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bpo-24882: eagerly spawn threads in ThreadPoolExecutor #6530

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bpo-24882: eagerly spawn threads in ThreadPoolExecutor #6530

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41 changes: 19 additions & 22 deletions Lib/concurrent/futures/thread.py
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Original file line number Diff line number Diff line change
Expand Up @@ -142,6 +142,25 @@ def __init__(self, max_workers=None, thread_name_prefix='',
self._initializer = initializer
self._initargs = initargs

# eagerly create all worker threads

# When the executor gets destroyed, the weakref callback will wake up
# the worker threads.
def weakref_cb(_, q=self._work_queue):
q.put(None)
for thread_num in range(max_workers):
thread_name = '%s_%d' % (self._thread_name_prefix or self,
thread_num)
t = threading.Thread(name=thread_name, target=_worker,
args=(weakref.ref(self, weakref_cb),
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@methane methane Apr 19, 2018

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Can't the weakref shared among all threads?
If weakref_cb can call q.put(None) max_workers times. Or we can use __del__ method for it.

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@methane methane Apr 19, 2018

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Oh, there is no need to call q.put(None) max_workers times.
First worker received None calls q.put(None) for next worker.

cpython/Lib/concurrent/futures/thread.py

Lines 94 to 95 in 6952482

# Notice other workers
work_queue.put(None)

self._work_queue,
self._initializer,
self._initargs))
t.daemon = True
t.start()
self._threads.add(t)
_threads_queues[t] = self._work_queue

def submit(self, fn, *args, **kwargs):
with self._shutdown_lock:
if self._broken:
Expand All @@ -157,31 +176,9 @@ def submit(self, fn, *args, **kwargs):
w = _WorkItem(f, fn, args, kwargs)

self._work_queue.put(w)
self._adjust_thread_count()
return f
submit.__doc__ = _base.Executor.submit.__doc__

def _adjust_thread_count(self):
# When the executor gets lost, the weakref callback will wake up
# the worker threads.
def weakref_cb(_, q=self._work_queue):
q.put(None)
# TODO(bquinlan): Should avoid creating new threads if there are more
# idle threads than items in the work queue.
num_threads = len(self._threads)
if num_threads < self._max_workers:
thread_name = '%s_%d' % (self._thread_name_prefix or self,
num_threads)
t = threading.Thread(name=thread_name, target=_worker,
args=(weakref.ref(self, weakref_cb),
self._work_queue,
self._initializer,
self._initargs))
t.daemon = True
t.start()
self._threads.add(t)
_threads_queues[t] = self._work_queue

def _initializer_failed(self):
with self._shutdown_lock:
self._broken = ('A thread initializer failed, the thread pool '
Expand Down
3 changes: 2 additions & 1 deletion Lib/test/test_concurrent_futures.py
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Expand Up @@ -346,7 +346,7 @@ def test_threads_terminate(self):
self.executor.submit(mul, 21, 2)
self.executor.submit(mul, 6, 7)
self.executor.submit(mul, 3, 14)
self.assertEqual(len(self.executor._threads), 3)
self.assertEqual(len(self.executor._threads), self.worker_count)
self.executor.shutdown()
for t in self.executor._threads:
t.join()
Expand Down Expand Up @@ -742,6 +742,7 @@ def test_default_workers(self):
executor = self.executor_type()
self.assertEqual(executor._max_workers,
(os.cpu_count() or 1) * 5)
executor.shutdown()


class ProcessPoolExecutorTest(ExecutorTest):
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2018-04-18-17-23-46.bpo-24882.3JXIwK.rst
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@@ -0,0 +1,2 @@
Modify ThreadPoolExecutor to spawn worker threads on creation, instead of on
work submission.