file --batch <obj >/dev/null
Time (mean ± σ): 17.2 ms ± 0.7 ms [User: 2.8 ms, System: 5.5 ms]
Range (min … max): 15.6 ms … 18.2 ms 45 runs
(Numbers taken in the kernel after cheating and using the next patch to
generate a reverse index). There are a couple of approaches to improve
cold cache performance not pursued here:
- We could include the object offsets in the reverse index format.
Predictably, this does result in fewer page faults, but it triples
the size of the file, while simultaneously duplicating a ton of data
already available in the .idx file. (This was the original way I
implemented the format, and it did show
`--batch-check='%(objectsize:disk)'` winning out against `--batch`.)
On the other hand, this increase in size also results in a large
block-cache footprint, which could potentially hurt other workloads.
- We could store the mapping from pack to index position in more
cache-friendly way, like constructing a binary search tree from the
table and writing the values in breadth-first order. This would
result in much better locality, but the price you pay is trading
O(1) lookup in 'pack_pos_to_index()' for an O(log n) one (since you
can no longer directly index the table).
So, neither of these approaches are taken here. (Thankfully, the format
is versioned, so we are free to pursue these in the future.) But, cold
cache performance likely isn't interesting outside of one-off cases like
asking for the size of an object directly. In real-world usage, Git is
often performing many operations in the revindex (i.e., asking about
many objects rather than a single one).
The trade-off is worth it, since we will avoid the vast majority of the
cost of generating the revindex that the extra pointer chase will look
like noise in the following patch's benchmarks.
This patch describes the format and prepares callers (like in
pack-revindex.c) to be able to read *.rev files once they exist. An
implementation of the writer will appear in the next patch, and callers
will gradually begin to start using the writer in the patches that
follow after that.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-26 07:37:14 +08:00
|
|
|
static int create_pack_revindex_in_memory(struct packed_git *p)
|
2008-02-28 13:25:17 +08:00
|
|
|
{
|
2021-01-26 07:37:46 +08:00
|
|
|
if (git_env_bool(GIT_TEST_REV_INDEX_DIE_IN_MEMORY, 0))
|
|
|
|
|
die("dying as requested by '%s'",
|
|
|
|
|
GIT_TEST_REV_INDEX_DIE_IN_MEMORY);
|
packfile: prepare for the existence of '*.rev' files
Specify the format of the on-disk reverse index 'pack-*.rev' file, as
well as prepare the code for the existence of such files.
The reverse index maps from pack relative positions (i.e., an index into
the array of object which is sorted by their offsets within the
packfile) to their position within the 'pack-*.idx' file. Today, this is
done by building up a list of (off_t, uint32_t) tuples for each object
(the off_t corresponding to that object's offset, and the uint32_t
corresponding to its position in the index). To convert between pack and
index position quickly, this array of tuples is radix sorted based on
its offset.
This has two major drawbacks:
First, the in-memory cost scales linearly with the number of objects in
a pack. Each 'struct revindex_entry' is sizeof(off_t) +
sizeof(uint32_t) + padding bytes for a total of 16.
To observe this, force Git to load the reverse index by, for e.g.,
running 'git cat-file --batch-check="%(objectsize:disk)"'. When asking
for a single object in a fresh clone of the kernel, Git needs to
allocate 120+ MB of memory in order to hold the reverse index in memory.
Second, the cost to sort also scales with the size of the pack.
Luckily, this is a linear function since 'load_pack_revindex()' uses a
radix sort, but this cost still must be paid once per pack per process.
As an example, it takes ~60x longer to print the _size_ of an object as
it does to print that entire object's _contents_:
Benchmark #1: git.compile cat-file --batch <obj
Time (mean ± σ): 3.4 ms ± 0.1 ms [User: 3.3 ms, System: 2.1 ms]
Range (min … max): 3.2 ms … 3.7 ms 726 runs
Benchmark #2: git.compile cat-file --batch-check="%(objectsize:disk)" <obj
Time (mean ± σ): 210.3 ms ± 8.9 ms [User: 188.2 ms, System: 23.2 ms]
Range (min … max): 193.7 ms … 224.4 ms 13 runs
Instead, avoid computing and sorting the revindex once per process by
writing it to a file when the pack itself is generated.
The format is relatively straightforward. It contains an array of
uint32_t's, the length of which is equal to the number of objects in the
pack. The ith entry in this table contains the index position of the
ith object in the pack, where "ith object in the pack" is determined by
pack offset.
One thing that the on-disk format does _not_ contain is the full (up to)
eight-byte offset corresponding to each object. This is something that
the in-memory revindex contains (it stores an off_t in 'struct
revindex_entry' along with the same uint32_t that the on-disk format
has). Omit it in the on-disk format, since knowing the index position
for some object is sufficient to get a constant-time lookup in the
pack-*.idx file to ask for an object's offset within the pack.
This trades off between the on-disk size of the 'pack-*.rev' file for
runtime to chase down the offset for some object. Even though the lookup
is constant time, the constant is heavier, since it can potentially
involve two pointer walks in v2 indexes (one to access the 4-byte offset
table, and potentially a second to access the double wide offset table).
Consider trying to map an object's pack offset to a relative position
within that pack. In a cold-cache scenario, more page faults occur while
switching between binary searching through the reverse index and
searching through the *.idx file for an object's offset. Sure enough,
with a cold cache (writing '3' into '/proc/sys/vm/drop_caches' after
'sync'ing), printing out the entire object's contents is still
marginally faster than printing its size:
Benchmark #1: git.compile cat-file --batch-check="%(objectsize:disk)" <obj >/dev/null
Time (mean ± σ): 22.6 ms ± 0.5 ms [User: 2.4 ms, System: 7.9 ms]
Range (min … max): 21.4 ms … 23.5 ms 41 runs
Benchmark #2: git.compile cat-file --batch <obj >/dev/null
Time (mean ± σ): 17.2 ms ± 0.7 ms [User: 2.8 ms, System: 5.5 ms]
Range (min … max): 15.6 ms … 18.2 ms 45 runs
(Numbers taken in the kernel after cheating and using the next patch to
generate a reverse index). There are a couple of approaches to improve
cold cache performance not pursued here:
- We could include the object offsets in the reverse index format.
Predictably, this does result in fewer page faults, but it triples
the size of the file, while simultaneously duplicating a ton of data
already available in the .idx file. (This was the original way I
implemented the format, and it did show
`--batch-check='%(objectsize:disk)'` winning out against `--batch`.)
On the other hand, this increase in size also results in a large
block-cache footprint, which could potentially hurt other workloads.
- We could store the mapping from pack to index position in more
cache-friendly way, like constructing a binary search tree from the
table and writing the values in breadth-first order. This would
result in much better locality, but the price you pay is trading
O(1) lookup in 'pack_pos_to_index()' for an O(log n) one (since you
can no longer directly index the table).
So, neither of these approaches are taken here. (Thankfully, the format
is versioned, so we are free to pursue these in the future.) But, cold
cache performance likely isn't interesting outside of one-off cases like
asking for the size of an object directly. In real-world usage, Git is
often performing many operations in the revindex (i.e., asking about
many objects rather than a single one).
The trade-off is worth it, since we will avoid the vast majority of the
cost of generating the revindex that the extra pointer chase will look
like noise in the following patch's benchmarks.
This patch describes the format and prepares callers (like in
pack-revindex.c) to be able to read *.rev files once they exist. An
implementation of the writer will appear in the next patch, and callers
will gradually begin to start using the writer in the patches that
follow after that.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-26 07:37:14 +08:00
|
|
|
if (open_pack_index(p))
|
|
|
|
|
return -1;
|
|
|
|
|
create_pack_revindex(p);
|
2019-04-06 02:04:24 +08:00
|
|
|
return 0;
|
2013-10-25 02:00:36 +08:00
|
|
|
}
|
|
|
|
|
|
packfile: prepare for the existence of '*.rev' files
Specify the format of the on-disk reverse index 'pack-*.rev' file, as
well as prepare the code for the existence of such files.
The reverse index maps from pack relative positions (i.e., an index into
the array of object which is sorted by their offsets within the
packfile) to their position within the 'pack-*.idx' file. Today, this is
done by building up a list of (off_t, uint32_t) tuples for each object
(the off_t corresponding to that object's offset, and the uint32_t
corresponding to its position in the index). To convert between pack and
index position quickly, this array of tuples is radix sorted based on
its offset.
This has two major drawbacks:
First, the in-memory cost scales linearly with the number of objects in
a pack. Each 'struct revindex_entry' is sizeof(off_t) +
sizeof(uint32_t) + padding bytes for a total of 16.
To observe this, force Git to load the reverse index by, for e.g.,
running 'git cat-file --batch-check="%(objectsize:disk)"'. When asking
for a single object in a fresh clone of the kernel, Git needs to
allocate 120+ MB of memory in order to hold the reverse index in memory.
Second, the cost to sort also scales with the size of the pack.
Luckily, this is a linear function since 'load_pack_revindex()' uses a
radix sort, but this cost still must be paid once per pack per process.
As an example, it takes ~60x longer to print the _size_ of an object as
it does to print that entire object's _contents_:
Benchmark #1: git.compile cat-file --batch <obj
Time (mean ± σ): 3.4 ms ± 0.1 ms [User: 3.3 ms, System: 2.1 ms]
Range (min … max): 3.2 ms … 3.7 ms 726 runs
Benchmark #2: git.compile cat-file --batch-check="%(objectsize:disk)" <obj
Time (mean ± σ): 210.3 ms ± 8.9 ms [User: 188.2 ms, System: 23.2 ms]
Range (min … max): 193.7 ms … 224.4 ms 13 runs
Instead, avoid computing and sorting the revindex once per process by
writing it to a file when the pack itself is generated.
The format is relatively straightforward. It contains an array of
uint32_t's, the length of which is equal to the number of objects in the
pack. The ith entry in this table contains the index position of the
ith object in the pack, where "ith object in the pack" is determined by
pack offset.
One thing that the on-disk format does _not_ contain is the full (up to)
eight-byte offset corresponding to each object. This is something that
the in-memory revindex contains (it stores an off_t in 'struct
revindex_entry' along with the same uint32_t that the on-disk format
has). Omit it in the on-disk format, since knowing the index position
for some object is sufficient to get a constant-time lookup in the
pack-*.idx file to ask for an object's offset within the pack.
This trades off between the on-disk size of the 'pack-*.rev' file for
runtime to chase down the offset for some object. Even though the lookup
is constant time, the constant is heavier, since it can potentially
involve two pointer walks in v2 indexes (one to access the 4-byte offset
table, and potentially a second to access the double wide offset table).
Consider trying to map an object's pack offset to a relative position
within that pack. In a cold-cache scenario, more page faults occur while
switching between binary searching through the reverse index and
searching through the *.idx file for an object's offset. Sure enough,
with a cold cache (writing '3' into '/proc/sys/vm/drop_caches' after
'sync'ing), printing out the entire object's contents is still
marginally faster than printing its size:
Benchmark #1: git.compile cat-file --batch-check="%(objectsize:disk)" <obj >/dev/null
Time (mean ± σ): 22.6 ms ± 0.5 ms [User: 2.4 ms, System: 7.9 ms]
Range (min … max): 21.4 ms … 23.5 ms 41 runs
Benchmark #2: git.compile cat-file --batch <obj >/dev/null
Time (mean ± σ): 17.2 ms ± 0.7 ms [User: 2.8 ms, System: 5.5 ms]
Range (min … max): 15.6 ms … 18.2 ms 45 runs
(Numbers taken in the kernel after cheating and using the next patch to
generate a reverse index). There are a couple of approaches to improve
cold cache performance not pursued here:
- We could include the object offsets in the reverse index format.
Predictably, this does result in fewer page faults, but it triples
the size of the file, while simultaneously duplicating a ton of data
already available in the .idx file. (This was the original way I
implemented the format, and it did show
`--batch-check='%(objectsize:disk)'` winning out against `--batch`.)
On the other hand, this increase in size also results in a large
block-cache footprint, which could potentially hurt other workloads.
- We could store the mapping from pack to index position in more
cache-friendly way, like constructing a binary search tree from the
table and writing the values in breadth-first order. This would
result in much better locality, but the price you pay is trading
O(1) lookup in 'pack_pos_to_index()' for an O(log n) one (since you
can no longer directly index the table).
So, neither of these approaches are taken here. (Thankfully, the format
is versioned, so we are free to pursue these in the future.) But, cold
cache performance likely isn't interesting outside of one-off cases like
asking for the size of an object directly. In real-world usage, Git is
often performing many operations in the revindex (i.e., asking about
many objects rather than a single one).
The trade-off is worth it, since we will avoid the vast majority of the
cost of generating the revindex that the extra pointer chase will look
like noise in the following patch's benchmarks.
This patch describes the format and prepares callers (like in
pack-revindex.c) to be able to read *.rev files once they exist. An
implementation of the writer will appear in the next patch, and callers
will gradually begin to start using the writer in the patches that
follow after that.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-26 07:37:14 +08:00
|
|
|
static char *pack_revindex_filename(struct packed_git *p)
|
|
|
|
|
{
|
|
|
|
|
size_t len;
|
|
|
|
|
if (!strip_suffix(p->pack_name, ".pack", &len))
|
|
|
|
|
BUG("pack_name does not end in .pack");
|
|
|
|
|
return xstrfmt("%.*s.rev", (int)len, p->pack_name);
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
#define RIDX_HEADER_SIZE (12)
|
|
|
|
|
#define RIDX_MIN_SIZE (RIDX_HEADER_SIZE + (2 * the_hash_algo->rawsz))
|
|
|
|
|
|
|
|
|
|
struct revindex_header {
|
|
|
|
|
uint32_t signature;
|
|
|
|
|
uint32_t version;
|
|
|
|
|
uint32_t hash_id;
|
|
|
|
|
};
|
|
|
|
|
|
|
|
|
|
static int load_revindex_from_disk(char *revindex_name,
|
|
|
|
|
uint32_t num_objects,
|
|
|
|
|
const uint32_t **data_p, size_t *len_p)
|
|
|
|
|
{
|
|
|
|
|
int fd, ret = 0;
|
|
|
|
|
struct stat st;
|
|
|
|
|
void *data = NULL;
|
|
|
|
|
size_t revindex_size;
|
|
|
|
|
struct revindex_header *hdr;
|
|
|
|
|
|
2023-04-13 06:20:27 +08:00
|
|
|
if (git_env_bool(GIT_TEST_REV_INDEX_DIE_ON_DISK, 0))
|
|
|
|
|
die("dying as requested by '%s'", GIT_TEST_REV_INDEX_DIE_ON_DISK);
|
|
|
|
|
|
packfile: prepare for the existence of '*.rev' files
Specify the format of the on-disk reverse index 'pack-*.rev' file, as
well as prepare the code for the existence of such files.
