Mercurial > hg
view tests/test-no-symlinks.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 | 2d49d2eb1ff2 |
children | 0612e4c6fda0 |
line wrap: on
line source
#require no-symlink # The following script was used to create the bundle: # # hg init symlinks # cd symlinks # echo a > a # mkdir d # echo b > d/b # ln -s a a.lnk # ln -s d/b d/b.lnk # hg ci -Am t # hg bundle --base null ../test-no-symlinks.hg Extract a symlink on a platform not supporting them $ hg init t $ cd t $ hg pull -q "$TESTDIR/bundles/test-no-symlinks.hg" $ hg update 4 files updated, 0 files merged, 0 files removed, 0 files unresolved $ cat a.lnk && echo a $ cat d/b.lnk && echo d/b Copy a symlink and move another $ hg copy a.lnk d/a2.lnk $ hg mv d/b.lnk b2.lnk $ hg ci -Am copy $ cat d/a2.lnk && echo a $ cat b2.lnk && echo d/b Bundle and extract again $ hg bundle --base null ../symlinks.hg 2 changesets found $ cd .. $ hg init t2 $ cd t2 $ hg pull ../symlinks.hg pulling from ../symlinks.hg requesting all changes adding changesets adding manifests adding file changes added 2 changesets with 6 changes to 6 files new changesets d326ae2d01ee:71d85cf3ba90 (run 'hg update' to get a working copy) $ hg update 5 files updated, 0 files merged, 0 files removed, 0 files unresolved $ cat a.lnk && echo a $ cat d/a2.lnk && echo a $ cat b2.lnk && echo d/b