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  r68462 | antoine.pitrou | 2009-01-09 22:40:55 +0100 (ven., 09 janv. 2009) | 6 lines

  Issue #4074: Change the criteria for doing a full garbage collection (i.e.
  collecting the oldest generation) so that allocating lots of objects without
  destroying them does not show quadratic performance. Based on a proposal by
  Martin von Löwis at http://mail.python.org/pipermail/python-dev/2008-June/080579.html.
........
This commit is contained in:
Antoine Pitrou 2009-01-09 22:27:08 +00:00
parent 9169641b8b
commit 14b78f5fc5
2 changed files with 71 additions and 1 deletions
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6
Misc/NEWS
View File

@ -12,6 +12,12 @@ What's New in Python 3.1 alpha 0
Core and Builtins
-----------------
- Issue #4074: Change the criteria for doing a full garbage collection (i.e.
collecting the oldest generation) so that allocating lots of objects without
destroying them does not show quadratic performance. Based on a proposal by
Martin von Löwis at
http://mail.python.org/pipermail/python-dev/2008-June/080579.html.
- Issue #4604: Some objects of the I/O library could still be used after
having been closed (for instance, a read() call could return some
previously buffered data). Patch by Dmitry Vasiliev.

66
Modules/gcmodule.c
View File

@ -68,6 +68,55 @@ static PyObject *gc_str = NULL;
/* Python string used to look for __del__ attribute. */
static PyObject *delstr = NULL;
/* This is the number of objects who survived the last full collection. It
approximates the number of long lived objects tracked by the GC.
(by "full collection", we mean a collection of the oldest generation).
*/
static Py_ssize_t long_lived_total = 0;
/* This is the number of objects who survived all "non-full" collections,
and are awaiting to undergo a full collection for the first time.
*/
static Py_ssize_t long_lived_pending = 0;
/*
NOTE: about the counting of long-lived objects.
To limit the cost of garbage collection, there are two strategies;
- make each collection faster, e.g. by scanning fewer objects
- do less collections
This heuristic is about the latter strategy.
In addition to the various configurable thresholds, we only trigger a
full collection if the ratio
long_lived_pending / long_lived_total
is above a given value (hardwired to 25%).
The reason is that, while "non-full" collections (i.e., collections of
the young and middle generations) will always examine roughly the same
number of objects -- determined by the aforementioned thresholds --,
the cost of a full collection is proportional to the total number of
long-lived objects, which is virtually unbounded.
Indeed, it has been remarked that doing a full collection every
<constant number> of object creations entails a dramatic performance
degradation in workloads which consist in creating and storing lots of
long-lived objects (e.g. building a large list of GC-tracked objects would
show quadratic performance, instead of linear as expected: see issue #4074).
Using the above ratio, instead, yields amortized linear performance in
the total number of objects (the effect of which can be summarized
thusly: "each full garbage collection is more and more costly as the
number of objects grows, but we do fewer and fewer of them").
This heuristic was suggested by Martin von Löwis on python-dev in
June 2008. His original analysis and proposal can be found at:
http://mail.python.org/pipermail/python-dev/2008-June/080579.html
*/
/* set for debugging information */
#define DEBUG_STATS (1<<0) /* print collection statistics */
#define DEBUG_COLLECTABLE (1<<1) /* print collectable objects */
@ -795,8 +844,16 @@ collect(int generation)
move_unreachable(young, &unreachable);
/* Move reachable objects to next generation. */
if (young != old)
if (young != old) {
if (generation == NUM_GENERATIONS - 2) {
long_lived_pending += gc_list_size(young);
}
gc_list_merge(young, old);
}
else {
long_lived_pending = 0;
long_lived_total = gc_list_size(young);
}
/* All objects in unreachable are trash, but objects reachable from
* finalizers can't safely be deleted. Python programmers should take
@ -890,6 +947,13 @@ collect_generations(void)
* generations younger than it will be collected. */
for (i = NUM_GENERATIONS-1; i >= 0; i--) {
if (generations[i].count > generations[i].threshold) {
/* Avoid quadratic performance degradation in number
of tracked objects. See comments at the beginning
of this file, and issue #4074.
*/
if (i == NUM_GENERATIONS - 1
&& long_lived_pending < long_lived_total / 4)
continue;
n = collect(i);
break;
}