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
============================================
Testing obsolescence markers push: Cases B.6
============================================
Mercurial pushes obsolescences markers relevant to the "pushed-set", the set of
all changesets that requested to be "in sync" after the push (even if they are
already on both side).
This test belongs to a series of tests checking such set is properly computed
and applied. This does not tests "obsmarkers" discovery capabilities.
Category B: pruning case
TestCase 6: Pruned changeset with precursors not in pushed set
B.6 Pruned changeset with precursors not in pushed set
======================================================
.. {{{
.. B ø⇠⊗ B'
.. | |
.. A ○ |
.. |/
.. ● O
.. }}}
..
.. Marker exist from:
..
.. * `B ø⇠⊗ B'`
.. * B' prune
..
.. Command run:
..
.. * hg push -r O
..
.. Expected exchange:
..
.. * `B ø⇠⊗ B'`
.. * B' prune
Setup
-----
$ . $TESTDIR/testlib/exchange-obsmarker-util.sh
Initial
$ setuprepos B.6
creating test repo for test case B.6
- pulldest
- main
- pushdest
cd into `main` and proceed with env setup
$ cd main
$ mkcommit A
$ mkcommit B0
$ hg up --quiet 0
$ mkcommit B1
created new head
$ hg debugobsolete `getid 'desc(B0)'` `getid 'desc(B1)'`
1 new obsolescence markers
obsoleted 1 changesets
$ hg prune -qd '0 0' .
$ hg log -G --hidden
x f6298a8ac3a4 (draft): B1
|
| x 962ecf6b1afc (draft): B0
| |
| o f5bc6836db60 (draft): A
|/
@ a9bdc8b26820 (public): O
$ inspect_obsmarkers
obsstore content
================
962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
$ cd ..
$ cd ..
Actual Test
-------------------------------------
$ dotest B.6 O
## Running testcase B.6
# testing echange of "O" (a9bdc8b26820)
## initial state
# obstore: main
962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
# obstore: pushdest
# obstore: pulldest
## pushing "O" from main to pushdest
pushing to pushdest
searching for changes
no changes found
remote: 2 new obsolescence markers
## post push state
# obstore: main
962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
# obstore: pushdest
962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
# obstore: pulldest
## pulling "a9bdc8b26820" from main into pulldest
pulling from main
no changes found
2 new obsolescence markers
## post pull state
# obstore: main
962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
# obstore: pushdest
962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
# obstore: pulldest
962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}