Mercurial > hg
view tests/test-diff-reverse.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 | c586cb50872b |
children | 55c6ebd11cb9 |
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$ hg init $ cat > a <<EOF > a > b > c > EOF $ hg ci -Am adda adding a $ cat > a <<EOF > d > e > f > EOF $ hg ci -m moda $ hg diff --reverse -r0 -r1 diff -r 2855cdcfcbb7 -r 8e1805a3cf6e a --- a/a Thu Jan 01 00:00:00 1970 +0000 +++ b/a Thu Jan 01 00:00:00 1970 +0000 @@ -1,3 +1,3 @@ -d -e -f +a +b +c $ cat >> a <<EOF > g > h > EOF $ hg diff --reverse --nodates diff -r 2855cdcfcbb7 a --- a/a +++ b/a @@ -1,5 +1,3 @@ d e f -g -h should show removed file 'a' as being added $ hg revert a $ hg rm a $ hg diff --reverse --nodates a diff -r 2855cdcfcbb7 a --- /dev/null +++ b/a @@ -0,0 +1,3 @@ +d +e +f should show added file 'b' as being removed $ echo b >> b $ hg add b $ hg diff --reverse --nodates b diff -r 2855cdcfcbb7 b --- a/b +++ /dev/null @@ -1,1 +0,0 @@ -b