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gh-64192: Make imap()/imap_unordered() in multiprocessing.pool actually lazy #136871

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Merged
encukou merged 35 commits into from
Jul 29, 2026
Merged

gh-64192: Make imap()/imap_unordered() in multiprocessing.pool actually lazy #136871

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ccd0bbc
draft: impl lazy input consumption in mp.Pool.imap(_unordered)
Jul 20, 2025
002ef46
Use semaphore to synchronize threads
Jul 20, 2025
6e0bc58
Update buffersize behavior to match concurrent.futures.Executor behavior
Jul 21, 2025
62b2b6a
Release all `buffersize_lock` obj from the parent thread when terminate
Jul 21, 2025
0b6ba41
Add 2 basic `ThreadPool.imap()` tests w/ and w/o buffersize
Jul 21, 2025
aade15e
Fix accidental swap in imports
Jul 21, 2025
fb38a72
clear Pool._taskqueue_buffersize_semaphores safely
Jul 21, 2025
6ef488b
Slightly optimize Pool._taskqueue_buffersize_semaphores terminate
Jul 21, 2025
1716725
Rename `Pool.imap()` buffersize-related tests
Jul 21, 2025
9b43cd0
Fix typo in `IMapIterator.__init__()`
Jul 22, 2025
2d89341
Add tests for buffersize combinations with other kwargs
Jul 22, 2025
9ab2705
Remove if-branch in `_terminate_pool`
Jul 27, 2025
a955003
Add more edge-case tests for `imap` and `imap_unodered`
Jul 27, 2025
80efd6e
Split inf iterable test for `imap` and `imap_unordered`
Jul 27, 2025
83d6930
Add doc for `buffersize` argument of `imap` and `imap_unordered`
Jul 27, 2025
995ad8c
add *versionadded* for `imap_unordered`
Jul 28, 2025
3b6ad65
Remove ambiguity in `buffersize` description.
Jul 28, 2025
c941c16
Set *versionadded* as next in docs
Jul 28, 2025
d09e891
Add whatsnew entry
Jul 28, 2025
9c6d89d
Fix aggreed comments on code formatting/minor refactoring
Jul 28, 2025
4550a01
Remove `imap` and `imap_unordered` body code duplication
Jul 28, 2025
77bde4d
Merge branch 'main' into feature/add-buffersize-to-multiprocessing
obaltian Aug 31, 2025
aec39fc
Merge branch 'main' into feature/add-buffersize-to-multiprocessing
obaltian Sep 3, 2025
a469f2d
Merge remote-tracking branch 'origin/main' into feature/add-buffersiz…
obaltian Jul 17, 2026
bcda6e7
Remove extra import
obaltian Jul 17, 2026
1441eaa
Update Lib/multiprocessing/pool.py
obaltian Jul 18, 2026
6d61bd1
Update Doc/library/multiprocessing.rst
obaltian Jul 18, 2026
0e43d9c
Apply suggestion from @encukou
obaltian Jul 18, 2026
4ed84bc
Make `buffersize` argument kwarg-only
Jul 18, 2026
f7aeb2e
Use set for `._taskqueue_buffersize_semaphores`
Jul 18, 2026
78b1f58
Make `buffersize` arg check inline
Jul 18, 2026
49b03d9
Simplify `.imap()` and `.imap_undered()` tests
Jul 18, 2026
455b748
Fix iterator instantiation in in `.imap()`
Jul 18, 2026
56eaf3f
Mark kw-only arg in `imap`/`imap_unordered` in doc
Jul 18, 2026
ea540bf
Merge branch 'main' into feature/add-buffersize-to-multiprocessing
obaltian Jul 18, 2026
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Use set for ._taskqueue_buffersize_semaphores
  • Loading branch information
Sasha Baltian
Sasha Baltian committed Jul 18, 2026
commit f7aeb2eeb68cabf235e337172478e4af151e3f6b
16 changes: 6 additions & 10 deletions Lib/multiprocessing/pool.py
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Original file line number Diff line number Diff line change
Expand Up @@ -192,9 +192,9 @@ def __init__(self, processes=None, initializer=None, initargs=(),
self._taskqueue = queue.SimpleQueue()
# The _taskqueue_buffersize_semaphores exist to allow calling .release()
# on every active semaphore when the pool is terminating to let task_handler
# wake up to stop. It's a dict so that each iterator object can efficiently
# wake up to stop. It's a set so that each iterator object can efficiently
# deregister its semaphore when iterator finishes.
self._taskqueue_buffersize_semaphores = {}
self._taskqueue_buffersize_semaphores = set()
# The _change_notifier queue exist to wake up self._handle_workers()
# when the cache (self._cache) is empty or when there is a change in
# the _state variable of the thread that runs _handle_workers.
Expand Down Expand Up @@ -712,11 +712,9 @@ def _terminate_pool(cls, taskqueue, inqueue, outqueue, pool, change_notifier,

task_handler._state = TERMINATE
# Release all semaphores to wake up task_handler to stop.
for job_id, buffersize_sema in tuple(
taskqueue_buffersize_semaphores.items()
):
for buffersize_sema in tuple(taskqueue_buffersize_semaphores):
buffersize_sema.release()
taskqueue_buffersize_semaphores.pop(job_id, None)
taskqueue_buffersize_semaphores.discard(buffersize_sema)

util.debug('helping task handler/workers to finish')
cls._help_stuff_finish(inqueue, task_handler, len(pool))
Expand Down Expand Up @@ -877,9 +875,7 @@ def __init__(self, pool, *, buffersize=None):
self._buffersize_sema = None
else:
self._buffersize_sema = threading.Semaphore(buffersize)
self._pool._taskqueue_buffersize_semaphores[self._job] = (
self._buffersize_sema
)
self._pool._taskqueue_buffersize_semaphores.add(self._buffersize_sema)

def __iter__(self):
return self
Expand Down Expand Up @@ -910,7 +906,7 @@ def next(self, timeout=None):
def _stop_iterator(self):
if self._pool is not None:
# `self._pool` could be set to `None` in previous `.next()` calls
self._pool._taskqueue_buffersize_semaphores.pop(self._job, None)
self._pool._taskqueue_buffersize_semaphores.discard(self._buffersize_sema)
self._pool = None
raise StopIteration from None

Expand Down