mirror of
https://github.com/edk2-porting/linux-next.git
synced 2024-12-24 05:04:00 +08:00
8b898fd114
Remove implementation details from sysfs parameter descriptions. Also move the paragraph discussing fragmentation issues and their possible solution to the "Design" section. Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
262 lines
11 KiB
ReStructuredText
262 lines
11 KiB
ReStructuredText
.. _ksm:
|
|
|
|
=======================
|
|
Kernel Samepage Merging
|
|
=======================
|
|
|
|
Overview
|
|
========
|
|
|
|
KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y,
|
|
added to the Linux kernel in 2.6.32. See ``mm/ksm.c`` for its implementation,
|
|
and http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/
|
|
|
|
KSM was originally developed for use with KVM (where it was known as
|
|
Kernel Shared Memory), to fit more virtual machines into physical memory,
|
|
by sharing the data common between them. But it can be useful to any
|
|
application which generates many instances of the same data.
|
|
|
|
The KSM daemon ksmd periodically scans those areas of user memory
|
|
which have been registered with it, looking for pages of identical
|
|
content which can be replaced by a single write-protected page (which
|
|
is automatically copied if a process later wants to update its
|
|
content). The amount of pages that KSM daemon scans in a single pass
|
|
and the time between the passes are configured using :ref:`sysfs
|
|
intraface <ksm_sysfs>`
|
|
|
|
KSM only merges anonymous (private) pages, never pagecache (file) pages.
|
|
KSM's merged pages were originally locked into kernel memory, but can now
|
|
be swapped out just like other user pages (but sharing is broken when they
|
|
are swapped back in: ksmd must rediscover their identity and merge again).
|
|
|
|
Controlling KSM with madvise
|
|
============================
|
|
|
|
KSM only operates on those areas of address space which an application
|
|
has advised to be likely candidates for merging, by using the madvise(2)
|
|
system call::
|
|
|
|
int madvise(addr, length, MADV_MERGEABLE)
|
|
|
|
The app may call
|
|
|
|
::
|
|
|
|
int madvise(addr, length, MADV_UNMERGEABLE)
|
|
|
|
to cancel that advice and restore unshared pages: whereupon KSM
|
|
unmerges whatever it merged in that range. Note: this unmerging call
|
|
may suddenly require more memory than is available - possibly failing
|
|
with EAGAIN, but more probably arousing the Out-Of-Memory killer.
|
|
|
|
If KSM is not configured into the running kernel, madvise MADV_MERGEABLE
|
|
and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was
|
|
built with CONFIG_KSM=y, those calls will normally succeed: even if the
|
|
the KSM daemon is not currently running, MADV_MERGEABLE still registers
|
|
the range for whenever the KSM daemon is started; even if the range
|
|
cannot contain any pages which KSM could actually merge; even if
|
|
MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE.
|
|
|
|
If a region of memory must be split into at least one new MADV_MERGEABLE
|
|
or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
|
|
will exceed ``vm.max_map_count`` (see Documentation/sysctl/vm.txt).
|
|
|
|
Like other madvise calls, they are intended for use on mapped areas of
|
|
the user address space: they will report ENOMEM if the specified range
|
|
includes unmapped gaps (though working on the intervening mapped areas),
|
|
and might fail with EAGAIN if not enough memory for internal structures.
|
|
|
|
Applications should be considerate in their use of MADV_MERGEABLE,
|
|
restricting its use to areas likely to benefit. KSM's scans may use a lot
|
|
of processing power: some installations will disable KSM for that reason.
|
|
|
|
.. _ksm_sysfs:
|
|
|
|
KSM daemon sysfs interface
|
|
==========================
|
|
|
|
The KSM daemon is controlled by sysfs files in ``/sys/kernel/mm/ksm/``,
|
|
readable by all but writable only by root:
|
|
|
|
pages_to_scan
|
|
how many pages to scan before ksmd goes to sleep
|
|
e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``.
|
|
|
|
Default: 100 (chosen for demonstration purposes)
|
|
|
|
sleep_millisecs
|
|
how many milliseconds ksmd should sleep before next scan
|
|
e.g. ``echo 20 > /sys/kernel/mm/ksm/sleep_millisecs``
|
|
|
|
Default: 20 (chosen for demonstration purposes)
|
|
|
|
merge_across_nodes
|
|
specifies if pages from different NUMA nodes can be merged.
