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
This test tries to exercise the ssh functionality with a dummy script
#testcases sshv1 sshv2
#if sshv2
$ cat >> $HGRCPATH << EOF
> [experimental]
> sshpeer.advertise-v2 = true
> sshserver.support-v2 = true
> EOF
#endif
creating 'remote' repo
$ hg init remote
$ cd remote
$ hg unbundle "$TESTDIR/bundles/remote.hg"
adding changesets
adding manifests
adding file changes
added 9 changesets with 7 changes to 4 files (+1 heads)
new changesets bfaf4b5cbf01:916f1afdef90 (9 drafts)
(run 'hg heads' to see heads, 'hg merge' to merge)
$ hg up tip
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd ..
clone remote via stream
$ for i in 0 1 2 3 4 5 6 7 8; do
> hg clone -e "\"$PYTHON\" \"$TESTDIR/dummyssh\"" --stream -r "$i" ssh://user@dummy/remote test-"$i"
> if cd test-"$i"; then
> hg verify
> cd ..
> fi
> done
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
new changesets bfaf4b5cbf01
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 1 changesets with 1 changes to 1 files
adding changesets
adding manifests
adding file changes
added 2 changesets with 2 changes to 1 files
new changesets bfaf4b5cbf01:21f32785131f
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 2 changesets with 2 changes to 1 files
adding changesets
adding manifests
adding file changes
added 3 changesets with 3 changes to 1 files
new changesets bfaf4b5cbf01:4ce51a113780
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 3 changesets with 3 changes to 1 files
adding changesets
adding manifests
adding file changes
added 4 changesets with 4 changes to 1 files
new changesets bfaf4b5cbf01:93ee6ab32777
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 4 changesets with 4 changes to 1 files
adding changesets
adding manifests
adding file changes
added 2 changesets with 2 changes to 1 files
new changesets bfaf4b5cbf01:c70afb1ee985
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 2 changesets with 2 changes to 1 files
adding changesets
adding manifests
adding file changes
added 3 changesets with 3 changes to 1 files
new changesets bfaf4b5cbf01:f03ae5a9b979
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 3 changesets with 3 changes to 1 files
adding changesets
adding manifests
adding file changes
added 4 changesets with 5 changes to 2 files
new changesets bfaf4b5cbf01:095cb14b1b4d
updating to branch default
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 4 changesets with 5 changes to 2 files
adding changesets
adding manifests
adding file changes
added 5 changesets with 6 changes to 3 files
new changesets bfaf4b5cbf01:faa2e4234c7a
updating to branch default
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 5 changesets with 6 changes to 3 files
adding changesets
adding manifests
adding file changes
added 5 changesets with 5 changes to 2 files
new changesets bfaf4b5cbf01:916f1afdef90
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 5 changesets with 5 changes to 2 files
$ cd test-8
$ hg pull ../test-7
pulling from ../test-7
searching for changes
adding changesets
adding manifests
adding file changes
added 4 changesets with 2 changes to 3 files (+1 heads)
new changesets c70afb1ee985:faa2e4234c7a
(run 'hg heads' to see heads, 'hg merge' to merge)
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 9 changesets with 7 changes to 4 files
$ cd ..
$ cd test-1
$ hg pull -e "\"$PYTHON\" \"$TESTDIR/dummyssh\"" -r 4 ssh://user@dummy/remote
pulling from ssh://user@dummy/remote
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 0 changes to 0 files (+1 heads)
new changesets c70afb1ee985
(run 'hg heads' to see heads, 'hg merge' to merge)
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 3 changesets with 2 changes to 1 files
$ hg pull -e "\"$PYTHON\" \"$TESTDIR/dummyssh\"" ssh://user@dummy/remote
pulling from ssh://user@dummy/remote
searching for changes
adding changesets
adding manifests
adding file changes
added 6 changesets with 5 changes to 4 files
new changesets 4ce51a113780:916f1afdef90
(run 'hg update' to get a working copy)
$ cd ..
$ cd test-2
$ hg pull -e "\"$PYTHON\" \"$TESTDIR/dummyssh\"" -r 5 ssh://user@dummy/remote
pulling from ssh://user@dummy/remote
searching for changes
adding changesets
adding manifests
adding file changes
added 2 changesets with 0 changes to 0 files (+1 heads)
new changesets c70afb1ee985:f03ae5a9b979
(run 'hg heads' to see heads, 'hg merge' to merge)
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 5 changesets with 3 changes to 1 files
$ hg pull -e "\"$PYTHON\" \"$TESTDIR/dummyssh\"" ssh://user@dummy/remote
pulling from ssh://user@dummy/remote
searching for changes
adding changesets
adding manifests
adding file changes
added 4 changesets with 4 changes to 4 files
new changesets 93ee6ab32777:916f1afdef90
(run 'hg update' to get a working copy)
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 9 changesets with 7 changes to 4 files
$ cd ..