Mercurial > hg
view tests/test-revset-dirstate-parents.t @ 38732:be4984261611
merge: mark file gets as not thread safe (issue5933)
In default installs, this has the effect of disabling the thread-based
worker on Windows when manifesting files in the working directory. My
measurements have shown that with revlog-based repositories, Mercurial
spends a lot of CPU time in revlog code resolving file data. This ends
up incurring a lot of context switching across threads and slows down
`hg update` operations when going from an empty working directory to
the tip of the repo.
On mozilla-unified (246,351 files) on an i7-6700K (4+4 CPUs):
before: 487s wall
after: 360s wall (equivalent to worker.enabled=false)
cpus=2: 379s wall
Even with only 2 threads, the thread pool is still slower.
The introduction of the thread-based worker (02b36e860e0b) states that
it resulted in a "~50%" speedup for `hg sparse --enable-profile` and
`hg sparse --disable-profile`. This disagrees with my measurement
above. I theorize a few reasons for this:
1) Removal of files from the working directory is I/O - not CPU - bound
and should benefit from a thread pool (unless I/O is insanely fast
and the GIL release is near instantaneous). So tests like `hg sparse
--enable-profile` may exercise deletion throughput and aren't good
benchmarks for worker tasks that are CPU heavy.
2) The patch was authored by someone at Facebook. The results were
likely measured against a repository using remotefilelog. And I
believe that revision retrieval during working directory updates with
remotefilelog will often use a remote store, thus being I/O and not
CPU bound. This probably resulted in an overstated performance gain.
Since there appears to be a need to enable the thread-based worker with
some stores, I've made the flagging of file gets as thread safe
configurable. I've made it experimental because I don't want to formalize
a boolean flag for this option and because this attribute is best
captured against the store implementation. But we don't have a proper
store API for this yet. I'd rather cross this bridge later.
It is possible there are revlog-based repositories that do benefit from
a thread-based worker. I didn't do very comprehensive testing. If there
are, we may want to devise a more proper algorithm for whether to use
the thread-based worker, including possibly config options to limit the
number of threads to use. But until I see evidence that justifies
complexity, simplicity wins.
Differential Revision: https://phab.mercurial-scm.org/D3963
author | Gregory Szorc <gregory.szorc@gmail.com> |
---|---|
date | Wed, 18 Jul 2018 09:49:34 -0700 |
parents | 7bbc4e113e5f |
children |
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$ HGENCODING=utf-8 $ export HGENCODING $ try() { > hg debugrevspec --debug $@ > } $ log() { > hg log --template '{rev}\n' -r "$1" > } $ hg init repo $ cd repo $ try 'p1()' (func (symbol 'p1') None) * set: <baseset []> $ try 'p2()' (func (symbol 'p2') None) * set: <baseset []> $ try 'parents()' (func (symbol 'parents') None) * set: <baseset+ []> null revision $ log 'p1()' $ log 'p2()' $ log 'parents()' working dir with a single parent $ echo a > a $ hg ci -Aqm0 $ log 'p1()' 0 $ log 'tag() and p1()' $ log 'p2()' $ log 'parents()' 0 $ log 'tag() and parents()' merge in progress $ echo b > b $ hg ci -Aqm1 $ hg up -q 0 $ echo c > c $ hg ci -Aqm2 $ hg merge -q $ log 'p1()' 2 $ log 'p2()' 1 $ log 'tag() and p2()' $ log 'parents()' 1 2 $ cd ..