The reverse index maps from pack relative positions (i.e., an index into
the array of object which is sorted by their offsets within the
packfile) to their position within the 'pack-*.idx' file. Today, this is
done by building up a list of (off_t, uint32_t) tuples for each object
(the off_t corresponding to that object's offset, and the uint32_t
corresponding to its position in the index). To convert between pack and
index position quickly, this array of tuples is radix sorted based on
its offset.
This has two major drawbacks:
First, the in-memory cost scales linearly with the number of objects in
a pack. Each 'struct revindex_entry' is sizeof(off_t) +
sizeof(uint32_t) + padding bytes for a total of 16.
To observe this, force Git to load the reverse index by, for e.g.,
running 'git cat-file --batch-check="%(objectsize:disk)"'. When asking
for a single object in a fresh clone of the kernel, Git needs to
allocate 120+ MB of memory in order to hold the reverse index in memory.
Second, the cost to sort also scales with the size of the pack.
Luckily, this is a linear function since 'load_pack_revindex()' uses a
radix sort, but this cost still must be paid once per pack per process.
As an example, it takes ~60x longer to print the _size_ of an object as
it does to print that entire object's _contents_:
Benchmark #1: git.compile cat-file --batch <obj
Time (mean ± σ): 3.4 ms ± 0.1 ms [User: 3.3 ms, System: 2.1 ms]
Range (min … max): 3.2 ms … 3.7 ms 726 runs
Benchmark #2: git.compile cat-file --batch-check="%(objectsize:disk)" <obj
Time (mean ± σ): 210.3 ms ± 8.9 ms [User: 188.2 ms, System: 23.2 ms]
Range (min … max): 193.7 ms … 224.4 ms 13 runs
Instead, avoid computing and sorting the revindex once per process by
writing it to a file when the pack itself is generated.
The format is relatively straightforward. It contains an array of
uint32_t's, the length of which is equal to the number of objects in the
pack. The ith entry in this table contains the index position of the
ith object in the pack, where "ith object in the pack" is determined by
pack offset.
One thing that the on-disk format does _not_ contain is the full (up to)
eight-byte offset corresponding to each object. This is something that
the in-memory revindex contains (it stores an off_t in 'struct
revindex_entry' along with the same uint32_t that the on-disk format
has). Omit it in the on-disk format, since knowing the index position
for some object is sufficient to get a constant-time lookup in the
pack-*.idx file to ask for an object's offset within the pack.
This trades off between the on-disk size of the 'pack-*.rev' file for
runtime to chase down the offset for some object. Even though the lookup
is constant time, the constant is heavier, since it can potentially
involve two pointer walks in v2 indexes (one to access the 4-byte offset
table, and potentially a second to access the double wide offset table).
Consider trying to map an object's pack offset to a relative position
within that pack. In a cold-cache scenario, more page faults occur while
switching between binary searching through the reverse index and
searching through the *.idx file for an object's offset. Sure enough,
with a cold cache (writing '3' into '/proc/sys/vm/drop_caches' after
'sync'ing), printing out the entire object's contents is still
marginally faster than printing its size:
Benchmark #1: git.compile cat-file --batch-check="%(objectsize:disk)" <obj >/dev/null
Time (mean ± σ): 22.6 ms ± 0.5 ms [User: 2.4 ms, System: 7.9 ms]
Range (min … max): 21.4 ms … 23.5 ms 41 runs
Benchmark #2: git.compile cat-file --batch <obj >/dev/null
Time (mean ± σ): 17.2 ms ± 0.7 ms [User: 2.8 ms, System: 5.5 ms]
Range (min … max): 15.6 ms … 18.2 ms 45 runs
(Numbers taken in the kernel after cheating and using the next patch to
generate a reverse index). There are a couple of approaches to improve
cold cache performance not pursued here:
- We could include the object offsets in the reverse index format.
Predictably, this does result in fewer page faults, but it triples
the size of the file, while simultaneously duplicating a ton of data
already available in the .idx file. (This was the original way I
implemented the format, and it did show
`--batch-check='%(objectsize:disk)'` winning out against `--batch`.)
On the other hand, this increase in size also results in a large
block-cache footprint, which could potentially hurt other workloads.
- We could store the mapping from pack to index position in more
cache-friendly way, like constructing a binary search tree from the
table and writing the values in breadth-first order. This would
result in much better locality, but the price you pay is trading
O(1) lookup in 'pack_pos_to_index()' for an O(log n) one (since you
can no longer directly index the table).
So, neither of these approaches are taken here. (Thankfully, the format
is versioned, so we are free to pursue these in the future.) But, cold
cache performance likely isn't interesting outside of one-off cases like
asking for the size of an object directly. In real-world usage, Git is
often performing many operations in the revindex (i.e., asking about
many objects rather than a single one).
The trade-off is worth it, since we will avoid the vast majority of the
cost of generating the revindex that the extra pointer chase will look
like noise in the following patch's benchmarks.
This patch describes the format and prepares callers (like in
pack-revindex.c) to be able to read *.rev files once they exist. An
implementation of the writer will appear in the next patch, and callers
will gradually begin to start using the writer in the patches that
follow after that.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-26 07:37:14 +08:00
|
|
|
fd = git_open(revindex_name);
|
|
|
|
|
|
|
|
|
|
if (fd < 0) {
|
2023-04-18 00:21:41 +08:00
|
|
|
/* "No file" means return 1. */
|
|
|
|
|
ret = 1;
|
packfile: prepare for the existence of '*.rev' files
Specify the format of the on-disk reverse index 'pack-*.rev' file, as
well as prepare the code for the existence of such files.
The reverse index maps from pack relative positions (i.e., an index into
the array of object which is sorted by their offsets within the
packfile) to their position within the 'pack-*.idx' file. Today, this is
done by building up a list of (off_t, uint32_t) tuples for each object
(the off_t corresponding to that object's offset, and the uint32_t
corresponding to its position in the index). To convert between pack and
index position quickly, this array of tuples is radix sorted based on
its offset.
This has two major drawbacks:
First, the in-memory cost scales linearly with the number of objects in
a pack. Each 'struct revindex_entry' is sizeof(off_t) +
sizeof(uint32_t) + padding bytes for a total of 16.
To observe this, force Git to load the reverse index by, for e.g.,
running 'git cat-file --batch-check="%(objectsize:disk)"'. When asking
for a single object in a fresh clone of the kernel, Git needs to
allocate 120+ MB of memory in order to hold the reverse index in memory.
Second, the cost to sort also scales with the size of the pack.
Luckily, this is a linear function since 'load_pack_revindex()' uses a
radix sort, but this cost still must be paid once per pack per process.
As an example, it takes ~60x longer to print the _size_ of an object as
it does to print that entire object's _contents_:
Benchmark #1: git.compile cat-file --batch <obj
Time (mean ± σ): 3.4 ms ± 0.1 ms [User: 3.3 ms, System: 2.1 ms]
Range (min … max): 3.2 ms … 3.7 ms 726 runs
Benchmark #2: git.compile cat-file --batch-check="%(objectsize:disk)" <obj
Time (mean ± σ): 210.3 ms ± 8.9 ms [User: 188.2 ms, System: 23.2 ms]
Range (min … max): 193.7 ms … 224.4 ms 13 runs
Instead, avoid computing and sorting the revindex once per process by
writing it to a file when the pack itself is generated.
The format is relatively straightforward. It contains an array of
uint32_t's, the length of which is equal to the number of objects in the
pack. The ith entry in this table contains the index position of the
ith object in the pack, where "ith object in the pack" is determined by
pack offset.
One thing that the on-disk format does _not_ contain is the full (up to)
eight-byte offset corresponding to each object. This is something that
the in-memory revindex contains (it stores an off_t in 'struct
revindex_entry' along with the same uint32_t that the on-disk format
has). Omit it in the on-disk format, since knowing the index position
for some object is sufficient to get a constant-time lookup in the
pack-*.idx file to ask for an object's offset within the pack.
This trades off between the on-disk size of the 'pack-*.rev' file for
runtime to chase down the offset for some object. Even though the lookup
is constant time, the constant is heavier, since it can potentially
involve two pointer walks in v2 indexes (one to access the 4-byte offset
table, and potentially a second to access the double wide offset table).
Consider trying to map an object's pack offset to a relative position
within that pack. In a cold-cache scenario, more page faults occur while
switching between binary searching through the reverse index and
searching through the *.idx file for an object's offset. Sure enough,
with a cold cache (writing '3' into '/proc/sys/vm/drop_caches' after
'sync'ing), printing out the entire object's contents is still
marginally faster than printing its size:
Benchmark #1: git.compile cat-file --batch-check="%(objectsize:disk)" <obj >/dev/null
Time (mean ± σ): 22.6 ms ± 0.5 ms [User: 2.4 ms, System: 7.9 ms]
Range (min … max): 21.4 ms … 23.5 ms 41 runs
Benchmark #2: git.compile cat-file --batch <obj >/dev/null
Time (mean ± σ): 17.2 ms ± 0.7 ms [User: 2.8 ms, System: 5.5 ms]
Range (min … max): 15.6 ms … 18.2 ms 45 runs
(Numbers taken in the kernel after cheating and using the next patch to
generate a reverse index). There are a couple of approaches to improve
cold cache performance not pursued here:
- We could include the object offsets in the reverse index format.
Predictably, this does result in fewer page faults, but it triples
the size of the file, while simultaneously duplicating a ton of data
already available in the .idx file. (This was the original way I
implemented the format, and it did show
`--batch-check='%(objectsize:disk)'` winning out against `--batch`.)
On the other hand, this increase in size also results in a large
block-cache footprint, which could potentially hurt other workloads.
- We could store the mapping from pack to index position in more
cache-friendly way, like constructing a binary search tree from the
table and writing the values in breadth-first order. This would
result in much better locality, but the price you pay is trading
O(1) lookup in 'pack_pos_to_index()' for an O(log n) one (since you
can no longer directly index the table).
So, neither of these approaches are taken here. (Thankfully, the format
is versioned, so we are free to pursue these in the future.) But, cold
cache performance likely isn't interesting outside of one-off cases like
asking for the size of an object directly. In real-world usage, Git is
often performing many operations in the revindex (i.e., asking about
many objects rather than a single one).
The trade-off is worth it, since we will avoid the vast majority of the
cost of generating the revindex that the extra pointer chase will look
like noise in the following patch's benchmarks.
This patch describes the format and prepares callers (like in
pack-revindex.c) to be able to read *.rev files once they exist. An
implementation of the writer will appear in the next patch, and callers
will gradually begin to start using the writer in the patches that
follow after that.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-26 07:37:14 +08:00
|
|
|
goto cleanup;
|
|
|
|
|
}
|
|
|
|
|
if (fstat(fd, &st)) {
|
|
|
|
|
ret = error_errno(_("failed to read %s"), revindex_name);
|
|
|
|
|
goto cleanup;
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
revindex_size = xsize_t(st.st_size);
|
|
|
|
|
|
|
|
|
|
if (revindex_size < RIDX_MIN_SIZE) {
|
|
|
|
|
ret = error(_("reverse-index file %s is too small"), revindex_name);
|
|
|
|
|
goto cleanup;
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
if (revindex_size - RIDX_MIN_SIZE != st_mult(sizeof(uint32_t), num_objects)) {
|
|
|
|
|
ret = error(_("reverse-index file %s is corrupt"), revindex_name);
|
|
|
|
|
goto cleanup;
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
data = xmmap(NULL, revindex_size, PROT_READ, MAP_PRIVATE, fd, 0);
|
|
|
|
|
hdr = data;
|
|
|
|
|
|
|
|
|
|
if (ntohl(hdr->signature) != RIDX_SIGNATURE) {
|
|
|
|
|
ret = error(_("reverse-index file %s has unknown signature"), revindex_name);
|
|
|
|
|
goto cleanup;
|
|
|
|
|
}
|
|
|
|
|
if (ntohl(hdr->version) != 1) {
|
|
|
|
|
ret = error(_("reverse-index file %s has unsupported version %"PRIu32),
|
|
|
|
|
revindex_name, ntohl(hdr->version));
|
|
|
|
|
goto cleanup;
|
|
|
|
|
}
|
|
|
|
|
if (!(ntohl(hdr->hash_id) == 1 || ntohl(hdr->hash_id) == 2)) {
|
|
|
|
|
ret = error(_("reverse-index file %s has unsupported hash id %"PRIu32),
|
|
|
|
|
revindex_name, ntohl(hdr->hash_id));
|
|
|
|
|
goto cleanup;
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
cleanup:
|
|
|
|
|
if (ret) {
|
|
|
|
|
if (data)
|
|
|
|
|
munmap(data, revindex_size);
|
|
|
|
|
} else {
|
|
|
|
|
*len_p = revindex_size;
|
|
|
|
|
*data_p = (const uint32_t *)data;
|
|
|
|
|
}
|
|
|
|
|
|
2021-02-27 00:31:02 +08:00
|
|
|
if (fd >= 0)
|
|
|
|
|
close(fd);
|
packfile: prepare for the existence of '*.rev' files
Specify the format of the on-disk reverse index 'pack-*.rev' file, as
well as prepare the code for the existence of such files.
The reverse index maps from pack relative positions (i.e., an index into
the array of object which is sorted by their offsets within the
packfile) to their position within the 'pack-*.idx' file. Today, this is
done by building up a list of (off_t, uint32_t) tuples for each object
(the off_t corresponding to that object's offset, and the uint32_t
corresponding to its position in the index). To convert between pack and
index position quickly, this array of tuples is radix sorted based on
its offset.
This has two major drawbacks:
First, the in-memory cost scales linearly with the number of objects in
a pack. Each 'struct revindex_entry' is sizeof(off_t) +
sizeof(uint32_t) + padding bytes for a total of 16.
To observe this, force Git to load the reverse index by, for e.g.,
running 'git cat-file --batch-check="%(objectsize:disk)"'. When asking
for a single object in a fresh clone of the kernel, Git needs to
allocate 120+ MB of memory in order to hold the reverse index in memory.
