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

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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
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Remove ambiguity in buffersize description. 
Previously it was a bit ambigious to say
"pass this to fully utilize pool's capacity" as the fastest
way would still be not passing `buffersize` at all.
Now this section clearly says "... when using this feature".
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@obaltian
Oleksandr Baltian authored and obaltian committed Aug 14, 2025
commit 3b6ad65b71a93da3a770f6a4c79921aa14457a5c
7 changes: 4 additions & 3 deletions Doc/library/multiprocessing.rst
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Expand Up @@ -2466,9 +2466,10 @@ with the :class:`Pool` class.
*buffersize* is specified to limit the number of submitted tasks whose
results have not yet been yielded. If the buffer is full, iteration over
the *iterables* pauses until a result is yielded from the buffer.
To fully utilize pool's capacity, set *buffersize* to the number of
processes in pool (to consume *iterable* as you go) or even higher
(to prefetch *buffersize - processes* arguments).
To fully utilize pool's capacity when using this feature,
set *buffersize* at least to the number of processes in pool
(to consume *iterable* as you go), or even higher
(to prefetch the next ``N=buffersize-processes`` arguments).

.. versionadded:: 3.15
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Added the *buffersize* parameter.
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7 changes: 4 additions & 3 deletions ...-07-28-00-37-22.gh-issue-64192.iCGeQ5.rst → ...-07-28-11-00-49.gh-issue-64192.7htLtg.rst
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Expand Up @@ -3,6 +3,7 @@ Add the optional ``buffersize`` parameter to
:meth:`multiprocessing.pool.Pool.imap_unordered` to limit the number of
submitted tasks whose results have not yet been yielded. If the buffer is
full, iteration over the *iterables* pauses until a result is yielded from
the buffer. To fully utilize pool's capacity, set *buffersize* to the number
of processes in pool (to consume *iterable* as you go) or even higher (to
prefetch *buffersize - processes* arguments).
the buffer. To fully utilize pool's capacity when using this feature, set
*buffersize* at least to the number of processes in pool (to consume
*iterable* as you go), or even higher (to prefetch the next
``N=buffersize-processes`` arguments).