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
# unit tests for mercuril.util utilities
from __future__ import absolute_import
import contextlib
import itertools
import unittest
from mercurial import pycompat, util, utils
@contextlib.contextmanager
def mocktimer(incr=0.1, *additional_targets):
"""Replaces util.timer and additional_targets with a mock
The timer starts at 0. On each call the time incremented by the value
of incr. If incr is an iterable, then the time is incremented by the
next value from that iterable, looping in a cycle when reaching the end.
additional_targets must be a sequence of (object, attribute_name) tuples;
the mock is set with setattr(object, attribute_name, mock).
"""
time = [0]
try:
incr = itertools.cycle(incr)
except TypeError:
incr = itertools.repeat(incr)
def timer():
time[0] += next(incr)
return time[0]
# record original values
orig = util.timer
additional_origs = [(o, a, getattr(o, a)) for o, a in additional_targets]
# mock out targets
util.timer = timer
for obj, attr in additional_targets:
setattr(obj, attr, timer)
try:
yield
finally:
# restore originals
util.timer = orig
for args in additional_origs:
setattr(*args)
# attr.s default factory for util.timedstats.start binds the timer we
# need to mock out.
_start_default = (util.timedcmstats.start.default, 'factory')
@contextlib.contextmanager
def capturestderr():
"""Replace utils.procutil.stderr with a pycompat.bytesio instance
The instance is made available as the return value of __enter__.
This contextmanager is reentrant.
"""
orig = utils.procutil.stderr
utils.procutil.stderr = pycompat.bytesio()
try:
yield utils.procutil.stderr
finally:
utils.procutil.stderr = orig
class timedtests(unittest.TestCase):
def testtimedcmstatsstr(self):
stats = util.timedcmstats()
self.assertEqual(str(stats), '<unknown>')
self.assertEqual(bytes(stats), b'<unknown>')
stats.elapsed = 12.34
self.assertEqual(str(stats), pycompat.sysstr(util.timecount(12.34)))
self.assertEqual(bytes(stats), util.timecount(12.34))
def testtimedcmcleanexit(self):
# timestamps 1, 4, elapsed time of 4 - 1 = 3
with mocktimer([1, 3], _start_default):
with util.timedcm('pass') as stats:
# actual context doesn't matter
pass
self.assertEqual(stats.start, 1)
self.assertEqual(stats.elapsed, 3)
self.assertEqual(stats.level, 1)
def testtimedcmnested(self):
# timestamps 1, 3, 6, 10, elapsed times of 6 - 3 = 3 and 10 - 1 = 9
with mocktimer([1, 2, 3, 4], _start_default):
with util.timedcm('outer') as outer_stats:
with util.timedcm('inner') as inner_stats:
# actual context doesn't matter
pass
self.assertEqual(outer_stats.start, 1)
self.assertEqual(outer_stats.elapsed, 9)
self.assertEqual(outer_stats.level, 1)
self.assertEqual(inner_stats.start, 3)
self.assertEqual(inner_stats.elapsed, 3)
self.assertEqual(inner_stats.level, 2)
def testtimedcmexception(self):
# timestamps 1, 4, elapsed time of 4 - 1 = 3
with mocktimer([1, 3], _start_default):
try:
with util.timedcm('exceptional') as stats:
raise ValueError()
except ValueError:
pass
self.assertEqual(stats.start, 1)
self.assertEqual(stats.elapsed, 3)
self.assertEqual(stats.level, 1)
def testtimeddecorator(self):
@util.timed
def testfunc(callcount=1):
callcount -= 1
if callcount:
testfunc(callcount)
# timestamps 1, 2, 3, 4, elapsed time of 3 - 2 = 1 and 4 - 1 = 3
with mocktimer(1, _start_default):
with capturestderr() as out:
testfunc(2)
self.assertEqual(
out.getvalue(),
(b' testfunc: 1.000 s\n' b' testfunc: 3.000 s\n'),
)
if __name__ == '__main__':
import silenttestrunner
silenttestrunner.main(__name__)