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
Test transaction safety
=======================
#testcases revlogv1 revlogv2 changelogv2
#if revlogv1
$ cat << EOF >> $HGRCPATH
> [experimental]
> revlogv2=no
> EOF
#endif
#if revlogv2
$ cat << EOF >> $HGRCPATH
> [experimental]
> revlogv2=enable-unstable-format-and-corrupt-my-data
> EOF
#endif
#if changelogv2
$ cat << EOF >> $HGRCPATH
> [format]
> exp-use-changelog-v2=enable-unstable-format-and-corrupt-my-data
> EOF
#endif
This test basic case to make sure external process do not see transaction
content until it is committed.
# TODO: also add an external reader accessing revlog files while they are written
# (instead of during transaction finalisation)
# TODO: also add stream clone and hardlink clone happening during these transaction.
setup
-----
synchronisation+output script:
$ mkdir sync
$ mkdir output
$ mkdir script
$ HG_TEST_FILE_EXT_WAITING=$TESTTMP/sync/ext_waiting
$ export HG_TEST_FILE_EXT_WAITING
$ HG_TEST_FILE_EXT_UNLOCK=$TESTTMP/sync/ext_unlock
$ export HG_TEST_FILE_EXT_UNLOCK
$ HG_TEST_FILE_EXT_DONE=$TESTTMP/sync/ext_done
$ export HG_TEST_FILE_EXT_DONE
$ cat << EOF > script/external.sh
> #!/bin/sh
> "$RUNTESTDIR/testlib/wait-on-file" 5 "$HG_TEST_FILE_EXT_UNLOCK" "$HG_TEST_FILE_EXT_WAITING"
> hg log --rev 'tip' -T 'external: {rev} {desc}\n' > "$TESTTMP/output/external.out"
> touch "$HG_TEST_FILE_EXT_DONE"
> EOF
$ cat << EOF > script/internal.sh
> #!/bin/sh
> hg log --rev 'tip' -T 'internal: {rev} {desc}\n' > "$TESTTMP/output/internal.out"
> "$RUNTESTDIR/testlib/wait-on-file" 5 "$HG_TEST_FILE_EXT_DONE" "$HG_TEST_FILE_EXT_UNLOCK"
> EOF
Automated commands:
$ make_one_commit() {
> rm -f $TESTTMP/sync/*
> rm -f $TESTTMP/output/*
> hg log --rev 'tip' -T 'pre-commit: {rev} {desc}\n'
> echo x >> a
> sh $TESTTMP/script/external.sh & hg commit -m "$1"
> cat $TESTTMP/output/external.out
> cat $TESTTMP/output/internal.out
> hg log --rev 'tip' -T 'post-tr: {rev} {desc}\n'
> }
$ make_one_pull() {
> rm -f $TESTTMP/sync/*
> rm -f $TESTTMP/output/*
> hg log --rev 'tip' -T 'pre-commit: {rev} {desc}\n'
> echo x >> a
> sh $TESTTMP/script/external.sh & hg pull ../other-repo/ --rev "$1" --force --quiet
> cat $TESTTMP/output/external.out
> cat $TESTTMP/output/internal.out
> hg log --rev 'tip' -T 'post-tr: {rev} {desc}\n'
> }
prepare a large source to which to pull from:
The source is large to unsure we don't use inline more after the pull
$ hg init other-repo
$ hg -R other-repo debugbuilddag .+500
prepare an empty repository where to make test:
$ hg init repo
$ cd repo
$ touch a
$ hg add a
prepare a small extension to controll inline size
$ mkdir $TESTTMP/ext
$ cat << EOF > $TESTTMP/ext/small_inline.py
> from mercurial import revlog
> revlog._maxinline = 64 * 100
> EOF
$ cat << EOF >> $HGRCPATH
> [extensions]
> small_inline=$TESTTMP/ext/small_inline.py
> [hooks]
> pretxnclose = sh $TESTTMP/script/internal.sh
> EOF
check this is true for the initial commit (inline → inline)
-----------------------------------------------------------
the repository should still be inline (for relevant format)
$ make_one_commit first
pre-commit: -1
external: -1
internal: 0 first
post-tr: 0 first
#if revlogv1
$ hg debugrevlog -c | grep inline
flags : inline
#endif
check this is true for extra commit (inline → inline)
-----------------------------------------------------
the repository should still be inline (for relevant format)
#if revlogv1
$ hg debugrevlog -c | grep inline
flags : inline
#endif
$ make_one_commit second
pre-commit: 0 first
external: 0 first
internal: 1 second
post-tr: 1 second
#if revlogv1
$ hg debugrevlog -c | grep inline
flags : inline
#endif
check this is true for a small pull (inline → inline)
-----------------------------------------------------
the repository should still be inline (for relevant format)
#if revlogv1
$ hg debugrevlog -c | grep inline
flags : inline
#endif
$ make_one_pull 3
pre-commit: 1 second
warning: repository is unrelated
external: 1 second
internal: 5 r3
post-tr: 5 r3
#if revlogv1
$ hg debugrevlog -c | grep inline
flags : inline
#endif
Make a large pull (inline → no-inline)
---------------------------------------
the repository should no longer be inline (for relevant format)
#if revlogv1
$ hg debugrevlog -c | grep inline
flags : inline
#endif
$ make_one_pull 400
pre-commit: 5 r3
external: 5 r3
internal: 402 r400
post-tr: 402 r400
#if revlogv1
$ hg debugrevlog -c | grep inline
[1]
#endif
check this is true for extra commit (no-inline → no-inline)
-----------------------------------------------------------
the repository should no longer be inline (for relevant format)
#if revlogv1
$ hg debugrevlog -c | grep inline
[1]
#endif
$ make_one_commit third
pre-commit: 402 r400
external: 402 r400
internal: 403 third
post-tr: 403 third
#if revlogv1
$ hg debugrevlog -c | grep inline
[1]
#endif
Make a pull (not-inline → no-inline)
-------------------------------------
the repository should no longer be inline (for relevant format)
#if revlogv1
$ hg debugrevlog -c | grep inline
[1]
#endif
$ make_one_pull tip
pre-commit: 403 third
external: 403 third
internal: 503 r500
post-tr: 503 r500
#if revlogv1
$ hg debugrevlog -c | grep inline
[1]
#endif