gh-142518: Improve mimalloc allocator docs (#145224)

lysnikolaou · web-flow · commit 7a1da4575b1d · 2026-03-11T15:14:47.000+01:00
diff --git a/Doc/c-api/memory.rst b/Doc/c-api/memory.rst
@@ -204,8 +204,11 @@ The following function sets, modeled after the ANSI C standard, but specifying
 behavior when requesting zero bytes, are available for allocating and releasing
 memory from the Python heap.
 
-The :ref:`default memory allocator <default-memory-allocators>` uses the
-:ref:`pymalloc memory allocator <pymalloc>`.
+In the GIL-enabled build (default build) the
+:ref:`default memory allocator <default-memory-allocators>` uses the
+:ref:`pymalloc memory allocator <pymalloc>`, whereas in the
+:term:`free-threaded build`, the default is the
+:ref:`mimalloc memory allocator <mimalloc>` instead.
 
 .. warning::
 
@@ -215,6 +218,11 @@ The :ref:`default memory allocator <default-memory-allocators>` uses the
 
    The default allocator is now pymalloc instead of system :c:func:`malloc`.
 
+.. versionchanged:: 3.13
+
+   In the :term:`free-threaded <free threading>` build, the default allocator
+   is now :ref:`mimalloc <mimalloc>`.
+
 .. c:function:: void* PyMem_Malloc(size_t n)
 
    Allocates *n* bytes and returns a pointer of type :c:expr:`void*` to the
@@ -340,7 +348,9 @@ memory from the Python heap.
     the :ref:`Customize Memory Allocators <customize-memory-allocators>` section.
 
 The :ref:`default object allocator <default-memory-allocators>` uses the
-:ref:`pymalloc memory allocator <pymalloc>`.
+:ref:`pymalloc memory allocator <pymalloc>`.  In the
+:term:`free-threaded <free threading>` build, the default is the
+:ref:`mimalloc memory allocator <mimalloc>` instead.
 
 .. warning::
 
@@ -420,23 +430,24 @@ Default Memory Allocators
 
 Default memory allocators:
 
-===============================  ====================  ==================  =====================  ====================
-Configuration                    Name                  PyMem_RawMalloc     PyMem_Malloc           PyObject_Malloc
-===============================  ====================  ==================  =====================  ====================
-Release build                    ``"pymalloc"``        ``malloc``          ``pymalloc``           ``pymalloc``
-Debug build                      ``"pymalloc_debug"``  ``malloc`` + debug  ``pymalloc`` + debug   ``pymalloc`` + debug
-Release build, without pymalloc  ``"malloc"``          ``malloc``          ``malloc``             ``malloc``
-Debug build, without pymalloc    ``"malloc_debug"``    ``malloc`` + debug  ``malloc`` + debug     ``malloc`` + debug
-===============================  ====================  ==================  =====================  ====================
+===================================  =======================  ====================  ======================  ======================
+Configuration                        Name                     PyMem_RawMalloc       PyMem_Malloc            PyObject_Malloc
+===================================  =======================  ====================  ======================  ======================
+Release build                        ``"pymalloc"``           ``malloc``            ``pymalloc``            ``pymalloc``
+Debug build                          ``"pymalloc_debug"``     ``malloc`` + debug    ``pymalloc`` + debug    ``pymalloc`` + debug
+Release build, without pymalloc      ``"malloc"``             ``malloc``            ``malloc``              ``malloc``
+Debug build, without pymalloc        ``"malloc_debug"``       ``malloc`` + debug    ``malloc`` + debug      ``malloc`` + debug
+Free-threaded build                  ``"mimalloc"``           ``mimalloc``          ``mimalloc``            ``mimalloc``
+Free-threaded debug build            ``"mimalloc_debug"``     ``mimalloc`` + debug  ``mimalloc`` + debug    ``mimalloc`` + debug
+===================================  =======================  ====================  ======================  ======================
 
 Legend:
 
