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
#testcases continuecommand continueflag
Rebasing using a single transaction
$ cat >> $HGRCPATH <<EOF
> [extensions]
> rebase=
> drawdag=$TESTDIR/drawdag.py
>
> [rebase]
> singletransaction=True
>
> [phases]
> publish=False
>
> [alias]
> tglog = log -G --template "{rev}: {desc}"
> EOF
#if continueflag
$ cat >> $HGRCPATH <<EOF
> [alias]
> continue = rebase --continue
> EOF
#endif
Check that a simple rebase works
$ hg init simple && cd simple
$ hg debugdrawdag <<'EOF'
> Z
> |
> | D
> | |
> | C
> | |
> Y B
> |/
> A
> EOF
- We should only see one status stored message. It comes from the start.
$ hg rebase --debug -b D -d Z | grep 'status stored'
rebase status stored
$ hg tglog
o 5: D
|
o 4: C
|
o 3: B
|
o 2: Z
|
o 1: Y
|
o 0: A
$ cd ..
Check that --collapse works
$ hg init collapse && cd collapse
$ hg debugdrawdag <<'EOF'
> Z
> |
> | D
> | |
> | C
> | |
> Y B
> |/
> A
> EOF
- We should only see two status stored messages. One from the start, one from
- cmdutil.commitforceeditor() which forces tr.writepending()
$ hg rebase --collapse --debug -b D -d Z | grep 'status stored'
rebase status stored
rebase status stored
$ hg tglog
o 3: Collapsed revision
| * B
| * C
| * D
o 2: Z
|
o 1: Y
|
o 0: A
$ cd ..
With --collapse, check that conflicts can be resolved and rebase can then be
continued
$ hg init collapse-conflict && cd collapse-conflict
$ hg debugdrawdag <<'EOF'
> Z # Z/conflict=Z
> |
> | D
> | |
> | C # C/conflict=C
> | |
> Y B
> |/
> A
> EOF
$ hg rebase --collapse -b D -d Z
rebasing 1:112478962961 B "B"
rebasing 3:c26739dbe603 C "C"
merging conflict
warning: conflicts while merging conflict! (edit, then use 'hg resolve --mark')
unresolved conflicts (see 'hg resolve', then 'hg rebase --continue')
[240]
$ hg tglog
o 5: D
|
| @ 4: Z
| |
% | 3: C
| |
| o 2: Y
| |
o | 1: B
|/
o 0: A
$ hg st
M conflict
A B
A C
? conflict.orig
$ echo resolved > conflict
$ hg resolve -m
(no more unresolved files)
continue: hg rebase --continue
$ hg continue
already rebased 1:112478962961 B "B" as 79bc8f4973ce
rebasing 3:c26739dbe603 C "C"
rebasing 5:d24bb333861c D tip "D"
saved backup bundle to $TESTTMP/collapse-conflict/.hg/strip-backup/112478962961-b5b34645-rebase.hg
$ hg tglog
o 3: Collapsed revision
| * B
| * C
| * D
o 2: Z
|
o 1: Y
|
o 0: A
$ cd ..
With --collapse, check that the commit message editing can be canceled and
rebase can then be continued
$ hg init collapse-cancel-editor && cd collapse-cancel-editor
$ hg debugdrawdag <<'EOF'
> Z
> |
> | D
> | |
> | C
> | |
> Y B
> |/
> A
> EOF
$ HGEDITOR=false hg --config ui.interactive=1 rebase --collapse -b D -d Z
rebasing 1:112478962961 B "B"
rebasing 3:26805aba1e60 C "C"
rebasing 5:f585351a92f8 D tip "D"
transaction abort!
rollback completed
abort: edit failed: false exited with status 1
[250]
$ hg tglog
o 5: D
|
| o 4: Z
| |
o | 3: C
| |
| o 2: Y
| |
o | 1: B
|/
o 0: A
$ hg continue
rebasing 1:112478962961 B "B"
rebasing 3:26805aba1e60 C "C"
rebasing 5:f585351a92f8 D tip "D"
saved backup bundle to $TESTTMP/collapse-cancel-editor/.hg/strip-backup/112478962961-cb2a9b47-rebase.hg
$ hg tglog
o 3: Collapsed revision
| * B
| * C
| * D
o 2: Z
|
o 1: Y
|
o 0: A
$ cd ..