[Python-Dev] cpython: Issue #3329: Add new APIs to customize memory allocators

[Victor Stinner]

2013/6/15 Antoine Pitrou <solipsis at pitrou.net>:
> On Sat, 15 Jun 2013 03:54:50 +0200
> Victor Stinner <victor.stinner at gmail.com> wrote:
>> The addition of PyMem_RawMalloc() is motivated by the issue #18203
>> (Replace calls to malloc() with PyMem_Malloc()). The goal is to be
>> able to setup a custom allocator for *all* allocation made by Python,
>> so malloc() should not be called directly. PyMem_RawMalloc() is
>> required in places where the GIL is not held (ex: in os.getcwd() on
>> Windows).
>
> We already had this discussion on IRC and this argument isn't very
> convincing to me. If os.getcwd() doesn't hold the GIL while allocating
> memory, then you should fix it to hold the GIL while allocating memory.

The GIL is released for best performances, holding the GIL would have
an impact on performances.

PyMem_RawMalloc() is needed when PyMem_Malloc() cannot be used because
the GIL was released. For example, for the issue #18227 (reuse the
custom allocator in external libraries), PyMem_Malloc() is usually not
appropriate. PyMem_RawMalloc() should also be used instead of
PyMem_Malloc() in the Python startup sequence, because PyMem_Malloc()
requires the GIL whereas the GIL does not exist yet.

PyMem_RawMalloc() also provides more accurate memory usage if it can
be replaced or hooked (with PyMem_SetRawAllocators).

The issue #18203 explains why I would like to replace direct call to
malloc() with PyMem_Malloc() or PyMem_RawMalloc().

> I don't like the idea of adding of third layer of allocation APIs. The
> dichotomy between PyObject_Malloc and PyMem_Malloc is already a bit
> gratuitous (i.e. not motivated by any actual real-world concern, as
> far as I can tell).

In Python 3.3, PyMem_Malloc() cannot be used instead of malloc() where
the GIL is not held. Instead of adding PyMem_RawMalloc(), an
alternative is to remove the "the GIL must be held" restriction from
PyMem_Malloc() by changing PyMem_Malloc() to make it always call
malloc() (instead of PyObject_Malloc() in debug mode).

With such change, a debug hook cannot rely on the GIL anymore: it
cannot inspect Python objects, get a frame or traceback, etc. To still
get accurate debug report, PyMem_Malloc() should be replaced with
PyObject_Malloc().

I don't understand yet the effect of such change on backport
compatibility. May it break applications?

> As for the debug functions you added: PyMem_GetRawAllocators(),
> PyMem_SetRawAllocators(), PyMem_GetAllocators(), PyMem_SetAllocators(),
> PyMem_SetupDebugHooks(), _PyObject_GetArenaAllocators(),
> _PyObject_SetArenaAllocators(). Well, do we need all *7* of them? Can't
> you try to make that 2 or 3?

Get/SetAllocators of PyMem, PyMem_Raw and PyObject can be grouped into
2 functions (get and set) with an argument to select the API.

It is what I proposed initially. I changed this when I had to choose a
name for the name of the argument ("api", "domain",  something else?)
because there were only two choices. With 3 family of functions
(PyMem, PyMem_Raw and PyObject), it becomes again interesting to have
generic functions.

The arena case is different: pymalloc only uses two functions to
allocate areneas: void* alloc(size_t) and void release(void*, size_t).
The release function has a size argument, which is unusual, but
require to implement it using munmap(). VirtualFree() on Windows
requires also the size.

An application can choose to replace PyObject_Malloc() with its own
allocator, but in my experience, it has an important impact on
performance (Python is slower). To benefit of pymalloc with a custom
memory allocator, _PyObject_SetArenaAllocators() can be used.

I kept _PyObject_SetArenaAllocators() private because I don't like its
API, it is not homogenous with the other SetAllocators functions. I'm
not sure that it would be used, so I prefer to keep it private until
it is tested by some projects.

"Private" functions can be used by applications, it's just that Python
doesn't give any backward compatibility warranty. Am I right?

Victor