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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 | c29efd272395 |
children |
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[This file is here for historical purposes, all recent contributors should appear in the changelog directly] Andrea Arcangeli <andrea at suse.de> Thomas Arendsen Hein <thomas at intevation.de> Goffredo Baroncelli <kreijack at libero.it> Muli Ben-Yehuda <mulix at mulix.org> Mikael Berthe <mikael at lilotux.net> Benoit Boissinot <bboissin at gmail.com> Brendan Cully <brendan at kublai.com> Vincent Danjean <vdanjean.ml at free.fr> Jake Edge <jake at edge2.net> Michael Fetterman <michael.fetterman at intel.com> Edouard Gomez <ed.gomez at free.fr> Eric Hopper <hopper at omnifarious.org> Alecs King <alecsk at gmail.com> Volker Kleinfeld <Volker.Kleinfeld at gmx.de> Vadim Lebedev <vadim at mbdsys.com> Christopher Li <hg at chrisli.org> Chris Mason <mason at suse.com> Colin McMillen <mcmillen at cs.cmu.edu> Wojciech Milkowski <wmilkowski at interia.pl> Chad Netzer <chad.netzer at gmail.com> Bryan O'Sullivan <bos at serpentine.com> Vicent SeguĂ Pascual <vseguip at gmail.com> Sean Perry <shaleh at speakeasy.net> Nguyen Anh Quynh <aquynh at gmail.com> Ollivier Robert <roberto at keltia.freenix.fr> Alexander Schremmer <alex at alexanderweb.de> Arun Sharma <arun at sharma-home.net> Josef "Jeff" Sipek <jeffpc at optonline.net> Kevin Smith <yarcs at qualitycode.com> TK Soh <teekaysoh at yahoo.com> Radoslaw Szkodzinski <astralstorm at gorzow.mm.pl> Samuel Tardieu <sam at rfc1149.net> K Thananchayan <thananck at yahoo.com> Andrew Thompson <andrewkt at aktzero.com> Michael S. Tsirkin <mst at mellanox.co.il> Rafael Villar Burke <pachi at mmn-arquitectos.com> Tristan Wibberley <tristan at wibberley.org> Mark Williamson <mark.williamson at cl.cam.ac.uk>