Second, the cost to sort also scales with the size of the pack.
Luckily, this is a linear function since 'load_pack_revindex()' uses a
radix sort, but this cost still must be paid once per pack per process.
As an example, it takes ~60x longer to print the _size_ of an object as
it does to print that entire object's _contents_:
Benchmark #1: git.compile cat-file --batch <obj
Time (mean ± σ): 3.4 ms ± 0.1 ms [User: 3.3 ms, System: 2.1 ms]
Range (min … max): 3.2 ms … 3.7 ms 726 runs
Benchmark #2: git.compile cat-file --batch-check="%(objectsize:disk)" <obj
Time (mean ± σ): 210.3 ms ± 8.9 ms [User: 188.2 ms, System: 23.2 ms]
Range (min … max): 193.7 ms … 224.4 ms 13 runs
Instead, avoid computing and sorting the revindex once per process by
writing it to a file when the pack itself is generated.
The format is relatively straightforward. It contains an array of
uint32_t's, the length of which is equal to the number of objects in the
pack. The ith entry in this table contains the index position of the
ith object in the pack, where "ith object in the pack" is determined by
pack offset.
One thing that the on-disk format does _not_ contain is the full (up to)
eight-byte offset corresponding to each object. This is something that
the in-memory revindex contains (it stores an off_t in 'struct
revindex_entry' along with the same uint32_t that the on-disk format
has). Omit it in the on-disk format, since knowing the index position
for some object is sufficient to get a constant-time lookup in the
pack-*.idx file to ask for an object's offset within the pack.
This trades off between the on-disk size of the 'pack-*.rev' file for
runtime to chase down the offset for some object. Even though the lookup
is constant time, the constant is heavier, since it can potentially
involve two pointer walks in v2 indexes (one to access the 4-byte offset
table, and potentially a second to access the double wide offset table).
Consider trying to map an object's pack offset to a relative position
within that pack. In a cold-cache scenario, more page faults occur while
switching between binary searching through the reverse index and
searching through the *.idx file for an object's offset. Sure enough,
with a cold cache (writing '3' into '/proc/sys/vm/drop_caches' after
'sync'ing), printing out the entire object's contents is still
marginally faster than printing its size:
Benchmark #1: git.compile cat-file --batch-check="%(objectsize:disk)" <obj >/dev/null
Time (mean ± σ): 22.6 ms ± 0.5 ms [User: 2.4 ms, System: 7.9 ms]
Range (min … max): 21.4 ms … 23.5 ms 41 runs
Benchmark #2: git.compile cat-file --batch <obj >/dev/null
Time (mean ± σ): 17.2 ms ± 0.7 ms [User: 2.8 ms, System: 5.5 ms]
Range (min … max): 15.6 ms … 18.2 ms 45 runs
(Numbers taken in the kernel after cheating and using the next patch to
generate a reverse index). There are a couple of approaches to improve
cold cache performance not pursued here:
- We could include the object offsets in the reverse index format.
Predictably, this does result in fewer page faults, but it triples
the size of the file, while simultaneously duplicating a ton of data
already available in the .idx file. (This was the original way I
implemented the format, and it did show
`--batch-check='%(objectsize:disk)'` winning out against `--batch`.)
On the other hand, this increase in size also results in a large
block-cache footprint, which could potentially hurt other workloads.
- We could store the mapping from pack to index position in more
cache-friendly way, like constructing a binary search tree from the
table and writing the values in breadth-first order. This would
result in much better locality, but the price you pay is trading
O(1) lookup in 'pack_pos_to_index()' for an O(log n) one (since you
can no longer directly index the table).
So, neither of these approaches are taken here. (Thankfully, the format
is versioned, so we are free to pursue these in the future.) But, cold
cache performance likely isn't interesting outside of one-off cases like
asking for the size of an object directly. In real-world usage, Git is
often performing many operations in the revindex (i.e., asking about
many objects rather than a single one).
The trade-off is worth it, since we will avoid the vast majority of the
cost of generating the revindex that the extra pointer chase will look
like noise in the following patch's benchmarks.
This patch describes the format and prepares callers (like in
pack-revindex.c) to be able to read *.rev files once they exist. An
implementation of the writer will appear in the next patch, and callers
will gradually begin to start using the writer in the patches that
follow after that.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-26 07:37:14 +08:00
|
|
|
return ret;
|
|
|
|
|
}
|
|
|
|
|
|
2023-04-18 00:21:41 +08:00
|
|
|
int load_pack_revindex_from_disk(struct packed_git *p)
|
packfile: prepare for the existence of '*.rev' files
Specify the format of the on-disk reverse index 'pack-*.rev' file, as
well as prepare the code for the existence of such files.
The reverse index maps from pack relative positions (i.e., an index into
the array of object which is sorted by their offsets within the
packfile) to their position within the 'pack-*.idx' file. Today, this is
done by building up a list of (off_t, uint32_t) tuples for each object
(the off_t corresponding to that object's offset, and the uint32_t
corresponding to its position in the index). To convert between pack and
index position quickly, this array of tuples is radix sorted based on
its offset.
This has two major drawbacks:
First, the in-memory cost scales linearly with the number of objects in
a pack. Each 'struct revindex_entry' is sizeof(off_t) +
sizeof(uint32_t) + padding bytes for a total of 16.
To observe this, force Git to load the reverse index by, for e.g.,
running 'git cat-file --batch-check="%(objectsize:disk)"'. When asking
for a single object in a fresh clone of the kernel, Git needs to
allocate 120+ MB of memory in order to hold the reverse index in memory.
Second, the cost to sort also scales with the size of the pack.
Luckily, this is a linear function since 'load_pack_revindex()' uses a
radix sort, but this cost still must be paid once per pack per process.
As an example, it takes ~60x longer to print the _size_ of an object as
it does to print that entire object's _contents_:
Benchmark #1: git.compile cat-file --batch <obj
Time (mean ± σ): 3.4 ms ± 0.1 ms [User: 3.3 ms, System: 2.1 ms]
Range (min … max): 3.2 ms … 3.7 ms 726 runs
Benchmark #2: git.compile cat-file --batch-check="%(objectsize:disk)" <obj
Time (mean ± σ): 210.3 ms ± 8.9 ms [User: 188.2 ms, System: 23.2 ms]
Range (min … max): 193.7 ms … 224.4 ms 13 runs
Instead, avoid computing and sorting the revindex once per process by
writing it to a file when the pack itself is generated.
The format is relatively straightforward. It contains an array of
uint32_t's, the length of which is equal to the number of objects in the
pack. The ith entry in this table contains the index position of the
ith object in the pack, where "ith object in the pack" is determined by
pack offset.
One thing that the on-disk format does _not_ contain is the full (up to)
eight-byte offset corresponding to each object. This is something that
the in-memory revindex contains (it stores an off_t in 'struct
revindex_entry' along with the same uint32_t that the on-disk format
has). Omit it in the on-disk format, since knowing the index position
for some object is sufficient to get a constant-time lookup in the
pack-*.idx file to ask for an object's offset within the pack.
This trades off between the on-disk size of the 'pack-*.rev' file for
runtime to chase down the offset for some object. Even though the lookup
is constant time, the constant is heavier, since it can potentially
involve two pointer walks in v2 indexes (one to access the 4-byte offset
table, and potentially a second to access the double wide offset table).
Consider trying to map an object's pack offset to a relative position
within that pack. In a cold-cache scenario, more page faults occur while
switching between binary searching through the reverse index and
searching through the *.idx file for an object's offset. Sure enough,
with a cold cache (writing '3' into '/proc/sys/vm/drop_caches' after
'sync'ing), printing out the entire object's contents is still
marginally faster than printing its size:
Benchmark #1: git.compile cat-file --batch-check="%(objectsize:disk)" <obj >/dev/null
Time (mean ± σ): 22.6 ms ± 0.5 ms [User: 2.4 ms, System: 7.9 ms]
Range (min … max): 21.4 ms … 23.5 ms 41 runs
Benchmark #2: git.compile cat-file --batch <obj >/dev/null
Time (mean ± σ): 17.2 ms ± 0.7 ms [User: 2.8 ms, System: 5.5 ms]
Range (min … max): 15.6 ms … 18.2 ms 45 runs
(Numbers taken in the kernel after cheating and using the next patch to
generate a reverse index). There are a couple of approaches to improve
cold cache performance not pursued here:
- We could include the object offsets in the reverse index format.
Predictably, this does result in fewer page faults, but it triples
the size of the file, while simultaneously duplicating a ton of data
already available in the .idx file. (This was the original way I
implemented the format, and it did show
`--batch-check='%(objectsize:disk)'` winning out against `--batch`.)
On the other hand, this increase in size also results in a large
block-cache footprint, which could potentially hurt other workloads.
- We could store the mapping from pack to index position in more
cache-friendly way, like constructing a binary search tree from the
table and writing the values in breadth-first order. This would
result in much better locality, but the price you pay is trading
O(1) lookup in 'pack_pos_to_index()' for an O(log n) one (since you
can no longer directly index the table).
So, neither of these approaches are taken here. (Thankfully, the format
is versioned, so we are free to pursue these in the future.) But, cold
cache performance likely isn't interesting outside of one-off cases like
asking for the size of an object directly. In real-world usage, Git is
often performing many operations in the revindex (i.e., asking about
many objects rather than a single one).
The trade-off is worth it, since we will avoid the vast majority of the
cost of generating the revindex that the extra pointer chase will look
like noise in the following patch's benchmarks.
This patch describes the format and prepares callers (like in
pack-revindex.c) to be able to read *.rev files once they exist. An
implementation of the writer will appear in the next patch, and callers
will gradually begin to start using the writer in the patches that
follow after that.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-26 07:37:14 +08:00
|
|
|
{
|
|
|
|
|
char *revindex_name;
|
|
|
|
|
int ret;
|
|
|
|
|
if (open_pack_index(p))
|
|
|
|
|
return -1;
|
|
|
|
|
|
|
|
|
|
revindex_name = pack_revindex_filename(p);
|
|
|
|
|
|
|
|
|
|
ret = load_revindex_from_disk(revindex_name,
|
|
|
|
|
p->num_objects,
|
|
|
|
|
&p->revindex_map,
|
|
|
|
|
&p->revindex_size);
|
|
|
|
|
if (ret)
|
|
|
|
|
goto cleanup;
|
|
|
|
|
|
|
|
|
|
p->revindex_data = (const uint32_t *)((const char *)p->revindex_map + RIDX_HEADER_SIZE);
|
|
|
|
|
|
|
|
|
|
cleanup:
|
|
|
|
|
free(revindex_name);
|
|
|
|
|
return ret;
|
|
|
|
|
}
|
|
|
|
|
|
2023-04-13 06:20:24 +08:00
|
|
|
int load_pack_revindex(struct repository *r, struct packed_git *p)
|
packfile: prepare for the existence of '*.rev' files
Specify the format of the on-disk reverse index 'pack-*.rev' file, as
well as prepare the code for the existence of such files.
The reverse index maps from pack relative positions (i.e., an index into
the array of object which is sorted by their offsets within the
packfile) to their position within the 'pack-*.idx' file. Today, this is
done by building up a list of (off_t, uint32_t) tuples for each object
(the off_t corresponding to that object's offset, and the uint32_t
corresponding to its position in the index). To convert between pack and
index position quickly, this array of tuples is radix sorted based on
its offset.
This has two major drawbacks:
First, the in-memory cost scales linearly with the number of objects in
a pack. Each 'struct revindex_entry' is sizeof(off_t) +
sizeof(uint32_t) + padding bytes for a total of 16.
To observe this, force Git to load the reverse index by, for e.g.,
running 'git cat-file --batch-check="%(objectsize:disk)"'. When asking
for a single object in a fresh clone of the kernel, Git needs to
allocate 120+ MB of memory in order to hold the reverse index in memory.
Second, the cost to sort also scales with the size of the pack.
Luckily, this is a linear function since 'load_pack_revindex()' uses a
radix sort, but this cost still must be paid once per pack per process.
As an example, it takes ~60x longer to print the _size_ of an object as
it does to print that entire object's _contents_:
Benchmark #1: git.compile cat-file --batch <obj
Time (mean ± σ): 3.4 ms ± 0.1 ms [User: 3.3 ms, System: 2.1 ms]
Range (min … max): 3.2 ms … 3.7 ms 726 runs
Benchmark #2: git.compile cat-file --batch-check="%(objectsize:disk)" <obj
Time (mean ± σ): 210.3 ms ± 8.9 ms [User: 188.2 ms, System: 23.2 ms]
Range (min … max): 193.7 ms … 224.4 ms 13 runs
Instead, avoid computing and sorting the revindex once per process by
writing it to a file when the pack itself is generated.
The format is relatively straightforward. It contains an array of
uint32_t's, the length of which is equal to the number of objects in the
pack. The ith entry in this table contains the index position of the
ith object in the pack, where "ith object in the pack" is determined by
pack offset.
One thing that the on-disk format does _not_ contain is the full (up to)
eight-byte offset corresponding to each object. This is something that
the in-memory revindex contains (it stores an off_t in 'struct
revindex_entry' along with the same uint32_t that the on-disk format
has). Omit it in the on-disk format, since knowing the index position
for some object is sufficient to get a constant-time lookup in the
pack-*.idx file to ask for an object's offset within the pack.
This trades off between the on-disk size of the 'pack-*.rev' file for
runtime to chase down the offset for some object. Even though the lookup
is constant time, the constant is heavier, since it can potentially
involve two pointer walks in v2 indexes (one to access the 4-byte offset
table, and potentially a second to access the double wide offset table).
Consider trying to map an object's pack offset to a relative position
within that pack. In a cold-cache scenario, more page faults occur while
switching between binary searching through the reverse index and
searching through the *.idx file for an object's offset. Sure enough,
with a cold cache (writing '3' into '/proc/sys/vm/drop_caches' after
'sync'ing), printing out the entire object's contents is still
marginally faster than printing its size:
Benchmark #1: git.compile cat-file --batch-check="%(objectsize:disk)" <obj >/dev/null
Time (mean ± σ): 22.6 ms ± 0.5 ms [User: 2.4 ms, System: 7.9 ms]
Range (min … max): 21.4 ms … 23.5 ms 41 runs
Benchmark #2: git.compile cat-file --batch <obj >/dev/null
Time (mean ± σ): 17.2 ms ± 0.7 ms [User: 2.8 ms, System: 5.5 ms]
Range (min … max): 15.6 ms … 18.2 ms 45 runs
(Numbers taken in the kernel after cheating and using the next patch to
generate a reverse index). There are a couple of approaches to improve
cold cache performance not pursued here:
- We could include the object offsets in the reverse index format.
