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
#require bzr
$ . "$TESTDIR/bzr-definitions"
Work around https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=944379
$ mkdir -p "${HOME}/.config/breezy"
empty directory
$ mkdir test-empty
$ cd test-empty
$ brz init -q source
$ cd source
$ echo content > a
$ brz add -q a
$ brz commit -q -m 'Initial add'
$ mkdir empty
$ brz add -q empty
$ brz commit -q -m 'Empty directory added'
$ echo content > empty/something
$ brz add -q empty/something
$ brz commit -q -m 'Added file into directory'
$ cd ..
$ hg convert source source-hg
initializing destination source-hg repository
scanning source...
sorting...
converting...
2 Initial add
1 Empty directory added
0 Added file into directory
$ manifest source-hg 1
% manifest of 1
644 a
$ manifest source-hg tip
% manifest of tip
644 a
644 empty/something
$ cd ..
directory renames
$ mkdir test-dir-rename
$ cd test-dir-rename
$ brz init -q source
$ cd source
$ mkdir tpyo
$ echo content > tpyo/something
$ brz add -q tpyo
$ brz commit -q -m 'Added directory'
$ brz mv tpyo typo
tpyo => typo
$ brz commit -q -m 'Oops, typo'
$ cd ..
$ hg convert source source-hg
initializing destination source-hg repository
scanning source...
sorting...
converting...
1 Added directory
0 Oops, typo
$ manifest source-hg 0
% manifest of 0
644 tpyo/something
$ manifest source-hg tip
% manifest of tip
644 typo/something
$ cd ..
nested directory renames
$ mkdir test-nested-dir-rename
$ cd test-nested-dir-rename
$ brz init -q source
$ cd source
$ mkdir -p firstlevel/secondlevel/thirdlevel
$ echo content > firstlevel/secondlevel/file
$ echo this_needs_to_be_there_too > firstlevel/secondlevel/thirdlevel/stuff
$ brz add -q firstlevel
$ brz commit -q -m 'Added nested directories'
$ brz mv firstlevel/secondlevel secondlevel
firstlevel/secondlevel => secondlevel
$ brz commit -q -m 'Moved secondlevel one level up'
$ cd ..
$ hg convert source source-hg
initializing destination source-hg repository
scanning source...
sorting...
converting...
1 Added nested directories
0 Moved secondlevel one level up
$ manifest source-hg tip
% manifest of tip
644 secondlevel/file
644 secondlevel/thirdlevel/stuff
$ cd ..
directory remove
$ mkdir test-dir-remove
$ cd test-dir-remove
$ brz init -q source
$ cd source
$ mkdir src
$ echo content > src/sourcecode
$ brz add -q src
$ brz commit -q -m 'Added directory'
$ brz rm -q src
$ brz commit -q -m 'Removed directory'
$ cd ..
$ hg convert source source-hg
initializing destination source-hg repository
scanning source...
sorting...
converting...
1 Added directory
0 Removed directory
$ manifest source-hg 0
% manifest of 0
644 src/sourcecode
$ manifest source-hg tip
% manifest of tip
$ cd ..
directory replace
$ mkdir test-dir-replace
$ cd test-dir-replace
$ brz init -q source
$ cd source
$ mkdir first second
$ echo content > first/file
$ echo morecontent > first/dummy
$ echo othercontent > second/something
$ brz add -q first second
$ brz commit -q -m 'Initial layout'
$ brz mv first/file second/file
first/file => second/file
$ brz mv first third
first => third
$ brz commit -q -m 'Some conflicting moves'
$ cd ..
$ hg convert source source-hg
initializing destination source-hg repository
scanning source...
sorting...
converting...
1 Initial layout
0 Some conflicting moves
$ manifest source-hg tip
% manifest of tip
644 second/file
644 second/something
644 third/dummy
$ cd ..
divergent nested renames (issue3089)
$ mkdir test-divergent-renames
$ cd test-divergent-renames
$ brz init -q source
$ cd source
$ mkdir -p a/c
$ echo a > a/fa
$ echo c > a/c/fc
$ brz add -q a
$ brz commit -q -m 'Initial layout'
$ brz mv a b
a => b
$ mkdir a
$ brz add a
add(ed|ing) a (re)
$ brz mv b/c a/c
b/c => a/c
$ brz status
added:
a/
renamed:
a/? => b/? (re)
a/c/? => a/c/? (re)
$ brz commit -q -m 'Divergent renames'
$ cd ..
$ hg convert source source-hg
initializing destination source-hg repository
scanning source...
sorting...
converting...
1 Initial layout
0 Divergent renames
$ hg -R source-hg st -C --change 1
A b/fa
a/fa
R a/fa
$ hg -R source-hg manifest -r 1
a/c/fc
b/fa
$ cd ..