|
|
When set to 0, ksm merges only pages which physically reside
|
|
in the memory area of same NUMA node. That brings lower
|
|
latency to access of shared pages. Systems with more nodes, at
|
|
significant NUMA distances, are likely to benefit from the
|
|
lower latency of setting 0. Smaller systems, which need to
|
|
minimize memory usage, are likely to benefit from the greater
|
|
sharing of setting 1 (default). You may wish to compare how
|
|
your system performs under each setting, before deciding on
|
|
which to use. ``merge_across_nodes`` setting can be changed only
|
|
when there are no ksm shared pages in the system: set run 2 to
|
|
unmerge pages first, then to 1 after changing
|
|
``merge_across_nodes``, to remerge according to the new setting.
|
|
|
|
Default: 1 (merging across nodes as in earlier releases)
|
|
|
|
run
|
|
* set to 0 to stop ksmd from running but keep merged pages,
|
|
* set to 1 to run ksmd e.g. ``echo 1 > /sys/kernel/mm/ksm/run``,
|
|
* set to 2 to stop ksmd and unmerge all pages currently merged, but
|
|
leave mergeable areas registered for next run.
|
|
|
|
Default: 0 (must be changed to 1 to activate KSM, except if
|
|
CONFIG_SYSFS is disabled)
|
|
|
|
use_zero_pages
|
|
specifies whether empty pages (i.e. allocated pages that only
|
|
contain zeroes) should be treated specially. When set to 1,
|
|
empty pages are merged with the kernel zero page(s) instead of
|
|
with each other as it would happen normally. This can improve
|
|
the performance on architectures with coloured zero pages,
|
|
depending on the workload. Care should be taken when enabling
|
|
this setting, as it can potentially degrade the performance of
|
|
KSM for some workloads, for example if the checksums of pages
|
|
candidate for merging match the checksum of an empty
|
|
page. This setting can be changed at any time, it is only
|
|
effective for pages merged after the change.
|
|
|
|
Default: 0 (normal KSM behaviour as in earlier releases)
|
|
|
|
max_page_sharing
|
|
Maximum sharing allowed for each KSM page. This enforces a
|
|
deduplication limit to avoid high latency for virtual memory
|
|
operations that involve traversal of the virtual mappings that
|
|
share the KSM page. The minimum value is 2 as a newly created
|
|
KSM page will have at least two sharers. The higher this value
|
|
the faster KSM will merge the memory and the higher the
|
|
deduplication factor will be, but the slower the worst case
|
|
virtual mappings traversal could be for any given KSM
|
|
page. Slowing down this traversal means there will be higher
|
|
latency for certain virtual memory operations happening during
|
|
swapping, compaction, NUMA balancing and page migration, in
|
|
turn decreasing responsiveness for the caller of those virtual
|
|
memory operations. The scheduler latency of other tasks not
|
|
involved with the VM operations doing the virtual mappings
|
|
traversal is not affected by this parameter as these
|
|
traversals are always schedule friendly themselves.
|
|
|
|
stable_node_chains_prune_millisecs
|
|
specifies how frequently KSM checks the metadata of the pages
|
|
that hit the deduplication limit for stale information.
|
|
Smaller milllisecs values will free up the KSM metadata with
|
|
lower latency, but they will make ksmd use more CPU during the
|
|
scan. It's a noop if not a single KSM page hit the
|
|
``max_page_sharing`` yet.
|
|
|
|
The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
|
|
|
|
pages_shared
|
|
how many shared pages are being used
|
|
pages_sharing
|
|
how many more sites are sharing them i.e. how much saved
|
|
pages_unshared
|
|
how many pages unique but repeatedly checked for merging
|
|
pages_volatile
|
|
how many pages changing too fast to be placed in a tree
|
|
full_scans
|
|
how many times all mergeable areas have been scanned
|
|
stable_node_chains
|
|
the number of KSM pages that hit the ``max_page_sharing`` limit
|
|
stable_node_dups
|
|
number of duplicated KSM pages
|
|
|
|
A high ratio of ``pages_sharing`` to ``pages_shared`` indicates good
|
|
sharing, but a high ratio of ``pages_unshared`` to ``pages_sharing``
|
|
indicates wasted effort. ``pages_volatile`` embraces several
|
|
different kinds of activity, but a high proportion there would also
|
|
indicate poor use of madvise MADV_MERGEABLE.
