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
$ cat >> $HGRCPATH<<EOF
> [extensions]
> rebase=
> drawdag=$TESTDIR/drawdag.py
> EOF
$ hg init non-merge
$ cd non-merge
$ hg debugdrawdag<<'EOS'
> F
> |
> E
> |
> D
> |
> B C
> |/
> A
> EOS
$ for i in C D E F; do
> hg bookmark -r $i -i BOOK-$i
> done
$ hg debugdrawdag<<'EOS'
> E
> |
> D
> |
> B
> EOS
$ hg log -G -T '{rev} {desc} {bookmarks}'
o 7 E
|
o 6 D
|
| o 5 F BOOK-F
| |
| o 4 E BOOK-E
| |
| o 3 D BOOK-D
| |
| o 2 C BOOK-C
| |
o | 1 B
|/
o 0 A
With --keep, bookmark should move
$ hg rebase -r 3+4 -d E --keep
rebasing 3:e7b3f00ed42e BOOK-D "D"
note: not rebasing 3:e7b3f00ed42e BOOK-D "D", its destination already has all its changes
rebasing 4:69a34c08022a BOOK-E "E"
note: not rebasing 4:69a34c08022a BOOK-E "E", its destination already has all its changes
$ hg log -G -T '{rev} {desc} {bookmarks}'
o 7 E BOOK-D BOOK-E
|
o 6 D
|
| o 5 F BOOK-F
| |
| o 4 E
| |
| o 3 D
| |
| o 2 C BOOK-C
| |
o | 1 B
|/
o 0 A
Move D and E back for the next test
$ hg bookmark BOOK-D -fqir 3
$ hg bookmark BOOK-E -fqir 4
Bookmark is usually an indication of a head. For changes that are introduced by
an ancestor of bookmark B, after moving B to B-NEW, the changes are ideally
still introduced by an ancestor of changeset on B-NEW. In the below case,
"BOOK-D", and "BOOK-E" include changes introduced by "C".
$ hg rebase -s 2 -d E
rebasing 2:dc0947a82db8 BOOK-C C "C"
rebasing 3:e7b3f00ed42e BOOK-D "D"
note: not rebasing 3:e7b3f00ed42e BOOK-D "D", its destination already has all its changes
rebasing 4:69a34c08022a BOOK-E "E"
note: not rebasing 4:69a34c08022a BOOK-E "E", its destination already has all its changes
rebasing 5:6b2aeab91270 BOOK-F F "F"
saved backup bundle to $TESTTMP/non-merge/.hg/strip-backup/dc0947a82db8-52bb4973-rebase.hg
$ hg log -G -T '{rev} {desc} {bookmarks}'
o 5 F BOOK-F
|
o 4 C BOOK-C BOOK-D BOOK-E
|
o 3 E
|
o 2 D
|
o 1 B
|
o 0 A
Merge and its ancestors all become empty
$ hg init $TESTTMP/merge1
$ cd $TESTTMP/merge1
$ hg debugdrawdag<<'EOS'
> E
> /|
> B C D
> \|/
> A
> EOS
$ for i in C D E; do
> hg bookmark -r $i -i BOOK-$i
> done
$ hg debugdrawdag<<'EOS'
> H
> |
> D
> |
> C
> |
> B
> EOS
Previously, there was a bug where the empty commit check compared the parent
branch name with the wdir branch name instead of the actual branch name (which
should stay unchanged if --keepbranches is passed), and erroneously assumed
that an otherwise empty changeset should be created because of the incorrectly
assumed branch name change.
$ hg update H -q
$ hg branch foo -q
$ hg rebase -r '(A::)-(B::)-A' -d H --keepbranches
rebasing 2:dc0947a82db8 BOOK-C "C"
note: not rebasing 2:dc0947a82db8 BOOK-C "C", its destination already has all its changes
rebasing 3:b18e25de2cf5 BOOK-D "D"
note: not rebasing 3:b18e25de2cf5 BOOK-D "D", its destination already has all its changes
rebasing 4:86a1f6686812 BOOK-E E "E"
note: not rebasing 4:86a1f6686812 BOOK-E E "E", its destination already has all its changes
saved backup bundle to $TESTTMP/merge1/.hg/strip-backup/b18e25de2cf5-1fd0a4ba-rebase.hg
$ hg update null -q
$ hg log -G -T '{rev} {desc} {bookmarks}'
o 4 H BOOK-C BOOK-D BOOK-E
|
o 3 D
|
o 2 C
|
o 1 B
|
o 0 A
Part of ancestors of a merge become empty
$ hg init $TESTTMP/merge2
$ cd $TESTTMP/merge2
$ hg debugdrawdag<<'EOS'
> G
> /|
> E F
> | |
> B C D
> \|/
> A
> EOS
$ for i in C D E F G; do
> hg bookmark -r $i -i BOOK-$i
> done
$ hg debugdrawdag<<'EOS'
> H
> |
> F
> |
> C
> |
> B
> EOS
$ hg rebase -r '(A::)-(B::)-A' -d H
rebasing 2:dc0947a82db8 BOOK-C "C"
note: not rebasing 2:dc0947a82db8 BOOK-C "C", its destination already has all its changes
rebasing 3:b18e25de2cf5 BOOK-D D "D"
rebasing 4:03ca77807e91 BOOK-E E "E"
rebasing 5:ad6717a6a58e BOOK-F "F"
note: not rebasing 5:ad6717a6a58e BOOK-F "F", its destination already has all its changes
rebasing 6:c58e8bdac1f4 BOOK-G G "G"
saved backup bundle to $TESTTMP/merge2/.hg/strip-backup/b18e25de2cf5-2d487005-rebase.hg
$ hg log -G -T '{rev} {desc} {bookmarks}'
o 7 G BOOK-G
|\
| o 6 E BOOK-E
| |
o | 5 D BOOK-D BOOK-F
|/
o 4 H BOOK-C
|
o 3 F
|
o 2 C
|
o 1 B
|
o 0 A