Predictably, this does result in fewer page faults, but it triples
the size of the file, while simultaneously duplicating a ton of data
already available in the .idx file. (This was the original way I
implemented the format, and it did show
`--batch-check='%(objectsize:disk)'` winning out against `--batch`.)
On the other hand, this increase in size also results in a large
block-cache footprint, which could potentially hurt other workloads.
- We could store the mapping from pack to index position in more
cache-friendly way, like constructing a binary search tree from the
table and writing the values in breadth-first order. This would
result in much better locality, but the price you pay is trading
O(1) lookup in 'pack_pos_to_index()' for an O(log n) one (since you
can no longer directly index the table).
So, neither of these approaches are taken here. (Thankfully, the format
is versioned, so we are free to pursue these in the future.) But, cold
cache performance likely isn't interesting outside of one-off cases like
asking for the size of an object directly. In real-world usage, Git is
often performing many operations in the revindex (i.e., asking about
many objects rather than a single one).
The trade-off is worth it, since we will avoid the vast majority of the
cost of generating the revindex that the extra pointer chase will look
like noise in the following patch's benchmarks.
This patch describes the format and prepares callers (like in
pack-revindex.c) to be able to read *.rev files once they exist. An
implementation of the writer will appear in the next patch, and callers
will gradually begin to start using the writer in the patches that
follow after that.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-26 07:37:14 +08:00
|
|
|
{
|
|
|
|
|
if (p->revindex || p->revindex_data)
|
|
|
|
|
return 0;
|
|
|
|
|
|
pack-revindex: introduce `pack.readReverseIndex`
Since 1615c567b8 (Documentation/config/pack.txt: advertise
'pack.writeReverseIndex', 2021-01-25), we have had the
`pack.writeReverseIndex` configuration option, which tells Git whether
or not it is allowed to write a ".rev" file when indexing a pack.
Introduce a complementary configuration knob, `pack.readReverseIndex` to
control whether or not Git will read any ".rev" file(s) that may be
available on disk.
This option is useful for debugging, as well as disabling the effect of
".rev" files in certain instances.
This is useful because of the trade-off[^1] between the time it takes to
generate a reverse index (slow from scratch, fast when reading an
existing ".rev" file), and the time it takes to access a record (the
opposite).
For example, even though it is faster to use the on-disk reverse index
when computing the on-disk size of a packed object, it is slower to
enumerate the same value for all objects.
Here are a couple of examples from linux.git. When computing the above
for a single object, using the on-disk reverse index is significantly
faster:
$ git rev-parse HEAD >in
$ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} cat-file --batch-check="%(objectsize:disk)" <in'
Benchmark 1: git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" <in
Time (mean ± σ): 302.5 ms ± 12.5 ms [User: 258.7 ms, System: 43.6 ms]
Range (min … max): 291.1 ms … 328.1 ms 10 runs
Benchmark 2: git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" <in
Time (mean ± σ): 3.9 ms ± 0.3 ms [User: 1.6 ms, System: 2.4 ms]
Range (min … max): 2.0 ms … 4.4 ms 801 runs
Summary
'git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" <in' ran
77.29 ± 7.14 times faster than 'git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" <in'
, but when instead trying to compute the on-disk object size for all
objects in the repository, using the ".rev" file is a disadvantage over
creating the reverse index from scratch:
$ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} cat-file --batch-check="%(objectsize:disk)" --batch-all-objects'
Benchmark 1: git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" --batch-all-objects
Time (mean ± σ): 8.258 s ± 0.035 s [User: 7.949 s, System: 0.308 s]
Range (min … max): 8.199 s … 8.293 s 10 runs
Benchmark 2: git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" --batch-all-objects
Time (mean ± σ): 16.976 s ± 0.107 s [User: 16.706 s, System: 0.268 s]
Range (min … max): 16.839 s … 17.105 s 10 runs
Summary
'git.compile -c pack.readReverseIndex=false cat-file --batch-check="%(objectsize:disk)" --batch-all-objects' ran
2.06 ± 0.02 times faster than 'git.compile -c pack.readReverseIndex=true cat-file --batch-check="%(objectsize:disk)" --batch-all-objects'
Luckily, the results when running `git cat-file` with `--unordered` are
closer together:
$ hyperfine -L v false,true 'git.compile -c pack.readReverseIndex={v} cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects'
Benchmark 1: git.compile -c pack.readReverseIndex=false cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects
Time (mean ± σ): 5.066 s ± 0.105 s [User: 4.792 s, System: 0.274 s]
Range (min … max): 4.943 s … 5.220 s 10 runs
Benchmark 2: git.compile -c pack.readReverseIndex=true cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects
Time (mean ± σ): 6.193 s ± 0.069 s [User: 5.937 s, System: 0.255 s]
Range (min … max): 6.145 s … 6.356 s 10 runs
Summary
'git.compile -c pack.readReverseIndex=false cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects' ran
1.22 ± 0.03 times faster than 'git.compile -c pack.readReverseIndex=true cat-file --unordered --batch-check="%(objectsize:disk)" --batch-all-objects'
Because the equilibrium point between these two is highly machine- and
repository-dependent, allow users to configure whether or not they will
read any ".rev" file(s) with this configuration knob.
[^1]: Generating a reverse index in memory takes O(N) time (where N is
the number of objects in the repository), since we use a radix sort.
Reading an entry from an on-disk ".rev" file is slower since each
operation is bound by disk I/O instead of memory I/O.
In order to compute the on-disk size of a packed object, we need to
find the offset of our object, and the adjacent object (the on-disk
size difference of these two). Finding the first offset requires a
binary search. Finding the latter involves a single .rev lookup.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2023-04-13 06:20:30 +08:00
|
|
|
prepare_repo_settings(r);
|
|
|
|
|
|
|
|
|
|
if (r->settings.pack_read_reverse_index &&
|
|
|
|
|
!load_pack_revindex_from_disk(p))
|
packfile: prepare for the existence of '*.rev' files
Specify the format of the on-disk reverse index 'pack-*.rev' file, as
well as prepare the code for the existence of such files.
The reverse index maps from pack relative positions (i.e., an index into
the array of object which is sorted by their offsets within the
packfile) to their position within the 'pack-*.idx' file. Today, this is
done by building up a list of (off_t, uint32_t) tuples for each object
(the off_t corresponding to that object's offset, and the uint32_t
corresponding to its position in the index). To convert between pack and
index position quickly, this array of tuples is radix sorted based on
its offset.
This has two major drawbacks:
First, the in-memory cost scales linearly with the number of objects in
a pack. Each 'struct revindex_entry' is sizeof(off_t) +
sizeof(uint32_t) + padding bytes for a total of 16.
To observe this, force Git to load the reverse index by, for e.g.,
running 'git cat-file --batch-check="%(objectsize:disk)"'. When asking
for a single object in a fresh clone of the kernel, Git needs to
allocate 120+ MB of memory in order to hold the reverse index in memory.
Second, the cost to sort also scales with the size of the pack.
Luckily, this is a linear function since 'load_pack_revindex()' uses a
radix sort, but this cost still must be paid once per pack per process.
As an example, it takes ~60x longer to print the _size_ of an object as
it does to print that entire object's _contents_:
Benchmark #1: git.compile cat-file --batch <obj
Time (mean ± σ): 3.4 ms ± 0.1 ms [User: 3.3 ms, System: 2.1 ms]
Range (min … max): 3.2 ms … 3.7 ms 726 runs
Benchmark #2: git.compile cat-file --batch-check="%(objectsize:disk)" <obj
Time (mean ± σ): 210.3 ms ± 8.9 ms [User: 188.2 ms, System: 23.2 ms]
Range (min … max): 193.7 ms … 224.4 ms 13 runs
Instead, avoid computing and sorting the revindex once per process by
writing it to a file when the pack itself is generated.
The format is relatively straightforward. It contains an array of
uint32_t's, the length of which is equal to the number of objects in the
pack. The ith entry in this table contains the index position of the
ith object in the pack, where "ith object in the pack" is determined by
pack offset.
One thing that the on-disk format does _not_ contain is the full (up to)
eight-byte offset corresponding to each object. This is something that
the in-memory revindex contains (it stores an off_t in 'struct
revindex_entry' along with the same uint32_t that the on-disk format
has). Omit it in the on-disk format, since knowing the index position
for some object is sufficient to get a constant-time lookup in the
pack-*.idx file to ask for an object's offset within the pack.
This trades off between the on-disk size of the 'pack-*.rev' file for
runtime to chase down the offset for some object. Even though the lookup
is constant time, the constant is heavier, since it can potentially
involve two pointer walks in v2 indexes (one to access the 4-byte offset
table, and potentially a second to access the double wide offset table).
Consider trying to map an object's pack offset to a relative position
within that pack. In a cold-cache scenario, more page faults occur while
switching between binary searching through the reverse index and
searching through the *.idx file for an object's offset. Sure enough,
with a cold cache (writing '3' into '/proc/sys/vm/drop_caches' after
'sync'ing), printing out the entire object's contents is still
marginally faster than printing its size:
Benchmark #1: git.compile cat-file --batch-check="%(objectsize:disk)" <obj >/dev/null
Time (mean ± σ): 22.6 ms ± 0.5 ms [User: 2.4 ms, System: 7.9 ms]
Range (min … max): 21.4 ms … 23.5 ms 41 runs
Benchmark #2: git.compile cat-file --batch <obj >/dev/null
Time (mean ± σ): 17.2 ms ± 0.7 ms [User: 2.8 ms, System: 5.5 ms]
Range (min … max): 15.6 ms … 18.2 ms 45 runs
(Numbers taken in the kernel after cheating and using the next patch to
generate a reverse index). There are a couple of approaches to improve
cold cache performance not pursued here:
- We could include the object offsets in the reverse index format.
Predictably, this does result in fewer page faults, but it triples
the size of the file, while simultaneously duplicating a ton of data
already available in the .idx file. (This was the original way I
implemented the format, and it did show
`--batch-check='%(objectsize:disk)'` winning out against `--batch`.)
On the other hand, this increase in size also results in a large
block-cache footprint, which could potentially hurt other workloads.
- We could store the mapping from pack to index position in more
cache-friendly way, like constructing a binary search tree from the
table and writing the values in breadth-first order. This would
result in much better locality, but the price you pay is trading
O(1) lookup in 'pack_pos_to_index()' for an O(log n) one (since you
can no longer directly index the table).
So, neither of these approaches are taken here. (Thankfully, the format
is versioned, so we are free to pursue these in the future.) But, cold
cache performance likely isn't interesting outside of one-off cases like
asking for the size of an object directly. In real-world usage, Git is
often performing many operations in the revindex (i.e., asking about
many objects rather than a single one).
The trade-off is worth it, since we will avoid the vast majority of the
cost of generating the revindex that the extra pointer chase will look
like noise in the following patch's benchmarks.
This patch describes the format and prepares callers (like in
pack-revindex.c) to be able to read *.rev files once they exist. An
implementation of the writer will appear in the next patch, and callers
will gradually begin to start using the writer in the patches that
follow after that.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-26 07:37:14 +08:00
|
|
|
return 0;
|
|
|
|
|
else if (!create_pack_revindex_in_memory(p))
|
|
|
|
|
return 0;
|
|
|
|
|
return -1;
|
|
|
|
|
}
|
|
|
|
|
|
2023-04-18 00:21:38 +08:00
|
|
|
/*
|
|
|
|
|
* verify_pack_revindex verifies that the on-disk rev-index for the given
|
|
|
|
|
* pack-file is the same that would be created if written from scratch.
|
|
|
|
|
*
|
|
|
|
|
* A negative number is returned on error.
|
|
|
|
|
*/
|
|
|
|
|
int verify_pack_revindex(struct packed_git *p)
|
|
|
|
|
{
|
2023-04-18 00:21:40 +08:00
|
|
|
int res = 0;
|
|
|
|
|
|
2023-04-18 00:21:39 +08:00
|
|
|
/* Do not bother checking if not initialized. */
|
2023-04-18 00:21:40 +08:00
|
|
|
if (!p->revindex_map || !p->revindex_data)
|
|
|
|
|
return res;
|
2023-04-18 00:21:39 +08:00
|
|
|
|
|
|
|
|
if (!hashfile_checksum_valid((const unsigned char *)p->revindex_map, p->revindex_size)) {
|
|
|
|
|
error(_("invalid checksum"));
|
2023-04-18 00:21:40 +08:00
|
|
|
res = -1;
|
2023-04-18 00:21:39 +08:00
|
|
|
}
|
|
|
|
|
|
2023-04-18 00:21:40 +08:00
|
|
|
/* This may fail due to a broken .idx. */
|
|
|
|
|
if (create_pack_revindex_in_memory(p))
|
|
|
|
|
return res;
|
|
|
|
|
|
|
|
|
|
for (size_t i = 0; i < p->num_objects; i++) {
|
|
|
|
|
uint32_t nr = p->revindex[i].nr;
|
|
|
|
|
uint32_t rev_val = get_be32(p->revindex_data + i);
|
|
|
|
|
|
|
|
|
|
if (nr != rev_val) {
|
|
|
|
|
error(_("invalid rev-index position at %"PRIu64": %"PRIu32" != %"PRIu32""),
|
|
|
|
|
(uint64_t)i, nr, rev_val);
|
|
|
|
|
res = -1;
|
|
|
|
|
}
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
return res;
|
2023-04-18 00:21:38 +08:00
|
|
|
}
|
|
|
|
|
|
2023-10-10 05:05:33 +08:00
|
|
|
static int can_use_midx_ridx_chunk(struct multi_pack_index *m)
|
|
|
|
|
{
|
|
|
|
|
if (!m->chunk_revindex)
|
|
|
|
|
return 0;
|
|
|
|
|
if (m->chunk_revindex_len != st_mult(sizeof(uint32_t), m->num_objects)) {
|
|
|
|
|
error(_("multi-pack-index reverse-index chunk is the wrong size"));
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
return 1;
|
|
|
|
|
}
|
|
|
|
|
|
pack-revindex: read multi-pack reverse indexes
Implement reading for multi-pack reverse indexes, as described in the
previous patch.
Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.
There are three new functions exposed by the revindex API:
- load_midx_revindex(): loads the reverse index corresponding to the
given multi-pack index.
- midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
multi-pack index and pseudo-pack order.
load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.
load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.
pack_pos_to_midx() simply reads the corresponding entry out of the
table.
midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.
So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-03-30 23:04:26 +08:00
|
|
|
int load_midx_revindex(struct multi_pack_index *m)
|
|
|
|
|
{
|
midx.c: write MIDX filenames to strbuf
To ask for the name of a MIDX and its corresponding .rev file, callers
invoke get_midx_filename() and get_midx_rev_filename(), respectively.
These both invoke xstrfmt(), allocating a chunk of memory which must be
freed later on.
This makes callers in pack-bitmap.c somewhat awkward. Specifically,
midx_bitmap_filename(), which is implemented like:
return xstrfmt("%s-%s.bitmap",
get_midx_filename(midx->object_dir),
hash_to_hex(get_midx_checksum(midx)));
this leaks the second argument to xstrfmt(), which itself was allocated
with xstrfmt(). This caller could assign both the result of
get_midx_filename() and the outer xstrfmt() to a temporary variable,
remembering to free() the former before returning. But that involves a
wasteful copy.
Instead, get_midx_filename() and get_midx_rev_filename() take a strbuf
as an output parameter. This way midx_bitmap_filename() can manipulate
and pass around a temporary buffer which it detaches back to its caller.
That allows us to implement the function without copying or open-coding
get_midx_filename() in a way that doesn't leak.
Update the other callers of get_midx_filename() and
get_midx_rev_filename() accordingly.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-10-27 05:01:21 +08:00
|
|
|
struct strbuf revindex_name = STRBUF_INIT;
|
pack-revindex: read multi-pack reverse indexes
Implement reading for multi-pack reverse indexes, as described in the
previous patch.
Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.
There are three new functions exposed by the revindex API:
- load_midx_revindex(): loads the reverse index corresponding to the
given multi-pack index.
- midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
multi-pack index and pseudo-pack order.
load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.
load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.
pack_pos_to_midx() simply reads the corresponding entry out of the
table.
midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.
So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-03-30 23:04:26 +08:00
|
|
|
int ret;
|
midx: read `RIDX` chunk when present
When a MIDX contains the new `RIDX` chunk, ensure that the reverse index
is read from it instead of the on-disk .rev file. Since we need to
encode the object order in the MIDX itself for correctness reasons,
there is no point in storing the same data again outside of the MIDX.
So, this patch stops writing separate .rev files, and reads it out of
the MIDX itself. This is possible to do with relatively little new code,
since the format of the RIDX chunk is identical to the data in the .rev
file. In other words, we can implement this by pointing the
`revindex_data` field at the reverse index chunk of the MIDX instead of
the .rev file without any other changes.
Note that we have two knobs that are adjusted for the new tests:
GIT_TEST_MIDX_WRITE_REV and GIT_TEST_MIDX_READ_RIDX. The former controls
whether the MIDX .rev is written at all, and the latter controls whether
we read the MIDX's RIDX chunk.
Both are necessary to ensure that the test added at the beginning of
this series continues to work. This is because we always need to write
the RIDX chunk in the MIDX in order to change its checksum, but we want
to make sure reading the existing .rev file still works (since the RIDX
chunk takes precedence by default).
Arguably this isn't a very interesting mode to test, because the
precedence rules mean that we'll always read the RIDX chunk over the
.rev file. But it makes it impossible for a user to induce corruption in
their repository by adjusting the test knobs (since if we had an
either/or knob they could stop writing the RIDX chunk, allowing them to
tweak the MIDX's object order without changing its checksum).
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2022-01-26 06:41:17 +08:00
|
|
|
|
pack-revindex: read multi-pack reverse indexes
Implement reading for multi-pack reverse indexes, as described in the
previous patch.
Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.
There are three new functions exposed by the revindex API:
- load_midx_revindex(): loads the reverse index corresponding to the
given multi-pack index.
- midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
multi-pack index and pseudo-pack order.
load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.
load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.
pack_pos_to_midx() simply reads the corresponding entry out of the
table.
midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.
So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-03-30 23:04:26 +08:00
|
|
|
if (m->revindex_data)
|
|
|
|
|
return 0;
|
|
|
|
|
|
2023-10-10 05:05:33 +08:00
|
|
|
if (can_use_midx_ridx_chunk(m)) {
|
midx: read `RIDX` chunk when present
When a MIDX contains the new `RIDX` chunk, ensure that the reverse index
is read from it instead of the on-disk .rev file. Since we need to
encode the object order in the MIDX itself for correctness reasons,
there is no point in storing the same data again outside of the MIDX.
So, this patch stops writing separate .rev files, and reads it out of
the MIDX itself. This is possible to do with relatively little new code,
since the format of the RIDX chunk is identical to the data in the .rev
file. In other words, we can implement this by pointing the
`revindex_data` field at the reverse index chunk of the MIDX instead of
the .rev file without any other changes.
Note that we have two knobs that are adjusted for the new tests:
GIT_TEST_MIDX_WRITE_REV and GIT_TEST_MIDX_READ_RIDX. The former controls
whether the MIDX .rev is written at all, and the latter controls whether
we read the MIDX's RIDX chunk.
Both are necessary to ensure that the test added at the beginning of
this series continues to work. This is because we always need to write
the RIDX chunk in the MIDX in order to change its checksum, but we want
to make sure reading the existing .rev file still works (since the RIDX
chunk takes precedence by default).
Arguably this isn't a very interesting mode to test, because the
precedence rules mean that we'll always read the RIDX chunk over the
.rev file. But it makes it impossible for a user to induce corruption in
their repository by adjusting the test knobs (since if we had an
either/or knob they could stop writing the RIDX chunk, allowing them to
tweak the MIDX's object order without changing its checksum).
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2022-01-26 06:41:17 +08:00
|
|
|
/*
|
|
|
|
|
* If the MIDX `m` has a `RIDX` chunk, then use its contents for
|
|
|
|
|
* the reverse index instead of trying to load a separate `.rev`
|
|
|
|
|
* file.
|
|
|
|
|
*
|
|
|
|
|
* Note that we do *not* set `m->revindex_map` here, since we do
|
|
|
|
|
* not want to accidentally call munmap() in the middle of the
|
|
|
|
|
* MIDX.
|
|
|
|
|
*/
|
|
|
|
|
trace2_data_string("load_midx_revindex", the_repository,
|
|
|
|
|
"source", "midx");
|
|
|
|
|
m->revindex_data = (const uint32_t *)m->chunk_revindex;
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
|
2022-01-26 06:41:05 +08:00
|
|
|
trace2_data_string("load_midx_revindex", the_repository,
|
|
|
|
|
"source", "rev");
|
|
|
|
|
|
2024-05-30 06:55:42 +08:00
|
|
|
get_midx_filename_ext(&revindex_name, m->object_dir,
|
|
|
|
|
get_midx_checksum(m), MIDX_EXT_REV);
|
pack-revindex: read multi-pack reverse indexes
Implement reading for multi-pack reverse indexes, as described in the
previous patch.
Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.
There are three new functions exposed by the revindex API:
- load_midx_revindex(): loads the reverse index corresponding to the
given multi-pack index.
- midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
multi-pack index and pseudo-pack order.
load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.
load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.
pack_pos_to_midx() simply reads the corresponding entry out of the
table.
midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.
So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-03-30 23:04:26 +08:00
|
|
|
|
midx.c: write MIDX filenames to strbuf
To ask for the name of a MIDX and its corresponding .rev file, callers
invoke get_midx_filename() and get_midx_rev_filename(), respectively.
These both invoke xstrfmt(), allocating a chunk of memory which must be
freed later on.
This makes callers in pack-bitmap.c somewhat awkward. Specifically,
midx_bitmap_filename(), which is implemented like:
return xstrfmt("%s-%s.bitmap",
get_midx_filename(midx->object_dir),
hash_to_hex(get_midx_checksum(midx)));
this leaks the second argument to xstrfmt(), which itself was allocated
with xstrfmt(). This caller could assign both the result of
get_midx_filename() and the outer xstrfmt() to a temporary variable,
remembering to free() the former before returning. But that involves a
wasteful copy.
Instead, get_midx_filename() and get_midx_rev_filename() take a strbuf
as an output parameter. This way midx_bitmap_filename() can manipulate
and pass around a temporary buffer which it detaches back to its caller.
That allows us to implement the function without copying or open-coding
get_midx_filename() in a way that doesn't leak.
Update the other callers of get_midx_filename() and
get_midx_rev_filename() accordingly.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-10-27 05:01:21 +08:00
|
|
|
ret = load_revindex_from_disk(revindex_name.buf,
|
pack-revindex: read multi-pack reverse indexes
Implement reading for multi-pack reverse indexes, as described in the
previous patch.
Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.
There are three new functions exposed by the revindex API:
- load_midx_revindex(): loads the reverse index corresponding to the
given multi-pack index.
- midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
multi-pack index and pseudo-pack order.
load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.
load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.
pack_pos_to_midx() simply reads the corresponding entry out of the
table.
midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.
So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-03-30 23:04:26 +08:00
|
|
|
m->num_objects,
|
|
|
|
|
&m->revindex_map,
|
|
|
|
|
&m->revindex_len);
|
|
|
|
|
if (ret)
|
|
|
|
|
goto cleanup;
|
|
|
|
|
|
|
|
|
|
m->revindex_data = (const uint32_t *)((const char *)m->revindex_map + RIDX_HEADER_SIZE);
|
|
|
|
|
|
|
|
|
|
cleanup:
|
midx.c: write MIDX filenames to strbuf
To ask for the name of a MIDX and its corresponding .rev file, callers
invoke get_midx_filename() and get_midx_rev_filename(), respectively.
These both invoke xstrfmt(), allocating a chunk of memory which must be
freed later on.
This makes callers in pack-bitmap.c somewhat awkward. Specifically,
midx_bitmap_filename(), which is implemented like:
return xstrfmt("%s-%s.bitmap",
get_midx_filename(midx->object_dir),
hash_to_hex(get_midx_checksum(midx)));
this leaks the second argument to xstrfmt(), which itself was allocated
with xstrfmt(). This caller could assign both the result of
get_midx_filename() and the outer xstrfmt() to a temporary variable,
remembering to free() the former before returning. But that involves a
wasteful copy.
Instead, get_midx_filename() and get_midx_rev_filename() take a strbuf
as an output parameter. This way midx_bitmap_filename() can manipulate
and pass around a temporary buffer which it detaches back to its caller.
That allows us to implement the function without copying or open-coding
get_midx_filename() in a way that doesn't leak.
Update the other callers of get_midx_filename() and
get_midx_rev_filename() accordingly.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-10-27 05:01:21 +08:00
|
|
|
strbuf_release(&revindex_name);
|
pack-revindex: read multi-pack reverse indexes
Implement reading for multi-pack reverse indexes, as described in the
previous patch.
Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.
There are three new functions exposed by the revindex API:
- load_midx_revindex(): loads the reverse index corresponding to the
given multi-pack index.
- midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
multi-pack index and pseudo-pack order.
load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.
load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.
pack_pos_to_midx() simply reads the corresponding entry out of the
table.
midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.
So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-03-30 23:04:26 +08:00
|
|
|
return ret;
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
int close_midx_revindex(struct multi_pack_index *m)
|
|
|
|
|
{
|
|
|
|
|
if (!m || !m->revindex_map)
|
|
|
|
|
return 0;
|
|
|
|
|
|
|
|
|
|
munmap((void*)m->revindex_map, m->revindex_len);
|
|
|
|
|
|
|
|
|
|
m->revindex_map = NULL;
|
|
|
|
|
m->revindex_data = NULL;
|
|
|
|
|
m->revindex_len = 0;
|
|
|
|
|
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
|
2021-01-14 06:25:02 +08:00
|
|
|
int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos)
|
2013-10-25 02:00:36 +08:00
|
|
|
{
|
2021-01-14 06:25:02 +08:00
|
|
|
unsigned lo, hi;
|
|
|
|
|
|
2023-04-13 06:20:24 +08:00
|
|
|
if (load_pack_revindex(the_repository, p) < 0)
|
2021-01-14 06:25:02 +08:00
|
|
|
return -1;
|
|
|
|
|
|
|
|
|
|
lo = 0;
|
|
|
|
|
hi = p->num_objects + 1;
|
2013-10-25 02:00:36 +08:00
|
|
|
|
2008-02-28 13:25:17 +08:00
|
|
|
do {
|
2019-02-03 07:16:45 +08:00
|
|
|
const unsigned mi = lo + (hi - lo) / 2;
|
2021-01-14 06:25:10 +08:00
|
|
|
off_t got = pack_pos_to_offset(p, mi);
|
|
|
|
|
|
|
|
|
|
if (got == ofs) {
|
2021-01-14 06:25:02 +08:00
|
|
|
*pos = mi;
|
|
|
|
|
return 0;
|
2021-01-14 06:25:10 +08:00
|
|
|
} else if (ofs < got)
|
2008-02-28 13:25:17 +08:00
|
|
|
hi = mi;
|
|
|
|
|
else
|
|
|
|
|
lo = mi + 1;
|
|
|
|
|
} while (lo < hi);
|
2013-10-25 02:00:36 +08:00
|
|
|
|
2008-10-30 07:02:49 +08:00
|
|
|
error("bad offset for revindex");
|
2013-10-25 02:00:36 +08:00
|
|
|
return -1;
|
|
|
|
|
}
|
|
|
|
|
|
pack-revindex: introduce a new API
In the next several patches, we will prepare for loading a reverse index
either in memory (mapping the inverse of the .idx's contents in-core),
or directly from a yet-to-be-introduced on-disk format. To prepare for
that, we'll introduce an API that avoids the caller explicitly indexing
the revindex pointer in the packed_git structure.
There are four ways to interact with the reverse index. Accordingly,
four functions will be exported from 'pack-revindex.h' by the time that
the existing API is removed. A caller may:
1. Load the pack's reverse index. This involves opening up the index,
generating an array, and then sorting it. Since opening the index
can fail, this function ('load_pack_revindex()') returns an int.
Accordingly, it takes only a single argument: the 'struct
packed_git' the caller wants to build a reverse index for.
This function is well-suited for both the current and new API.
