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gh-68663: Explanation of the difference between maxtasksperchild and chunksize for map method in multiprocessing.pool.Pool #110111

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gh-68663: Explanation of the difference between maxtasksperchild and chunksize for map method in multiprocessing.pool.Pool #110111

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Add explanation of chunksize for map method in multiprocessing.rst
Jamesgo1 Sep 29, 2023
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Merge branch 'main' of https://github.com/python/cpython
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10 changes: 10 additions & 0 deletions Doc/library/multiprocessing.rst
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Expand Up @@ -2293,6 +2293,16 @@ with the :class:`Pool` class.
the process pool as separate tasks. The (approximate) size of these
chunks can be specified by setting *chunksize* to a positive integer.

It is important to distinguish between *maxtasksperchild* in the
:class:`Pool` class and *chunksize*. Whereas *maxtasksperchild* denotes
how many tasks can be run at once before those resources are freed,
*chunksize* defines how many separate chunks in which to split all of the
iterations of the function. In order for each iteration of the function to
be executed separately, *chunksize* needs to be set to 1.

Whereas :meth:`imap` defaults to a *chunksize* of 1, :meth:`map` defaults
to ``None`` and needs to be declared explicitly.

Note that it may cause high memory usage for very long iterables. Consider
using :meth:`imap` or :meth:`imap_unordered` with explicit *chunksize*
option for better efficiency.
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