view tests/test-update-dest.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

Test update.requiredest
  $ cd $TESTTMP
  $ cat >> $HGRCPATH <<EOF
  > [commands]
  > update.requiredest = True
  > EOF
  $ hg init repo
  $ cd repo
  $ echo a >> a
  $ hg commit -qAm aa
  $ hg up
  abort: you must specify a destination
  (for example: hg update ".::")
  [255]
  $ hg up .
  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ HGPLAIN=1 hg up
  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg --config commands.update.requiredest=False up
  0 files updated, 0 files merged, 0 files removed, 0 files unresolved

  $ cd ..

Check update.requiredest interaction with pull --update
  $ hg clone repo clone
  updating to branch default
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ cd repo
  $ echo a >> a
  $ hg commit -qAm aa
  $ cd ../clone
  $ hg pull --update
  abort: update destination required by configuration
  (use hg pull followed by hg update DEST)
  [255]

  $ cd ..

update.requiredest should silent the "hg update" text after pull
  $ hg init repo1
  $ cd repo1
  $ hg pull ../repo
  pulling from ../repo
  requesting all changes
  adding changesets
  adding manifests
  adding file changes
  added 2 changesets with 2 changes to 1 files
  new changesets 8f0162e483d0:048c2cb95949