Callers will have to continue to open the reverse index explicitly,
but this function will eventually learn how to detect and load a
reverse index from the on-disk format, if one exists. Otherwise, it
will fallback to generating one in memory from scratch.
2. Convert a pack position into an offset. This operation is now
called `pack_pos_to_offset()`. It takes a pack and a position, and
returns the corresponding off_t.
Any error simply calls BUG(), since the callers are not well-suited
to handle a failure and keep going.
3. Convert a pack position into an index position. Same as above; this
takes a pack and a position, and returns a uint32_t. This operation
is known as `pack_pos_to_index()`. The same thinking about error
conditions applies here as well.
4. Find the pack position for a given offset. This operation is now
known as `offset_to_pack_pos()`. It takes a pack, an offset, and a
pointer to a uint32_t where the position is written, if an object
exists at that offset. Otherwise, -1 is returned to indicate
failure.
Unlike some of the callers that used to access '->offset' and '->nr'
directly, the error checking around this call is somewhat more
robust. This is important since callers should always pass an offset
which points at the boundary of two objects. The API, unlike direct
access, enforces that that is the case.
This will become important in a subsequent patch where a caller
which does not but could check the return value treats the signed
`-1` from `find_revindex_position()` as an index into the 'revindex'
array.
Two design warts are carried over into the new API:
- Asking for the index position of an out-of-bounds object will result
in a BUG() (since no such object exists), but asking for the offset
of the non-existent object at the end of the pack returns the total
size of the pack.
This makes it convenient for callers who always want to take the
difference of two adjacent object's offsets (to compute the on-disk
size) but don't want to worry about boundaries at the end of the
pack.
- offset_to_pack_pos() lazily loads the reverse index, but
pack_pos_to_index() doesn't (callers of the former are well-suited
to handle errors, but callers of the latter are not).
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-14 06:23:31 +08:00
|
|
|
uint32_t pack_pos_to_index(struct packed_git *p, uint32_t pos)
|
|
|
|
|
{
|
packfile: prepare for the existence of '*.rev' files
Specify the format of the on-disk reverse index 'pack-*.rev' file, as
well as prepare the code for the existence of such files.
The reverse index maps from pack relative positions (i.e., an index into
the array of object which is sorted by their offsets within the
packfile) to their position within the 'pack-*.idx' file. Today, this is
done by building up a list of (off_t, uint32_t) tuples for each object
(the off_t corresponding to that object's offset, and the uint32_t
corresponding to its position in the index). To convert between pack and
index position quickly, this array of tuples is radix sorted based on
its offset.
This has two major drawbacks:
First, the in-memory cost scales linearly with the number of objects in
a pack. Each 'struct revindex_entry' is sizeof(off_t) +
sizeof(uint32_t) + padding bytes for a total of 16.
To observe this, force Git to load the reverse index by, for e.g.,
running 'git cat-file --batch-check="%(objectsize:disk)"'. When asking
for a single object in a fresh clone of the kernel, Git needs to
allocate 120+ MB of memory in order to hold the reverse index in memory.
Second, the cost to sort also scales with the size of the pack.
Luckily, this is a linear function since 'load_pack_revindex()' uses a
radix sort, but this cost still must be paid once per pack per process.
As an example, it takes ~60x longer to print the _size_ of an object as
it does to print that entire object's _contents_:
Benchmark #1: git.compile cat-file --batch <obj
Time (mean ± σ): 3.4 ms ± 0.1 ms [User: 3.3 ms, System: 2.1 ms]
Range (min … max): 3.2 ms … 3.7 ms 726 runs
Benchmark #2: git.compile cat-file --batch-check="%(objectsize:disk)" <obj
Time (mean ± σ): 210.3 ms ± 8.9 ms [User: 188.2 ms, System: 23.2 ms]
Range (min … max): 193.7 ms … 224.4 ms 13 runs
Instead, avoid computing and sorting the revindex once per process by
writing it to a file when the pack itself is generated.
The format is relatively straightforward. It contains an array of
uint32_t's, the length of which is equal to the number of objects in the
pack. The ith entry in this table contains the index position of the
ith object in the pack, where "ith object in the pack" is determined by
pack offset.
One thing that the on-disk format does _not_ contain is the full (up to)
eight-byte offset corresponding to each object. This is something that
the in-memory revindex contains (it stores an off_t in 'struct
revindex_entry' along with the same uint32_t that the on-disk format
has). Omit it in the on-disk format, since knowing the index position
for some object is sufficient to get a constant-time lookup in the
pack-*.idx file to ask for an object's offset within the pack.
This trades off between the on-disk size of the 'pack-*.rev' file for
runtime to chase down the offset for some object. Even though the lookup
is constant time, the constant is heavier, since it can potentially
involve two pointer walks in v2 indexes (one to access the 4-byte offset
table, and potentially a second to access the double wide offset table).
Consider trying to map an object's pack offset to a relative position
within that pack. In a cold-cache scenario, more page faults occur while
switching between binary searching through the reverse index and
searching through the *.idx file for an object's offset. Sure enough,
with a cold cache (writing '3' into '/proc/sys/vm/drop_caches' after
'sync'ing), printing out the entire object's contents is still
marginally faster than printing its size:
Benchmark #1: git.compile cat-file --batch-check="%(objectsize:disk)" <obj >/dev/null
Time (mean ± σ): 22.6 ms ± 0.5 ms [User: 2.4 ms, System: 7.9 ms]
Range (min … max): 21.4 ms … 23.5 ms 41 runs
Benchmark #2: git.compile cat-file --batch <obj >/dev/null
Time (mean ± σ): 17.2 ms ± 0.7 ms [User: 2.8 ms, System: 5.5 ms]
Range (min … max): 15.6 ms … 18.2 ms 45 runs
(Numbers taken in the kernel after cheating and using the next patch to
generate a reverse index). There are a couple of approaches to improve
cold cache performance not pursued here:
- We could include the object offsets in the reverse index format.
Predictably, this does result in fewer page faults, but it triples
the size of the file, while simultaneously duplicating a ton of data
already available in the .idx file. (This was the original way I
implemented the format, and it did show
`--batch-check='%(objectsize:disk)'` winning out against `--batch`.)
On the other hand, this increase in size also results in a large
block-cache footprint, which could potentially hurt other workloads.
- We could store the mapping from pack to index position in more
cache-friendly way, like constructing a binary search tree from the
table and writing the values in breadth-first order. This would
result in much better locality, but the price you pay is trading
O(1) lookup in 'pack_pos_to_index()' for an O(log n) one (since you
can no longer directly index the table).
So, neither of these approaches are taken here. (Thankfully, the format
is versioned, so we are free to pursue these in the future.) But, cold
cache performance likely isn't interesting outside of one-off cases like
asking for the size of an object directly. In real-world usage, Git is
often performing many operations in the revindex (i.e., asking about
many objects rather than a single one).
The trade-off is worth it, since we will avoid the vast majority of the
cost of generating the revindex that the extra pointer chase will look
like noise in the following patch's benchmarks.
This patch describes the format and prepares callers (like in
pack-revindex.c) to be able to read *.rev files once they exist. An
implementation of the writer will appear in the next patch, and callers
will gradually begin to start using the writer in the patches that
follow after that.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-26 07:37:14 +08:00
|
|
|
if (!(p->revindex || p->revindex_data))
|
pack-revindex: introduce a new API
In the next several patches, we will prepare for loading a reverse index
either in memory (mapping the inverse of the .idx's contents in-core),
or directly from a yet-to-be-introduced on-disk format. To prepare for
that, we'll introduce an API that avoids the caller explicitly indexing
the revindex pointer in the packed_git structure.
There are four ways to interact with the reverse index. Accordingly,
four functions will be exported from 'pack-revindex.h' by the time that
the existing API is removed. A caller may:
1. Load the pack's reverse index. This involves opening up the index,
generating an array, and then sorting it. Since opening the index
can fail, this function ('load_pack_revindex()') returns an int.
Accordingly, it takes only a single argument: the 'struct
packed_git' the caller wants to build a reverse index for.
This function is well-suited for both the current and new API.
Callers will have to continue to open the reverse index explicitly,
but this function will eventually learn how to detect and load a
reverse index from the on-disk format, if one exists. Otherwise, it
will fallback to generating one in memory from scratch.
2. Convert a pack position into an offset. This operation is now
called `pack_pos_to_offset()`. It takes a pack and a position, and
returns the corresponding off_t.
Any error simply calls BUG(), since the callers are not well-suited
to handle a failure and keep going.
3. Convert a pack position into an index position. Same as above; this
takes a pack and a position, and returns a uint32_t. This operation
is known as `pack_pos_to_index()`. The same thinking about error
conditions applies here as well.
4. Find the pack position for a given offset. This operation is now
known as `offset_to_pack_pos()`. It takes a pack, an offset, and a
pointer to a uint32_t where the position is written, if an object
exists at that offset. Otherwise, -1 is returned to indicate
failure.
Unlike some of the callers that used to access '->offset' and '->nr'
directly, the error checking around this call is somewhat more
robust. This is important since callers should always pass an offset
which points at the boundary of two objects. The API, unlike direct
access, enforces that that is the case.
This will become important in a subsequent patch where a caller
which does not but could check the return value treats the signed
`-1` from `find_revindex_position()` as an index into the 'revindex'
array.
Two design warts are carried over into the new API:
- Asking for the index position of an out-of-bounds object will result
in a BUG() (since no such object exists), but asking for the offset
of the non-existent object at the end of the pack returns the total
size of the pack.
This makes it convenient for callers who always want to take the
difference of two adjacent object's offsets (to compute the on-disk
size) but don't want to worry about boundaries at the end of the
pack.
- offset_to_pack_pos() lazily loads the reverse index, but
pack_pos_to_index() doesn't (callers of the former are well-suited
to handle errors, but callers of the latter are not).
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-14 06:23:31 +08:00
|
|
|
BUG("pack_pos_to_index: reverse index not yet loaded");
|
|
|
|
|
if (p->num_objects <= pos)
|
|
|
|
|
BUG("pack_pos_to_index: out-of-bounds object at %"PRIu32, pos);
|
packfile: prepare for the existence of '*.rev' files
Specify the format of the on-disk reverse index 'pack-*.rev' file, as
well as prepare the code for the existence of such files.
The reverse index maps from pack relative positions (i.e., an index into
the array of object which is sorted by their offsets within the
packfile) to their position within the 'pack-*.idx' file. Today, this is
done by building up a list of (off_t, uint32_t) tuples for each object
(the off_t corresponding to that object's offset, and the uint32_t
corresponding to its position in the index). To convert between pack and
index position quickly, this array of tuples is radix sorted based on
its offset.
This has two major drawbacks:
First, the in-memory cost scales linearly with the number of objects in
a pack. Each 'struct revindex_entry' is sizeof(off_t) +
sizeof(uint32_t) + padding bytes for a total of 16.
To observe this, force Git to load the reverse index by, for e.g.,
running 'git cat-file --batch-check="%(objectsize:disk)"'. When asking
for a single object in a fresh clone of the kernel, Git needs to
allocate 120+ MB of memory in order to hold the reverse index in memory.
Second, the cost to sort also scales with the size of the pack.
Luckily, this is a linear function since 'load_pack_revindex()' uses a
radix sort, but this cost still must be paid once per pack per process.
As an example, it takes ~60x longer to print the _size_ of an object as
it does to print that entire object's _contents_:
Benchmark #1: git.compile cat-file --batch <obj
Time (mean ± σ): 3.4 ms ± 0.1 ms [User: 3.3 ms, System: 2.1 ms]
Range (min … max): 3.2 ms … 3.7 ms 726 runs
Benchmark #2: git.compile cat-file --batch-check="%(objectsize:disk)" <obj
Time (mean ± σ): 210.3 ms ± 8.9 ms [User: 188.2 ms, System: 23.2 ms]
Range (min … max): 193.7 ms … 224.4 ms 13 runs
Instead, avoid computing and sorting the revindex once per process by
writing it to a file when the pack itself is generated.
The format is relatively straightforward. It contains an array of
uint32_t's, the length of which is equal to the number of objects in the
pack. The ith entry in this table contains the index position of the
ith object in the pack, where "ith object in the pack" is determined by
pack offset.
One thing that the on-disk format does _not_ contain is the full (up to)
eight-byte offset corresponding to each object. This is something that
the in-memory revindex contains (it stores an off_t in 'struct
revindex_entry' along with the same uint32_t that the on-disk format
has). Omit it in the on-disk format, since knowing the index position
for some object is sufficient to get a constant-time lookup in the
pack-*.idx file to ask for an object's offset within the pack.
This trades off between the on-disk size of the 'pack-*.rev' file for
runtime to chase down the offset for some object. Even though the lookup
is constant time, the constant is heavier, since it can potentially
involve two pointer walks in v2 indexes (one to access the 4-byte offset
table, and potentially a second to access the double wide offset table).
Consider trying to map an object's pack offset to a relative position
within that pack. In a cold-cache scenario, more page faults occur while
switching between binary searching through the reverse index and
searching through the *.idx file for an object's offset. Sure enough,
with a cold cache (writing '3' into '/proc/sys/vm/drop_caches' after
'sync'ing), printing out the entire object's contents is still
marginally faster than printing its size:
Benchmark #1: git.compile cat-file --batch-check="%(objectsize:disk)" <obj >/dev/null
Time (mean ± σ): 22.6 ms ± 0.5 ms [User: 2.4 ms, System: 7.9 ms]
Range (min … max): 21.4 ms … 23.5 ms 41 runs
Benchmark #2: git.compile cat-file --batch <obj >/dev/null
Time (mean ± σ): 17.2 ms ± 0.7 ms [User: 2.8 ms, System: 5.5 ms]
Range (min … max): 15.6 ms … 18.2 ms 45 runs
(Numbers taken in the kernel after cheating and using the next patch to
generate a reverse index). There are a couple of approaches to improve
cold cache performance not pursued here:
- We could include the object offsets in the reverse index format.
Predictably, this does result in fewer page faults, but it triples
the size of the file, while simultaneously duplicating a ton of data
already available in the .idx file. (This was the original way I
implemented the format, and it did show
`--batch-check='%(objectsize:disk)'` winning out against `--batch`.)
On the other hand, this increase in size also results in a large
block-cache footprint, which could potentially hurt other workloads.
- We could store the mapping from pack to index position in more
cache-friendly way, like constructing a binary search tree from the
table and writing the values in breadth-first order. This would
result in much better locality, but the price you pay is trading
O(1) lookup in 'pack_pos_to_index()' for an O(log n) one (since you
can no longer directly index the table).
