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
$ hg init t
$ cd t
$ echo a > a
$ hg add a
$ hg commit -m test
$ rm .hg/requires
$ hg tip
abort: unknown version (65535) in revlog 00changelog
[50]
$ echo indoor-pool > .hg/requires
$ hg tip
abort: repository requires features unknown to this Mercurial: indoor-pool
(see https://mercurial-scm.org/wiki/MissingRequirement for more information)
[255]
$ echo outdoor-pool >> .hg/requires
$ hg tip
abort: repository requires features unknown to this Mercurial: indoor-pool outdoor-pool
(see https://mercurial-scm.org/wiki/MissingRequirement for more information)
[255]
$ cd ..
Test checking between features supported locally and ones required in
another repository of push/pull/clone on localhost:
$ mkdir supported-locally
$ cd supported-locally
$ hg init supported
$ echo a > supported/a
$ hg -R supported commit -Am '#0 at supported'
adding a
$ echo 'featuresetup-test' >> supported/.hg/requires
$ cat > $TESTTMP/supported-locally/supportlocally.py <<EOF
> from __future__ import absolute_import
> from mercurial import extensions, localrepo
> def featuresetup(ui, supported):
> for name, module in extensions.extensions(ui):
> if __name__ == module.__name__:
> # support specific feature locally
> supported |= {b'featuresetup-test'}
> return
> def uisetup(ui):
> localrepo.featuresetupfuncs.add(featuresetup)
> EOF
$ cat > supported/.hg/hgrc <<EOF
> [extensions]
> # enable extension locally
> supportlocally = $TESTTMP/supported-locally/supportlocally.py
> EOF
$ hg -R supported debugrequirements
dotencode
exp-dirstate-v2 (dirstate-v2 !)
featuresetup-test
fncache
generaldelta
persistent-nodemap (rust !)
revlog-compression-zstd (zstd !)
revlogv1
sparserevlog
store
$ hg -R supported status
$ hg init push-dst
$ hg -R supported push push-dst
pushing to push-dst
abort: required features are not supported in the destination: featuresetup-test
[255]
$ hg init pull-src
$ hg -R pull-src pull supported
pulling from supported
abort: required features are not supported in the destination: featuresetup-test
[255]
$ hg clone supported clone-dst
abort: repository requires features unknown to this Mercurial: featuresetup-test
(see https://mercurial-scm.org/wiki/MissingRequirement for more information)
[255]
$ hg clone --pull supported clone-dst
abort: required features are not supported in the destination: featuresetup-test
[255]
$ cd ..