Mercurial > hg
view tests/test-hgwebdir-gc.py @ 47802:de2e04fe4897
hgwebdir: avoid systematic full garbage collection
Forcing a systematic full garbage collection upon each request
can serioulsy harm performance. This is reported as
https://bz.mercurial-scm.org/show_bug.cgi?id=6075
With this change we're performing the full collection according
to a new setting, `experimental.web.full-garbage-collection-rate`.
The default value is 1, which doesn't change the behavior and will
allow us to test on real use cases. If the value is 0, no full garbage
collection occurs.
Regardless of the value of the setting, a partial garbage collection
still occurs upon each request (not attempting to collect objects from
the oldest generation). This should be enough to take care of
reference cycles that have been created by the last request
(assessment of this requires changing the setting, not to be 1).
In my experience chasing memory leaks in Mercurial servers,
the full collection never reclaimed any memory, but this is with
Python 3 and biased towards small repositories.
On the other hand, as explained in the Python developer docs [1],
frequent full collections are very harmful in terms of performance if
lots of objects survive the collection, and hence stay in the
oldest generation. Note that `gc.collect()` is indeed trying to
collect the oldest generation [2]. This happens usually in two cases:
- unwanted lingering objects (i.e., an actual memory leak that
the GC cannot do anything about). Sadly, we have lots of those
these days.
- desireable long-term objects, typically in caches (not inner caches
carried by repositories, which should be collected with them). This
is a subject of interest for the Heptapod project.
In short, the flat rate that this change still permits is
probably a bad idea in most cases, and the default value can
be tweaked later on (or even be set to 0) according to experiments
in the wild.
The test is inspired from test-hgwebdir-paths.py
[1] https://devguide.python.org/garbage_collector/#collecting-the-oldest-generation
[2] https://docs.python.org/3/library/gc.html#gc.collect
Differential Revision: https://phab.mercurial-scm.org/D11204
author | Georges Racinet <georges.racinet@octobus.net> |
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date | Tue, 20 Jul 2021 17:20:19 +0200 |
parents | |
children | 6000f5b25c9b |
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from __future__ import absolute_import import os from mercurial.hgweb import hgwebdir_mod hgwebdir = hgwebdir_mod.hgwebdir os.mkdir(b'webdir') os.chdir(b'webdir') webdir = os.path.realpath(b'.') def trivial_response(req, res): return [] def make_hgwebdir(gc_rate=None): config = os.path.join(webdir, b'hgwebdir.conf') with open(config, 'wb') as configfile: configfile.write(b'[experimental]\n') if gc_rate is not None: configfile.write(b'web.full-garbage-collection-rate=%d\n' % gc_rate) hg_wd = hgwebdir(config) hg_wd._runwsgi = trivial_response return hg_wd def process_requests(webdir_instance, number): # we don't care for now about passing realistic arguments for _ in range(number): for chunk in webdir_instance.run_wsgi(None, None): pass without_gc = make_hgwebdir(gc_rate=0) process_requests(without_gc, 5) assert without_gc.requests_count == 5 assert without_gc.gc_full_collections_done == 0 with_gc = make_hgwebdir(gc_rate=2) process_requests(with_gc, 5) assert with_gc.requests_count == 5 assert with_gc.gc_full_collections_done == 2 with_systematic_gc = make_hgwebdir() # default value of the setting process_requests(with_systematic_gc, 3) assert with_systematic_gc.requests_count == 3 assert with_systematic_gc.gc_full_collections_done == 3