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
$ echo a > a
$ hg ci -qAm 'add a'
$ hg init subrepo
$ echo 'subrepo = http://example.net/libfoo' > .hgsub
$ hg ci -qAm 'added subrepo'
$ hg up -qC 0
$ echo ax > a
$ hg ci -m 'changed a'
created new head
$ hg up -qC 1
$ cd subrepo
$ echo b > b
$ hg add b
$ cd ..
Should fail, since there are added files to subrepo:
$ hg merge
abort: uncommitted changes in subrepository "subrepo"
[255]
Deleted files trigger a '+' marker in top level repos. Deleted files are also
noticed by `update --check` in the top level repo.
$ hg ci -Sqm 'add b'
$ echo change > subrepo/b
$ hg ci -Sm 'change b'
committing subrepository subrepo
$ rm a
$ hg id
9bfe45a197d7+ tip
$ hg sum
parent: 4:9bfe45a197d7 tip
change b
branch: default
commit: 1 deleted (clean)
update: 1 new changesets, 2 branch heads (merge)
phases: 5 draft
$ hg up --check -r '.^'
abort: uncommitted changes
[20]
$ hg st -S
! a
$ hg up -Cq .
Test that dirty is consistent through subrepos
$ rm subrepo/b
A deleted subrepo file is flagged as dirty, like the top level repo
$ hg id --config extensions.blackbox= --config blackbox.dirty=True \
> --config blackbox.track='command commandfinish'
9bfe45a197d7+ tip
$ cat .hg/blackbox.log
* @9bfe45a197d7b0ab09bf287729dd57e9619c9da5+ (*)> serve --no-profile --cmdserver chgunix * (glob) (chg !)
* @9bfe45a197d7b0ab09bf287729dd57e9619c9da5+ (*)> id --config *extensions.blackbox=* --config *blackbox.dirty=True* (glob)
* @9bfe45a197d7b0ab09bf287729dd57e9619c9da5+ (*)> id --config *extensions.blackbox=* --config *blackbox.dirty=True* exited 0 * (glob)
TODO: a deleted file should be listed as such, like the top level repo
$ hg sum
parent: 4:9bfe45a197d7 tip
change b
branch: default
commit: (clean)
update: 1 new changesets, 2 branch heads (merge)
phases: 5 draft
Modified subrepo files are noticed by `update --check` and `summary`
$ echo mod > subrepo/b
$ hg st -S
M subrepo/b
$ hg up -r '.^' --check
abort: uncommitted changes in subrepository "subrepo"
[255]
$ hg sum
parent: 4:9bfe45a197d7 tip
change b
branch: default
commit: 1 subrepos
update: 1 new changesets, 2 branch heads (merge)
phases: 5 draft
TODO: why is -R needed here? If it's because the subrepo is treated as a
discrete unit, then this should probably warn or something.
$ hg revert -R subrepo --no-backup subrepo/b -r .
$ rm subrepo/b
$ hg st -S
! subrepo/b
`hg update --check` notices a subrepo with a missing file, like it notices a
missing file in the top level repo.
$ hg up -r '.^' --check
abort: uncommitted changes in subrepository "subrepo"
[255]
$ hg up -r '.^' --config ui.interactive=True << EOF
> d
> EOF
file 'b' was deleted in local [working copy] but was modified in other [destination].
You can use (c)hanged version, leave (d)eleted, or leave (u)nresolved.
What do you want to do? d
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
XXX: There's a difference between wdir() and '.', so there should be a status.
`hg files -S` from the top is also missing 'subrepo/b'. The files should be
seen as deleted (and, maybe even missing? in which case `hg files` should list
it)
$ hg st -S
R subrepo/b (missing-correct-output !)
$ hg st -R subrepo
R subrepo/b (missing-correct-output !)
(note: return [1] because no files "match" since the list is empty)
$ hg files -R subrepo
[1]
$ hg files -R subrepo -r '.'
subrepo/b
$ hg bookmark -r tip @other
$ echo xyz > subrepo/c
$ hg ci -SAm 'add c'
adding subrepo/c
committing subrepository subrepo
created new head
$ rm subrepo/c
Merge sees deleted subrepo files as an uncommitted change
$ hg merge @other
abort: uncommitted changes in subrepository "subrepo"
[255]