 * Name: value for :envvar:`PYTHONMALLOC` environment variable.
 * ``malloc``: system allocators from the standard C library, C functions:
   :c:func:`malloc`, :c:func:`calloc`, :c:func:`realloc` and :c:func:`free`.
 * ``pymalloc``: :ref:`pymalloc memory allocator <pymalloc>`.
-* ``mimalloc``: :ref:`mimalloc memory allocator <mimalloc>`.  The pymalloc
-  allocator will be used if mimalloc support isn't available.
+* ``mimalloc``: :ref:`mimalloc memory allocator <mimalloc>`.
 * "+ debug": with :ref:`debug hooks on the Python memory allocators
   <pymem-debug-hooks>`.
 * "Debug build": :ref:`Python build in debug mode <debug-build>`.
@@ -733,9 +744,27 @@ The mimalloc allocator
 
 .. versionadded:: 3.13
 
-Python supports the mimalloc allocator when the underlying platform support is available.
-mimalloc "is a general purpose allocator with excellent performance characteristics.
-Initially developed by Daan Leijen for the runtime systems of the Koka and Lean languages."
+Python supports the `mimalloc <https://github.com/microsoft/mimalloc/>`__
+allocator when the underlying platform support is available.
+mimalloc is a general purpose allocator with excellent performance
+characteristics, initially developed by Daan Leijen for the runtime systems
+of the Koka and Lean languages.
+
+Unlike :ref:`pymalloc <pymalloc>`, which is optimized for small objects (512
+bytes or fewer), mimalloc handles allocations of any size.
+
+In the :term:`free-threaded <free threading>` build, mimalloc is the default
+and **required** allocator for the :c:macro:`PYMEM_DOMAIN_MEM` and
+:c:macro:`PYMEM_DOMAIN_OBJ` domains.  It cannot be disabled in free-threaded
+builds.  The free-threaded build uses per-thread mimalloc heaps, which allows
+allocation and deallocation to proceed without locking in most cases.
+
+In the default (non-free-threaded) build, mimalloc is available but not the
+default allocator.  It can be selected at runtime using
+:envvar:`PYTHONMALLOC`\ ``=mimalloc`` (or ``mimalloc_debug`` to include
+:ref:`debug hooks <pymem-debug-hooks>`).  It can be disabled at build time
+using the :option:`--without-mimalloc` configure option, but this option
+cannot be combined with :option:`--disable-gil`.
 
 tracemalloc C API
 =================
diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
@@ -1085,6 +1085,13 @@ conflict.
    * ``pymalloc_debug``: same as ``pymalloc`` but also install debug hooks.
    * ``mimalloc_debug``: same as ``mimalloc`` but also install debug hooks.
 
+   .. note::
+
+      In the :term:`free-threaded <free threading>` build, the ``malloc``,
+      ``malloc_debug``, ``pymalloc``, and ``pymalloc_debug`` values are not
+      supported.  Only ``default``, ``debug``, ``mimalloc``, and
+      ``mimalloc_debug`` are accepted.
+
    .. versionadded:: 3.6
 
    .. versionchanged:: 3.7
@@ -1094,12 +1101,13 @@ conflict.
 .. envvar:: PYTHONMALLOCSTATS
 
    If set to a non-empty string, Python will print statistics of the
-   :ref:`pymalloc memory allocator <pymalloc>` every time a new pymalloc object
-   arena is created, and on shutdown.
+   :ref:`pymalloc memory allocator <pymalloc>` or the
+   :ref:`mimalloc memory allocator <mimalloc>` (whichever is in use)
+   every time a new object arena is created, and on shutdown.
 
    This variable is ignored if the :envvar:`PYTHONMALLOC` environment variable
    is used to force the :c:func:`malloc` allocator of the C library, or if
-   Python is configured without ``pymalloc`` support.
+   Python is configured without both ``pymalloc`` and ``mimalloc`` support.
 
    .. versionchanged:: 3.6
       This variable can now also be used on Python compiled in release mode.
diff --git a/Doc/using/configure.rst b/Doc/using/configure.rst
@@ -774,6 +774,9 @@ also be used to improve performance.
    Disable the fast :ref:`mimalloc <mimalloc>` allocator
    (enabled by default).
 
+   This option cannot be used together with :option:`--disable-gil`
+   because the :term:`free-threaded <free threading>` build requires mimalloc.
+
    See also :envvar:`PYTHONMALLOC` environment variable.
 
 .. option:: --without-pymalloc
