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
$ . "$TESTDIR/histedit-helpers.sh"
$ cat >> $HGRCPATH <<EOF
> [extensions]
> histedit=
> EOF
$ initrepo ()
> {
> hg init r
> cd r
> for x in a b c d e f ; do
> echo $x > $x
> hg add $x
> hg ci -m $x
> done
> }
$ initrepo
log before edit
$ hg log --graph
@ changeset: 5:652413bf663e
| tag: tip
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: f
|
o changeset: 4:e860deea161a
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: e
|
o changeset: 3:055a42cdd887
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: d
|
o changeset: 2:177f92b77385
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: c
|
o changeset: 1:d2ae7f538514
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: b
|
o changeset: 0:cb9a9f314b8b
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: a
edit the history
$ hg histedit 177f92b77385 --commands - 2>&1 << EOF | fixbundle
> drop 177f92b77385 c
> pick e860deea161a e
> pick 652413bf663e f
> pick 055a42cdd887 d
> EOF
log after edit
$ hg log --graph
@ changeset: 4:f518305ce889
| tag: tip
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: d
|
o changeset: 3:a4f7421b80f7
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: f
|
o changeset: 2:ee283cb5f2d5
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: e
|
o changeset: 1:d2ae7f538514
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: b
|
o changeset: 0:cb9a9f314b8b
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: a
Check histedit_source
$ hg log --debug --rev f518305ce889
changeset: 4:f518305ce889c07cb5bd05522176d75590ef3324
tag: tip
phase: draft
parent: 3:a4f7421b80f79fcc59fff01bcbf4a53d127dd6d3
parent: -1:0000000000000000000000000000000000000000
manifest: 4:d3d4f51c157ff242c32ff745d4799aaa26ccda44
user: test
date: Thu Jan 01 00:00:00 1970 +0000
files+: d
extra: branch=default
extra: histedit_source=055a42cdd88768532f9cf79daa407fc8d138de9b
description:
d
manifest after edit
$ hg manifest
a
b
d
e
f
Drop the last changeset
$ hg histedit ee283cb5f2d5 --commands - 2>&1 << EOF | fixbundle
> pick ee283cb5f2d5 e
> pick a4f7421b80f7 f
> drop f518305ce889 d
> EOF
$ hg log --graph
@ changeset: 3:a4f7421b80f7
| tag: tip
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: f
|
o changeset: 2:ee283cb5f2d5
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: e
|
o changeset: 1:d2ae7f538514
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: b
|
o changeset: 0:cb9a9f314b8b
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: a
$ hg histedit cb9a9f314b8b --commands - 2>&1 << EOF | fixbundle
> pick cb9a9f314b8b a
> pick ee283cb5f2d5 e
> EOF
hg: parse error: missing rules for changeset a4f7421b80f7
(use "drop a4f7421b80f7" to discard, see also: 'hg help -e histedit.config')
$ hg --config histedit.dropmissing=True histedit cb9a9f314b8b --commands - 2>&1 << EOF | fixbundle
> EOF
hg: parse error: no rules provided
(use strip extension to remove commits)
$ hg --config histedit.dropmissing=True histedit cb9a9f314b8b --commands - 2>&1 << EOF | fixbundle
> pick cb9a9f314b8b a
> pick ee283cb5f2d5 e
> EOF
$ hg log --graph
@ changeset: 1:e99c679bf03e
| tag: tip
| user: test
| date: Thu Jan 01 00:00:00 1970 +0000
| summary: e
|
o changeset: 0:cb9a9f314b8b
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: a