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

mercurial/configitems.py

@@ -623,6 +623,9 coreconfigitem('experimental', 'web.api.
 coreconfigitem('experimental', 'web.api.debugreflect',
     default=False,
 )
+coreconfigitem('experimental', 'worker.wdir-get-thread-safe',
+    default=False,
+)
 coreconfigitem('experimental', 'xdiff',
     default=False,
 )

mercurial/merge.py

@@ -1637,9 +1637,12 def applyupdates(repo, actions, wctx, mc
             wctx[f0].remove()
         progress.increment(item=f)
 
-    # get in parallel
+    # get in parallel.
+    threadsafe = repo.ui.configbool('experimental',
+                                    'worker.wdir-get-thread-safe')
     prog = worker.worker(repo.ui, cost, batchget, (repo, mctx, wctx),
-                         actions[ACTION_GET])
+                         actions[ACTION_GET],
+                         threadsafe=threadsafe)
     for i, item in prog:
         progress.increment(step=i, item=item)
     updated = len(actions[ACTION_GET])