So, neither of these approaches are taken here. (Thankfully, the format
is versioned, so we are free to pursue these in the future.) But, cold
cache performance likely isn't interesting outside of one-off cases like
asking for the size of an object directly. In real-world usage, Git is
often performing many operations in the revindex (i.e., asking about
many objects rather than a single one).
The trade-off is worth it, since we will avoid the vast majority of the
cost of generating the revindex that the extra pointer chase will look
like noise in the following patch's benchmarks.
This patch describes the format and prepares callers (like in
pack-revindex.c) to be able to read *.rev files once they exist. An
implementation of the writer will appear in the next patch, and callers
will gradually begin to start using the writer in the patches that
follow after that.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-26 07:37:14 +08:00
|
|
|
|
|
|
|
|
if (p->revindex)
|
|
|
|
|
return p->revindex[pos].nr;
|
|
|
|
|
else
|
|
|
|
|
return get_be32(p->revindex_data + pos);
|
pack-revindex: introduce a new API
In the next several patches, we will prepare for loading a reverse index
either in memory (mapping the inverse of the .idx's contents in-core),
or directly from a yet-to-be-introduced on-disk format. To prepare for
that, we'll introduce an API that avoids the caller explicitly indexing
the revindex pointer in the packed_git structure.
There are four ways to interact with the reverse index. Accordingly,
four functions will be exported from 'pack-revindex.h' by the time that
the existing API is removed. A caller may:
1. Load the pack's reverse index. This involves opening up the index,
generating an array, and then sorting it. Since opening the index
can fail, this function ('load_pack_revindex()') returns an int.
Accordingly, it takes only a single argument: the 'struct
packed_git' the caller wants to build a reverse index for.
This function is well-suited for both the current and new API.
Callers will have to continue to open the reverse index explicitly,
but this function will eventually learn how to detect and load a
reverse index from the on-disk format, if one exists. Otherwise, it
will fallback to generating one in memory from scratch.
2. Convert a pack position into an offset. This operation is now
called `pack_pos_to_offset()`. It takes a pack and a position, and
returns the corresponding off_t.
Any error simply calls BUG(), since the callers are not well-suited
to handle a failure and keep going.
3. Convert a pack position into an index position. Same as above; this
takes a pack and a position, and returns a uint32_t. This operation
is known as `pack_pos_to_index()`. The same thinking about error
conditions applies here as well.
4. Find the pack position for a given offset. This operation is now
known as `offset_to_pack_pos()`. It takes a pack, an offset, and a
pointer to a uint32_t where the position is written, if an object
exists at that offset. Otherwise, -1 is returned to indicate
failure.
Unlike some of the callers that used to access '->offset' and '->nr'
directly, the error checking around this call is somewhat more
robust. This is important since callers should always pass an offset
which points at the boundary of two objects. The API, unlike direct
access, enforces that that is the case.
This will become important in a subsequent patch where a caller
which does not but could check the return value treats the signed
`-1` from `find_revindex_position()` as an index into the 'revindex'
array.
Two design warts are carried over into the new API:
- Asking for the index position of an out-of-bounds object will result
in a BUG() (since no such object exists), but asking for the offset
of the non-existent object at the end of the pack returns the total
size of the pack.
This makes it convenient for callers who always want to take the
difference of two adjacent object's offsets (to compute the on-disk
size) but don't want to worry about boundaries at the end of the
pack.
- offset_to_pack_pos() lazily loads the reverse index, but
pack_pos_to_index() doesn't (callers of the former are well-suited
to handle errors, but callers of the latter are not).
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-14 06:23:31 +08:00
|
|
|
}
|
|
|
|
|
|
|
|
|
|
off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos)
|
|
|
|
|
{
|
packfile: prepare for the existence of '*.rev' files
Specify the format of the on-disk reverse index 'pack-*.rev' file, as
well as prepare the code for the existence of such files.
The reverse index maps from pack relative positions (i.e., an index into
the array of object which is sorted by their offsets within the
packfile) to their position within the 'pack-*.idx' file. Today, this is
done by building up a list of (off_t, uint32_t) tuples for each object
(the off_t corresponding to that object's offset, and the uint32_t
corresponding to its position in the index). To convert between pack and
index position quickly, this array of tuples is radix sorted based on
its offset.
This has two major drawbacks:
First, the in-memory cost scales linearly with the number of objects in
a pack. Each 'struct revindex_entry' is sizeof(off_t) +
sizeof(uint32_t) + padding bytes for a total of 16.
To observe this, force Git to load the reverse index by, for e.g.,
running 'git cat-file --batch-check="%(objectsize:disk)"'. When asking
for a single object in a fresh clone of the kernel, Git needs to
allocate 120+ MB of memory in order to hold the reverse index in memory.
Second, the cost to sort also scales with the size of the pack.
Luckily, this is a linear function since 'load_pack_revindex()' uses a
radix sort, but this cost still must be paid once per pack per process.
As an example, it takes ~60x longer to print the _size_ of an object as
it does to print that entire object's _contents_:
Benchmark #1: git.compile cat-file --batch <obj
Time (mean ± σ): 3.4 ms ± 0.1 ms [User: 3.3 ms, System: 2.1 ms]
Range (min … max): 3.2 ms … 3.7 ms 726 runs
Benchmark #2: git.compile cat-file --batch-check="%(objectsize:disk)" <obj
Time (mean ± σ): 210.3 ms ± 8.9 ms [User: 188.2 ms, System: 23.2 ms]
Range (min … max): 193.7 ms … 224.4 ms 13 runs
Instead, avoid computing and sorting the revindex once per process by
writing it to a file when the pack itself is generated.
The format is relatively straightforward. It contains an array of
uint32_t's, the length of which is equal to the number of objects in the
pack. The ith entry in this table contains the index position of the
ith object in the pack, where "ith object in the pack" is determined by
pack offset.
One thing that the on-disk format does _not_ contain is the full (up to)
eight-byte offset corresponding to each object. This is something that
the in-memory revindex contains (it stores an off_t in 'struct
revindex_entry' along with the same uint32_t that the on-disk format
has). Omit it in the on-disk format, since knowing the index position
for some object is sufficient to get a constant-time lookup in the
pack-*.idx file to ask for an object's offset within the pack.
This trades off between the on-disk size of the 'pack-*.rev' file for
runtime to chase down the offset for some object. Even though the lookup
is constant time, the constant is heavier, since it can potentially
involve two pointer walks in v2 indexes (one to access the 4-byte offset
table, and potentially a second to access the double wide offset table).
Consider trying to map an object's pack offset to a relative position
within that pack. In a cold-cache scenario, more page faults occur while
switching between binary searching through the reverse index and
searching through the *.idx file for an object's offset. Sure enough,
with a cold cache (writing '3' into '/proc/sys/vm/drop_caches' after
'sync'ing), printing out the entire object's contents is still
marginally faster than printing its size:
Benchmark #1: git.compile cat-file --batch-check="%(objectsize:disk)" <obj >/dev/null
Time (mean ± σ): 22.6 ms ± 0.5 ms [User: 2.4 ms, System: 7.9 ms]
Range (min … max): 21.4 ms … 23.5 ms 41 runs
Benchmark #2: git.compile cat-file --batch <obj >/dev/null
Time (mean ± σ): 17.2 ms ± 0.7 ms [User: 2.8 ms, System: 5.5 ms]
Range (min … max): 15.6 ms … 18.2 ms 45 runs
(Numbers taken in the kernel after cheating and using the next patch to
generate a reverse index). There are a couple of approaches to improve
cold cache performance not pursued here:
- We could include the object offsets in the reverse index format.
Predictably, this does result in fewer page faults, but it triples
the size of the file, while simultaneously duplicating a ton of data
already available in the .idx file. (This was the original way I
implemented the format, and it did show
`--batch-check='%(objectsize:disk)'` winning out against `--batch`.)
On the other hand, this increase in size also results in a large
block-cache footprint, which could potentially hurt other workloads.
- We could store the mapping from pack to index position in more
cache-friendly way, like constructing a binary search tree from the
table and writing the values in breadth-first order. This would
result in much better locality, but the price you pay is trading
O(1) lookup in 'pack_pos_to_index()' for an O(log n) one (since you
can no longer directly index the table).
So, neither of these approaches are taken here. (Thankfully, the format
is versioned, so we are free to pursue these in the future.) But, cold
cache performance likely isn't interesting outside of one-off cases like
asking for the size of an object directly. In real-world usage, Git is
often performing many operations in the revindex (i.e., asking about
many objects rather than a single one).
The trade-off is worth it, since we will avoid the vast majority of the
cost of generating the revindex that the extra pointer chase will look
like noise in the following patch's benchmarks.
This patch describes the format and prepares callers (like in
pack-revindex.c) to be able to read *.rev files once they exist. An
implementation of the writer will appear in the next patch, and callers
will gradually begin to start using the writer in the patches that
follow after that.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-26 07:37:14 +08:00
|
|
|
if (!(p->revindex || p->revindex_data))
|
pack-revindex: introduce a new API
In the next several patches, we will prepare for loading a reverse index
either in memory (mapping the inverse of the .idx's contents in-core),
or directly from a yet-to-be-introduced on-disk format. To prepare for
that, we'll introduce an API that avoids the caller explicitly indexing
the revindex pointer in the packed_git structure.
There are four ways to interact with the reverse index. Accordingly,
four functions will be exported from 'pack-revindex.h' by the time that
the existing API is removed. A caller may:
1. Load the pack's reverse index. This involves opening up the index,
generating an array, and then sorting it. Since opening the index
can fail, this function ('load_pack_revindex()') returns an int.
Accordingly, it takes only a single argument: the 'struct
packed_git' the caller wants to build a reverse index for.
This function is well-suited for both the current and new API.
Callers will have to continue to open the reverse index explicitly,
but this function will eventually learn how to detect and load a
reverse index from the on-disk format, if one exists. Otherwise, it
will fallback to generating one in memory from scratch.
2. Convert a pack position into an offset. This operation is now
called `pack_pos_to_offset()`. It takes a pack and a position, and
returns the corresponding off_t.
Any error simply calls BUG(), since the callers are not well-suited
to handle a failure and keep going.
3. Convert a pack position into an index position. Same as above; this
takes a pack and a position, and returns a uint32_t. This operation
is known as `pack_pos_to_index()`. The same thinking about error
conditions applies here as well.
4. Find the pack position for a given offset. This operation is now
known as `offset_to_pack_pos()`. It takes a pack, an offset, and a
pointer to a uint32_t where the position is written, if an object
exists at that offset. Otherwise, -1 is returned to indicate
failure.
Unlike some of the callers that used to access '->offset' and '->nr'
directly, the error checking around this call is somewhat more
robust. This is important since callers should always pass an offset
which points at the boundary of two objects. The API, unlike direct
access, enforces that that is the case.
This will become important in a subsequent patch where a caller
which does not but could check the return value treats the signed
`-1` from `find_revindex_position()` as an index into the 'revindex'
array.
Two design warts are carried over into the new API:
- Asking for the index position of an out-of-bounds object will result
in a BUG() (since no such object exists), but asking for the offset
of the non-existent object at the end of the pack returns the total
size of the pack.
This makes it convenient for callers who always want to take the
difference of two adjacent object's offsets (to compute the on-disk
size) but don't want to worry about boundaries at the end of the
pack.
- offset_to_pack_pos() lazily loads the reverse index, but
pack_pos_to_index() doesn't (callers of the former are well-suited
to handle errors, but callers of the latter are not).
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-14 06:23:31 +08:00
|
|
|
BUG("pack_pos_to_index: reverse index not yet loaded");
|
|
|
|
|
if (p->num_objects < pos)
|
|
|
|
|
BUG("pack_pos_to_offset: out-of-bounds object at %"PRIu32, pos);
|
packfile: prepare for the existence of '*.rev' files
Specify the format of the on-disk reverse index 'pack-*.rev' file, as
well as prepare the code for the existence of such files.
The reverse index maps from pack relative positions (i.e., an index into
the array of object which is sorted by their offsets within the
packfile) to their position within the 'pack-*.idx' file. Today, this is
done by building up a list of (off_t, uint32_t) tuples for each object
(the off_t corresponding to that object's offset, and the uint32_t
corresponding to its position in the index). To convert between pack and
index position quickly, this array of tuples is radix sorted based on
its offset.
This has two major drawbacks:
First, the in-memory cost scales linearly with the number of objects in
a pack. Each 'struct revindex_entry' is sizeof(off_t) +
sizeof(uint32_t) + padding bytes for a total of 16.
To observe this, force Git to load the reverse index by, for e.g.,
running 'git cat-file --batch-check="%(objectsize:disk)"'. When asking
for a single object in a fresh clone of the kernel, Git needs to
allocate 120+ MB of memory in order to hold the reverse index in memory.
Second, the cost to sort also scales with the size of the pack.
Luckily, this is a linear function since 'load_pack_revindex()' uses a
radix sort, but this cost still must be paid once per pack per process.
As an example, it takes ~60x longer to print the _size_ of an object as
it does to print that entire object's _contents_:
Benchmark #1: git.compile cat-file --batch <obj
Time (mean ± σ): 3.4 ms ± 0.1 ms [User: 3.3 ms, System: 2.1 ms]
Range (min … max): 3.2 ms … 3.7 ms 726 runs
Benchmark #2: git.compile cat-file --batch-check="%(objectsize:disk)" <obj
Time (mean ± σ): 210.3 ms ± 8.9 ms [User: 188.2 ms, System: 23.2 ms]
Range (min … max): 193.7 ms … 224.4 ms 13 runs
Instead, avoid computing and sorting the revindex once per process by
writing it to a file when the pack itself is generated.
The format is relatively straightforward. It contains an array of
uint32_t's, the length of which is equal to the number of objects in the
pack. The ith entry in this table contains the index position of the
ith object in the pack, where "ith object in the pack" is determined by
pack offset.
One thing that the on-disk format does _not_ contain is the full (up to)
eight-byte offset corresponding to each object. This is something that
the in-memory revindex contains (it stores an off_t in 'struct
revindex_entry' along with the same uint32_t that the on-disk format
has). Omit it in the on-disk format, since knowing the index position
for some object is sufficient to get a constant-time lookup in the
pack-*.idx file to ask for an object's offset within the pack.