|
|
|
|
The maximum possible ``pages_sharing/pages_shared`` ratio is limited by the
|
|
``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must
|
|
be increased accordingly.
|
|
|
|
Design
|
|
======
|
|
|
|
Overview
|
|
--------
|
|
|
|
.. kernel-doc:: mm/ksm.c
|
|
:DOC: Overview
|
|
|
|
Reverse mapping
|
|
---------------
|
|
KSM maintains reverse mapping information for KSM pages in the stable
|
|
tree.
|
|
|
|
If a KSM page is shared between less than ``max_page_sharing`` VMAs,
|
|
the node of the stable tree that represents such KSM page points to a
|
|
list of :c:type:`struct rmap_item` and the ``page->mapping`` of the
|
|
KSM page points to the stable tree node.
|
|
|
|
When the sharing passes this threshold, KSM adds a second dimension to
|
|
the stable tree. The tree node becomes a "chain" that links one or
|
|
more "dups". Each "dup" keeps reverse mapping information for a KSM
|
|
page with ``page->mapping`` pointing to that "dup".
|
|
|
|
Every "chain" and all "dups" linked into a "chain" enforce the
|
|
invariant that they represent the same write protected memory content,
|
|
even if each "dup" will be pointed by a different KSM page copy of
|
|
that content.
|
|
|
|
This way the stable tree lookup computational complexity is unaffected
|
|
if compared to an unlimited list of reverse mappings. It is still
|
|
enforced that there cannot be KSM page content duplicates in the
|
|
stable tree itself.
|
|
|
|
The deduplication limit enforced by ``max_page_sharing`` is required
|
|
to avoid the virtual memory rmap lists to grow too large. The rmap
|
|
walk has O(N) complexity where N is the number of rmap_items
|
|
(i.e. virtual mappings) that are sharing the page, which is in turn
|
|
capped by ``max_page_sharing``. So this effectively spreads the linear
|
|
O(N) computational complexity from rmap walk context over different
|
|
KSM pages. The ksmd walk over the stable_node "chains" is also O(N),
|
|
but N is the number of stable_node "dups", not the number of
|
|
rmap_items, so it has not a significant impact on ksmd performance. In
|
|
practice the best stable_node "dup" candidate will be kept and found
|
|
at the head of the "dups" list.
|
|
|
|
High values of ``max_page_sharing`` result in faster memory merging
|
|
(because there will be fewer stable_node dups queued into the
|
|
stable_node chain->hlist to check for pruning) and higher
|
|
deduplication factor at the expense of slower worst case for rmap
|
|
walks for any KSM page which can happen during swapping, compaction,
|
|
NUMA balancing and page migration.
|
|
|
|
The ``stable_node_dups/stable_node_chains`` ratio is also affected by the
|
|
``max_page_sharing`` tunable, and an high ratio may indicate fragmentation
|
|
in the stable_node dups, which could be solved by introducing
|
|
fragmentation algorithms in ksmd which would refile rmap_items from
|
|
one stable_node dup to another stable_node dup, in order to free up
|
|
stable_node "dups" with few rmap_items in them, but that may increase
|
|
the ksmd CPU usage and possibly slowdown the readonly computations on
|
|
the KSM pages of the applications.
|
|
|
|
The whole list of stable_node "dups" linked in the stable_node
|
|
"chains" is scanned periodically in order to prune stale stable_nodes.
|
|
The frequency of such scans is defined by
|
|
``stable_node_chains_prune_millisecs`` sysfs tunable.
|
|
|
|
Reference
|
|
---------
|
|
.. kernel-doc:: mm/ksm.c
|
|
:functions: mm_slot ksm_scan stable_node rmap_item
|
|
|
|
--
|
|
Izik Eidus,
|
|
Hugh Dickins, 17 Nov 2009
|