indygreg created this revision.
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REVISION SUMMARY
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
Even with the worker enabled with numcpus=2, I saw a slowdown (19s
slower than no worker threads).
The introduction of the thread-based worker
(https://phab.mercurial-scm.org/rHG02b36e860e0b893928d5f565417d55b5dd6495fc)
states that
it resulted in a "~50%" speedup for `hg sparse --enable-profile` and
`hg sparse --disable-profile`. 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`
which don't realize new files are measuring 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 CPU heavy
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.
REPOSITORY
rHG Mercurial
REVISION DETAIL
https://phab.mercurial-scm.org/D3963
AFFECTED FILES
mercurial/configitems.py
mercurial/merge.py
CHANGE DETAILS
diff --git a/mercurial/merge.py b/mercurial/merge.py
--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -1637,9 +1637,11 @@
wctx[f0].remove()
progress.increment(item=f)
-# get in parallel
+# get in parallel.
+cpuheavy = repo.ui.configbool('experimental', 'worker.wdir-get-cpu-heavy')
prog = worker.worker(repo.ui, cost, batchget, (repo, mctx, wctx),
- actions[ACTION_GET])
+ actions[ACTION_GET],
+ cpuheavy=cpuheavy)
for i, item in prog:
progress.increment(step=i, item=item)
updated = len(actions[ACTION_GET])
diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -623,6 +623,9 @@
coreconfigitem('experimental', 'web.api.debugreflect',
default=False,
)
+coreconfigitem('experimental', 'worker.wdir-get-cpu-heavy',
+default=True,
+)
coreconfigitem('experimental', 'xdiff',
default=False,
)
To: indygreg, #hg-reviewers
Cc: mercurial-devel
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