worker: ability to disable thread unsafe tasks

worker: ability to disable thread unsafe tasks

The worker on Windows is implemented using a thread pool. If worker
tasks are not thread safe, badness can occur. In addition, if tasks
are executing CPU bound code and holding onto the GIL, there will be
non-substantial overhead in Python context switching between active
threads. This can result in significant slowdowns of tasks.

This commit teaches the code for determining whether to use a worker
to take thread safety into account. Effectively, thread unsafe tasks
don't use the thread-based worker on Windows.

Differential Revision: https://phab.mercurial-scm.org/D3962

--- a/mercurial/worker.py	Tue Jul 17 16:57:27 2018 -0700
+++ b/mercurial/worker.py	Wed Jul 18 09:46:45 2018 -0700
@@ -63,18 +63,27 @@
 
 if pycompat.isposix or pycompat.iswindows:
     _STARTUP_COST = 0.01
+    # The Windows worker is thread based. If tasks are CPU bound, threads
+    # in the presence of the GIL result in excessive context switching and
+    # this overhead can slow down execution.
+    _DISALLOW_THREAD_UNSAFE = pycompat.iswindows
 else:
     _STARTUP_COST = 1e30
+    _DISALLOW_THREAD_UNSAFE = False
 
-def worthwhile(ui, costperop, nops):
+def worthwhile(ui, costperop, nops, threadsafe=True):
     '''try to determine whether the benefit of multiple processes can
     outweigh the cost of starting them'''
+
+    if not threadsafe and _DISALLOW_THREAD_UNSAFE:
+        return False
+
     linear = costperop * nops
     workers = _numworkers(ui)
     benefit = linear - (_STARTUP_COST * workers + linear / workers)
     return benefit >= 0.15
 
-def worker(ui, costperarg, func, staticargs, args):
+def worker(ui, costperarg, func, staticargs, args, threadsafe=True):
     '''run a function, possibly in parallel in multiple worker
     processes.
 
@@ -88,9 +97,13 @@
 
     args - arguments to split into chunks, to pass to individual
     workers
+
+    threadsafe - whether work items are thread safe and can be executed using
+    a thread-based worker. Should be disabled for CPU heavy tasks that don't
+    release the GIL.
     '''
     enabled = ui.configbool('worker', 'enabled')
-    if enabled and worthwhile(ui, costperarg, len(args)):
+    if enabled and worthwhile(ui, costperarg, len(args), threadsafe=threadsafe):
         return _platformworker(ui, func, staticargs, args)
     return func(*staticargs + (args,))
 

--- a/tests/test-simple-update.t	Tue Jul 17 16:57:27 2018 -0700
+++ b/tests/test-simple-update.t	Wed Jul 18 09:46:45 2018 -0700
@@ -65,7 +65,7 @@
 
   $ cat <<EOF > forceworker.py
   > from mercurial import extensions, worker
-  > def nocost(orig, ui, costperop, nops):
+  > def nocost(orig, ui, costperop, nops, threadsafe=True):
   >     return worker._numworkers(ui) > 1
   > def uisetup(ui):
   >     extensions.wrapfunction(worker, 'worthwhile', nocost)