view tests/test-rebase-issue-noparam-single-rev.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

  $ cat >> $HGRCPATH <<EOF
  > [extensions]
  > rebase=
  > 
  > [phases]
  > publish=False
  > 
  > [alias]
  > tglog = log -G --template "{rev}: {node|short} '{desc}' {branches}\n"
  > EOF


  $ hg init a
  $ cd a

  $ echo c1 > c1
  $ hg ci -Am c1
  adding c1

  $ echo c2 > c2
  $ hg ci -Am c2
  adding c2

  $ echo l1 > l1
  $ hg ci -Am l1
  adding l1

  $ hg up -q -C 1

  $ echo r1 > r1
  $ hg ci -Am r1
  adding r1
  created new head

  $ echo r2 > r2
  $ hg ci -Am r2
  adding r2

  $ hg tglog
  @  4: 225af64d03e6 'r2'
  |
  o  3: 8d0a8c99b309 'r1'
  |
  | o  2: 87c180a611f2 'l1'
  |/
  o  1: 56daeba07f4b 'c2'
  |
  o  0: e8faad3d03ff 'c1'
  
Rebase with no arguments - single revision in source branch:

  $ hg up -q -C 2

  $ hg rebase
  rebasing 2:87c180a611f2 "l1"
  saved backup bundle to $TESTTMP/a/.hg/strip-backup/87c180a611f2-a5be192d-rebase.hg

  $ hg tglog
  @  4: b1152cc99655 'l1'
  |
  o  3: 225af64d03e6 'r2'
  |
  o  2: 8d0a8c99b309 'r1'
  |
  o  1: 56daeba07f4b 'c2'
  |
  o  0: e8faad3d03ff 'c1'
  
  $ cd ..


  $ hg init b
  $ cd b

  $ echo c1 > c1
  $ hg ci -Am c1
  adding c1

  $ echo c2 > c2
  $ hg ci -Am c2
  adding c2

  $ echo l1 > l1
  $ hg ci -Am l1
  adding l1

  $ echo l2 > l2
  $ hg ci -Am l2
  adding l2

  $ hg up -q -C 1

  $ echo r1 > r1
  $ hg ci -Am r1
  adding r1
  created new head

  $ hg tglog
  @  4: 8d0a8c99b309 'r1'
  |
  | o  3: 1ac923b736ef 'l2'
  | |
  | o  2: 87c180a611f2 'l1'
  |/
  o  1: 56daeba07f4b 'c2'
  |
  o  0: e8faad3d03ff 'c1'
  
Rebase with no arguments - single revision in target branch:

  $ hg up -q -C 3

  $ hg rebase
  rebasing 2:87c180a611f2 "l1"
  rebasing 3:1ac923b736ef "l2"
  saved backup bundle to $TESTTMP/b/.hg/strip-backup/87c180a611f2-b980535c-rebase.hg

  $ hg tglog
  @  4: 023181307ed0 'l2'
  |
  o  3: 913ab52b43b4 'l1'
  |
  o  2: 8d0a8c99b309 'r1'
  |
  o  1: 56daeba07f4b 'c2'
  |
  o  0: e8faad3d03ff 'c1'
  

  $ cd ..