Mercurial > hg
view mercurial/rewriteutil.py @ 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 | 490df753894d |
children | 8c6329fa6038 |
line wrap: on
line source
# rewriteutil.py - utility functions for rewriting changesets # # Copyright 2017 Octobus <contact@octobus.net> # # This software may be used and distributed according to the terms of the # GNU General Public License version 2 or any later version. from __future__ import absolute_import from .i18n import _ from . import ( error, node, obsolete, revset, ) def precheck(repo, revs, action='rewrite'): """check if revs can be rewritten action is used to control the error message. Make sure this function is called after taking the lock. """ if node.nullrev in revs: msg = _("cannot %s null changeset") % (action) hint = _("no changeset checked out") raise error.Abort(msg, hint=hint) publicrevs = repo.revs('%ld and public()', revs) if len(repo[None].parents()) > 1: raise error.Abort(_("cannot %s while merging") % action) if publicrevs: msg = _("cannot %s public changesets") % (action) hint = _("see 'hg help phases' for details") raise error.Abort(msg, hint=hint) newunstable = disallowednewunstable(repo, revs) if newunstable: raise error.Abort(_("cannot %s changeset with children") % action) def disallowednewunstable(repo, revs): """Checks whether editing the revs will create new unstable changesets and are we allowed to create them. To allow new unstable changesets, set the config: `experimental.evolution.allowunstable=True` """ allowunstable = obsolete.isenabled(repo, obsolete.allowunstableopt) if allowunstable: return revset.baseset() return repo.revs("(%ld::) - %ld", revs, revs)