This trades off between the on-disk size of the 'pack-*.rev' file for
runtime to chase down the offset for some object. Even though the lookup
is constant time, the constant is heavier, since it can potentially
involve two pointer walks in v2 indexes (one to access the 4-byte offset
table, and potentially a second to access the double wide offset table).
Consider trying to map an object's pack offset to a relative position
within that pack. In a cold-cache scenario, more page faults occur while
switching between binary searching through the reverse index and
searching through the *.idx file for an object's offset. Sure enough,
with a cold cache (writing '3' into '/proc/sys/vm/drop_caches' after
'sync'ing), printing out the entire object's contents is still
marginally faster than printing its size:
Benchmark #1: git.compile cat-file --batch-check="%(objectsize:disk)" <obj >/dev/null
Time (mean ± σ): 22.6 ms ± 0.5 ms [User: 2.4 ms, System: 7.9 ms]
Range (min … max): 21.4 ms … 23.5 ms 41 runs
Benchmark #2: git.compile cat-file --batch <obj >/dev/null
Time (mean ± σ): 17.2 ms ± 0.7 ms [User: 2.8 ms, System: 5.5 ms]
Range (min … max): 15.6 ms … 18.2 ms 45 runs
(Numbers taken in the kernel after cheating and using the next patch to
generate a reverse index). There are a couple of approaches to improve
cold cache performance not pursued here:
- We could include the object offsets in the reverse index format.
Predictably, this does result in fewer page faults, but it triples
the size of the file, while simultaneously duplicating a ton of data
already available in the .idx file. (This was the original way I
implemented the format, and it did show
`--batch-check='%(objectsize:disk)'` winning out against `--batch`.)
On the other hand, this increase in size also results in a large
block-cache footprint, which could potentially hurt other workloads.
- We could store the mapping from pack to index position in more
cache-friendly way, like constructing a binary search tree from the
table and writing the values in breadth-first order. This would
result in much better locality, but the price you pay is trading
O(1) lookup in 'pack_pos_to_index()' for an O(log n) one (since you
can no longer directly index the table).
So, neither of these approaches are taken here. (Thankfully, the format
is versioned, so we are free to pursue these in the future.) But, cold
cache performance likely isn't interesting outside of one-off cases like
asking for the size of an object directly. In real-world usage, Git is
often performing many operations in the revindex (i.e., asking about
many objects rather than a single one).
The trade-off is worth it, since we will avoid the vast majority of the
cost of generating the revindex that the extra pointer chase will look
like noise in the following patch's benchmarks.
This patch describes the format and prepares callers (like in
pack-revindex.c) to be able to read *.rev files once they exist. An
implementation of the writer will appear in the next patch, and callers
will gradually begin to start using the writer in the patches that
follow after that.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-26 07:37:14 +08:00
|
|
|
|
|
|
|
|
if (p->revindex)
|
|
|
|
|
return p->revindex[pos].offset;
|
|
|
|
|
else if (pos == p->num_objects)
|
|
|
|
|
return p->pack_size - the_hash_algo->rawsz;
|
|
|
|
|
else
|
|
|
|
|
return nth_packed_object_offset(p, pack_pos_to_index(p, pos));
|
pack-revindex: introduce a new API
In the next several patches, we will prepare for loading a reverse index
either in memory (mapping the inverse of the .idx's contents in-core),
or directly from a yet-to-be-introduced on-disk format. To prepare for
that, we'll introduce an API that avoids the caller explicitly indexing
the revindex pointer in the packed_git structure.
There are four ways to interact with the reverse index. Accordingly,
four functions will be exported from 'pack-revindex.h' by the time that
the existing API is removed. A caller may:
1. Load the pack's reverse index. This involves opening up the index,
generating an array, and then sorting it. Since opening the index
can fail, this function ('load_pack_revindex()') returns an int.
Accordingly, it takes only a single argument: the 'struct
packed_git' the caller wants to build a reverse index for.
This function is well-suited for both the current and new API.
Callers will have to continue to open the reverse index explicitly,
but this function will eventually learn how to detect and load a
reverse index from the on-disk format, if one exists. Otherwise, it
will fallback to generating one in memory from scratch.
2. Convert a pack position into an offset. This operation is now
called `pack_pos_to_offset()`. It takes a pack and a position, and
returns the corresponding off_t.
Any error simply calls BUG(), since the callers are not well-suited
to handle a failure and keep going.
3. Convert a pack position into an index position. Same as above; this
takes a pack and a position, and returns a uint32_t. This operation
is known as `pack_pos_to_index()`. The same thinking about error
conditions applies here as well.
4. Find the pack position for a given offset. This operation is now
known as `offset_to_pack_pos()`. It takes a pack, an offset, and a
pointer to a uint32_t where the position is written, if an object
exists at that offset. Otherwise, -1 is returned to indicate
failure.
Unlike some of the callers that used to access '->offset' and '->nr'
directly, the error checking around this call is somewhat more
robust. This is important since callers should always pass an offset
which points at the boundary of two objects. The API, unlike direct
access, enforces that that is the case.
This will become important in a subsequent patch where a caller
which does not but could check the return value treats the signed
`-1` from `find_revindex_position()` as an index into the 'revindex'
array.
Two design warts are carried over into the new API:
- Asking for the index position of an out-of-bounds object will result
in a BUG() (since no such object exists), but asking for the offset
of the non-existent object at the end of the pack returns the total
size of the pack.
This makes it convenient for callers who always want to take the
difference of two adjacent object's offsets (to compute the on-disk
size) but don't want to worry about boundaries at the end of the
pack.
- offset_to_pack_pos() lazily loads the reverse index, but
pack_pos_to_index() doesn't (callers of the former are well-suited
to handle errors, but callers of the latter are not).
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-01-14 06:23:31 +08:00
|
|
|
}
|
pack-revindex: read multi-pack reverse indexes
Implement reading for multi-pack reverse indexes, as described in the
previous patch.
Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.
There are three new functions exposed by the revindex API:
- load_midx_revindex(): loads the reverse index corresponding to the
given multi-pack index.
- midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
multi-pack index and pseudo-pack order.
load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.
load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.
pack_pos_to_midx() simply reads the corresponding entry out of the
table.
midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.
So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-03-30 23:04:26 +08:00
|
|
|
|
|
|
|
|
uint32_t pack_pos_to_midx(struct multi_pack_index *m, uint32_t pos)
|
|
|
|
|
{
|
|
|
|
|
if (!m->revindex_data)
|
|
|
|
|
BUG("pack_pos_to_midx: reverse index not yet loaded");
|
|
|
|
|
if (m->num_objects <= pos)
|
|
|
|
|
BUG("pack_pos_to_midx: out-of-bounds object at %"PRIu32, pos);
|
|
|
|
|
return get_be32(m->revindex_data + pos);
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
struct midx_pack_key {
|
|
|
|
|
uint32_t pack;
|
|
|
|
|
off_t offset;
|
|
|
|
|
|
|
|
|
|
uint32_t preferred_pack;
|
|
|
|
|
struct multi_pack_index *midx;
|
|
|
|
|
};
|
|
|
|
|
|
|
|
|
|
static int midx_pack_order_cmp(const void *va, const void *vb)
|
|
|
|
|
{
|
|
|
|
|
const struct midx_pack_key *key = va;
|
|
|
|
|
struct multi_pack_index *midx = key->midx;
|
|
|
|
|
|
|
|
|
|
uint32_t versus = pack_pos_to_midx(midx, (uint32_t*)vb - (const uint32_t *)midx->revindex_data);
|
|
|
|
|
uint32_t versus_pack = nth_midxed_pack_int_id(midx, versus);
|
|
|
|
|
off_t versus_offset;
|
|
|
|
|
|
|
|
|
|
uint32_t key_preferred = key->pack == key->preferred_pack;
|
|
|
|
|
uint32_t versus_preferred = versus_pack == key->preferred_pack;
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* First, compare the preferred-ness, noting that the preferred pack
|
|
|
|
|
* comes first.
|
|
|
|
|
*/
|
|
|
|
|
if (key_preferred && !versus_preferred)
|
|
|
|
|
return -1;
|
|
|
|
|
else if (!key_preferred && versus_preferred)
|
|
|
|
|
return 1;
|
|
|
|
|
|
|
|
|
|
/* Then, break ties first by comparing the pack IDs. */
|
|
|
|
|
if (key->pack < versus_pack)
|
|
|
|
|
return -1;
|
|
|
|
|
else if (key->pack > versus_pack)
|
|
|
|
|
return 1;
|
|
|
|
|
|
|
|
|
|
/* Finally, break ties by comparing offsets within a pack. */
|
|
|
|
|
versus_offset = nth_midxed_offset(midx, versus);
|
|
|
|
|
if (key->offset < versus_offset)
|
|
|
|
|
return -1;
|
|
|
|
|
else if (key->offset > versus_offset)
|
|
|
|
|
return 1;
|
|
|
|
|
|
|
|
|
|
return 0;
|
|
|
|
|
}
|
|
|
|
|
|
2023-12-15 06:24:28 +08:00
|
|
|
static int midx_key_to_pack_pos(struct multi_pack_index *m,
|
|
|
|
|
struct midx_pack_key *key,
|
|
|
|
|
uint32_t *pos)
|
pack-revindex: read multi-pack reverse indexes
Implement reading for multi-pack reverse indexes, as described in the
previous patch.
Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.
There are three new functions exposed by the revindex API:
- load_midx_revindex(): loads the reverse index corresponding to the
given multi-pack index.
- midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
multi-pack index and pseudo-pack order.
load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.
load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.
pack_pos_to_midx() simply reads the corresponding entry out of the
table.
midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.
So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-03-30 23:04:26 +08:00
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{
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uint32_t *found;
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2024-06-12 01:28:24 +08:00
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if (key->pack >= m->num_packs)
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BUG("MIDX pack lookup out of bounds (%"PRIu32" >= %"PRIu32")",
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key->pack, m->num_packs);
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pack-revindex: read multi-pack reverse indexes
Implement reading for multi-pack reverse indexes, as described in the
previous patch.
Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.
There are three new functions exposed by the revindex API:
- load_midx_revindex(): loads the reverse index corresponding to the
given multi-pack index.
- midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
multi-pack index and pseudo-pack order.
load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.
load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.
pack_pos_to_midx() simply reads the corresponding entry out of the
table.
midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.
So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-03-30 23:04:26 +08:00
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/*
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* The preferred pack sorts first, so determine its identifier by
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* looking at the first object in pseudo-pack order.
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*
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* Note that if no --preferred-pack is explicitly given when writing a
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* multi-pack index, then whichever pack has the lowest identifier
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* implicitly is preferred (and includes all its objects, since ties are
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* broken first by pack identifier).
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*/
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2023-12-15 06:24:28 +08:00
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if (midx_preferred_pack(key->midx, &key->preferred_pack) < 0)
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midx: implement `midx_preferred_pack()`
When performing a binary search over the objects in a MIDX's bitmap
(i.e. in pseudo-pack order), the reader reconstructs the pseudo-pack
ordering using a combination of (a) the preferred pack, (b) the pack's
lexical position in the MIDX based on pack names, and (c) the object
offset within the pack.
In order to perform this binary search, the reader must know the
identity of the preferred pack. This could be stored in the MIDX, but
isn't for historical reasons, mostly because it can easily be inferred
at read-time by looking at the object in the first bit position and
finding out which pack it was selected from in the MIDX, like so:
nth_midxed_pack_int_id(m, pack_pos_to_midx(m, 0));
In midx_to_pack_pos() which performs this binary search, we look up the
identity of the preferred pack before each search. This is relatively
quick, since it involves two table-driven lookups (one in the MIDX's
revindex for `pack_pos_to_midx()`, and another in the MIDX's object
table for `nth_midxed_pack_int_id()`).
But since the preferred pack does not change after the MIDX is written,
it is safe to cache this value on the MIDX itself.
Write a helper to do just that, and rewrite all of the existing
call-sites that care about the identity of the preferred pack in terms
of this new helper.
This will prepare us for a subsequent patch where we will need to binary
search through the MIDX's pseudo-pack order multiple times.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2023-12-15 06:24:25 +08:00
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return error(_("could not determine preferred pack"));
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2023-12-15 06:24:28 +08:00
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found = bsearch(key, m->revindex_data, m->num_objects,
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sizeof(*m->revindex_data),
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midx_pack_order_cmp);
|
pack-revindex: read multi-pack reverse indexes
Implement reading for multi-pack reverse indexes, as described in the
previous patch.
Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.
There are three new functions exposed by the revindex API:
- load_midx_revindex(): loads the reverse index corresponding to the
given multi-pack index.
- midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
multi-pack index and pseudo-pack order.
load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.
load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.
pack_pos_to_midx() simply reads the corresponding entry out of the
table.
midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.
So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-03-30 23:04:26 +08:00
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if (!found)
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2023-12-15 06:24:28 +08:00
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return -1;
|
pack-revindex: read multi-pack reverse indexes
Implement reading for multi-pack reverse indexes, as described in the
previous patch.
Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.
There are three new functions exposed by the revindex API:
- load_midx_revindex(): loads the reverse index corresponding to the
given multi-pack index.
- midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
multi-pack index and pseudo-pack order.
load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.
load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.
pack_pos_to_midx() simply reads the corresponding entry out of the
table.
midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.
So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-03-30 23:04:26 +08:00
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*pos = found - m->revindex_data;
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return 0;
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}
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2023-12-15 06:24:28 +08:00
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int midx_to_pack_pos(struct multi_pack_index *m, uint32_t at, uint32_t *pos)
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{
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struct midx_pack_key key;
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if (!m->revindex_data)
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BUG("midx_to_pack_pos: reverse index not yet loaded");
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if (m->num_objects <= at)
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BUG("midx_to_pack_pos: out-of-bounds object at %"PRIu32, at);
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key.pack = nth_midxed_pack_int_id(m, at);
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key.offset = nth_midxed_offset(m, at);
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key.midx = m;
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return midx_key_to_pack_pos(m, &key, pos);
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}
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2023-12-15 06:24:30 +08:00
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int midx_pair_to_pack_pos(struct multi_pack_index *m, uint32_t pack_int_id,
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off_t ofs, uint32_t *pos)
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{
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struct midx_pack_key key = {
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.pack = pack_int_id,
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.offset = ofs,
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.midx = m,
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};
|
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|
|
return midx_key_to_pack_pos(m, &key, pos);
|
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}
|
