python-peps/pep-0445.txt

PEP: 445
Title: Add new APIs to customize memory allocators
Version: $Revision$
Last-Modified: $Date$
Author: Victor Stinner <victor.stinner@gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 15-june-2013
Python-Version: 3.4

Abstract
========

Add new APIs to customize memory allocators


Rationale
=========

Use cases:

* Application embedding Python wanting to use a custom memory allocator
  to allocate all Python memory somewhere else or with a different algorithm
* Python running on embedded devices with low memory and slow CPU.
  A custom memory allocator may be required to use efficiently the memory
  and/or to be able to use all memory of the device.
* Debug tool to track memory leaks
* Debug tool to detect buffer underflow, buffer overflow and misuse
  of Python allocator APIs
* Debug tool to inject bugs, simulate out of memory for example

API:

* Setup a custom memory allocator for all memory allocated by Python
* Hook memory allocator functions to call extra code before and/or after
  the underlying allocator function


Proposal
========

* Add a new ``PyMemAllocators`` structure

* Add new GIL-free memory allocator functions:

  - ``PyMem_RawMalloc()``
  - ``PyMem_RawRealloc()``
  - ``PyMem_RawFree()``

* Add new functions to get and set memory allocators:

  - ``PyMem_GetRawAllocators()``, ``PyMem_SetRawAllocators()``
  - ``PyMem_GetAllocators()``, ``PyMem_SetAllocators()``
  - ``PyObject_GetAllocators()``, ``PyObject_SetAllocators()``
  - ``_PyObject_GetArenaAllocators()``, ``_PyObject_SetArenaAllocators()``

* Add a new function to setup debug hooks after memory allocators were
  replaced:

  - ``PyMem_SetupDebugHooks()``

* ``PyObject_Malloc()`` now falls back on ``PyMem_Malloc()`` instead of
  ``malloc()`` if size is bigger than ``SMALL_REQUEST_THRESHOLD``, and
  ``PyObject_Realloc()`` falls back on ``PyMem_Realloc()`` instead of
  ``realloc()``

* ``PyMem_Malloc()`` and ``PyMem_Realloc()`` now always call ``malloc()`` and
  ``realloc()``, instead of calling ``PyObject_Malloc()`` and
  ``PyObject_Realloc()`` in debug mode


Performances
============

The Python benchmarks suite (-b 2n3): some tests are 1.04x faster, some tests
are 1.04 slower, significant is between 115 and -191. I don't understand these
output, but I guess that the overhead cannot be seen with such test.

pybench: "+0.1%" (diff between -4.9% and +5.6%).

Full output attached to the issue #3329.


Alternatives
============

Only one get and one set function
---------------------------------

Replace the 6 functions:

* ``PyMem_GetRawAllocators()``
* ``PyMem_GetAllocators()``
* ``PyObject_GetAllocators()``
* ``PyMem_SetRawAllocators(allocators)``
* ``PyMem_SetAllocators(allocators)``
* ``PyObject_SetAllocators(allocators)``

with 2 functions with an additional *domain* argument:

* ``PyMem_GetAllocators(domain)``
* ``PyMem_SetAllocators(domain, allocators)``


Setup Builtin Debug Hooks
-------------------------

To be able to use Python debug functions (like ``_PyMem_DebugMalloc()``) even
when a custom memory allocator is set, an environment variable
``PYDEBUGMALLOC`` can be added to set these debug function hooks, instead of
the new function ``PyMem_SetupDebugHooks()``.


Use macros to get customizable allocators
-----------------------------------------

To have no overhead in the default configuration, customizable allocators would
be an optional feature enabled by a configuration option or by macros.


Pass the C filename and line number
-----------------------------------

Use C macros using ``__FILE__`` and ``__LINE__`` to get the C filename
and line number of a memory allocation.


No context argument
-------------------

Simplify the signature of allocator functions, remove the context argument:

* ``void* malloc(size_t size)``
* ``void* realloc(void *ptr, size_t new_size)``
* ``void free(void *ptr)``

The context is a convinient way to reuse the same allocator for different APIs
(ex: PyMem and PyObject).


PyMem_Malloc() GIL-free
-----------------------

There is no real reason to require the GIL when calling PyMem_Malloc().


CCP API
-------

XXX To be done (Kristján Valur Jónsson) XXX


Links
=====

Memory allocators:

* `Issue #3329: Add new APIs to customize memory allocators
  <http://bugs.python.org/issue3329>`_
* `pytracemalloc
  <https://pypi.python.org/pypi/pytracemalloc>`_
* `Meliae: Python Memory Usage Analyzer
  <https://pypi.python.org/pypi/meliae>`_
* `Guppy-PE: umbrella package combining Heapy and GSL
  <http://guppy-pe.sourceforge.net/>`_
* `PySizer (developed for Python 2.4)
  <http://pysizer.8325.org/>`_

Other:

* `Python benchmark suite
  <http://hg.python.org/benchmarks>`_