gh-109649: Use os.process_cpu_count() (#110165)

Replace os.cpu_count() with os.process_cpu_count() in modules:

* compileall
* concurrent.futures
* multiprocessing

Replace os.cpu_count() with os.process_cpu_count() in programs:

* _decimal deccheck.py test
* freeze.py
* multissltests.py
* python -m test (regrtest)
* wasm_build.py

Other changes:

* test.pythoninfo logs os.process_cpu_count().
* regrtest gets os.process_cpu_count() / os.cpu_count() in headers.

4 files changed, 25 insertions, 6 deletions

diff --git a/Doc/library/compileall.rst b/Doc/library/compileall.rst
index a7455ae..b4723b9 100644
--- a/Doc/library/compileall.rst
+++ b/Doc/library/compileall.rst
@@ -90,7 +90,7 @@ compile Python sources.
 .. cmdoption:: -j N
 
    Use *N* workers to compile the files within the given directory.
-   If ``0`` is used, then the result of :func:`os.cpu_count()`
+   If ``0`` is used, then the result of :func:`os.process_cpu_count()`
    will be used.
 
 .. cmdoption:: --invalidation-mode [timestamp|checked-hash|unchecked-hash]
diff --git a/Doc/library/concurrent.futures.rst b/Doc/library/concurrent.futures.rst
index 6503d1f..dca5145 100644
--- a/Doc/library/concurrent.futures.rst
+++ b/Doc/library/concurrent.futures.rst
@@ -188,6 +188,10 @@ And::
       ThreadPoolExecutor now reuses idle worker threads before starting
       *max_workers* worker threads too.
 
+   .. versionchanged:: 3.13
+      Default value of *max_workers* is changed to
+      ``min(32, (os.process_cpu_count() or 1) + 4)``.
+
 
 .. _threadpoolexecutor-example:
 
@@ -243,7 +247,7 @@ to a :class:`ProcessPoolExecutor` will result in deadlock.
 
    An :class:`Executor` subclass that executes calls asynchronously using a pool
    of at most *max_workers* processes.  If *max_workers* is ``None`` or not
-   given, it will default to the number of processors on the machine.
+   given, it will default to :func:`os.process_cpu_count`.
    If *max_workers* is less than or equal to ``0``, then a :exc:`ValueError`
    will be raised.
    On Windows, *max_workers* must be less than or equal to ``61``. If it is not
@@ -301,6 +305,10 @@ to a :class:`ProcessPoolExecutor` will result in deadlock.
       different start method. See the :func:`os.fork` documentation for
       further explanation.
 
+   .. versionchanged:: 3.13
+      *max_workers* uses :func:`os.process_cpu_count` by default, instead of
+      :func:`os.cpu_count`.
+
 .. _processpoolexecutor-example:
 
 ProcessPoolExecutor Example
diff --git a/Doc/library/multiprocessing.rst b/Doc/library/multiprocessing.rst
index 2f0f1f8..d19f911 100644
--- a/Doc/library/multiprocessing.rst
+++ b/Doc/library/multiprocessing.rst
@@ -996,13 +996,13 @@ Miscellaneous
 
    This number is not equivalent to the number of CPUs the current process can
    use.  The number of usable CPUs can be obtained with
-   ``len(os.sched_getaffinity(0))``
+   :func:`os.process_cpu_count`.
 
    When the number of CPUs cannot be determined a :exc:`NotImplementedError`
    is raised.
 
    .. seealso::
-      :func:`os.cpu_count`
+      :func:`os.cpu_count` and :func:`os.process_cpu_count`
 
 .. function:: current_process()
 
@@ -2214,7 +2214,7 @@ with the :class:`Pool` class.
    callbacks and has a parallel map implementation.
 
    *processes* is the number of worker processes to use.  If *processes* is
-   ``None`` then the number returned by :func:`os.cpu_count` is used.
+   ``None`` then the number returned by :func:`os.process_cpu_count` is used.
 
    If *initializer* is not ``None`` then each worker process will call
    ``initializer(*initargs)`` when it starts.
@@ -2249,6 +2249,10 @@ with the :class:`Pool` class.
    .. versionadded:: 3.4
       *context*
 
+   .. versionchanged:: 3.13
+      *processes* uses :func:`os.process_cpu_count` by default, instead of
+      :func:`os.cpu_count`.
+
    .. note::
 
       Worker processes within a :class:`Pool` typically live for the complete
@@ -2775,7 +2779,7 @@ worker threads rather than worker processes.
    :meth:`~multiprocessing.pool.Pool.terminate` manually.
 
    *processes* is the number of worker threads to use.  If *processes* is
-   ``None`` then the number returned by :func:`os.cpu_count` is used.
+   ``None`` then the number returned by :func:`os.process_cpu_count` is used.
 
    If *initializer* is not ``None`` then each worker process will call
    ``initializer(*initargs)`` when it starts.
diff --git a/Doc/whatsnew/3.13.rst b/Doc/whatsnew/3.13.rst
index 484443a..a789084 100644
--- a/Doc/whatsnew/3.13.rst
+++ b/Doc/whatsnew/3.13.rst
@@ -91,6 +91,13 @@ Other Language Changes
   of the ``optimize`` argument.
   (Contributed by Irit Katriel in :gh:`108113`).
 
+* :mod:`multiprocessing`, :mod:`concurrent.futures`, :mod:`compileall`:
+  Replace :func:`os.cpu_count` with :func:`os.process_cpu_count` to select the
+  default number of worker threads and processes. Get the CPU affinity
+  if supported.
+  (Contributed by Victor Stinner in :gh:`109649`.)
+
+
 New Modules
 ===========