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

Nick Coghlan ncoghlan at gmail.com
Sat Jun 15 14:22:33 CEST 2013

On 15 June 2013 21:01, Antoine Pitrou <solipsis at pitrou.net> wrote:

On Sat, 15 Jun 2013 03:54:50 +0200 Victor Stinner <victor.stinner at gmail.com> wrote:

The addition of PyMemRawMalloc() is motivated by the issue #18203 (Replace calls to malloc() with PyMemMalloc()). The goal is to be able to setup a custom allocator for all allocation made by Python, so malloc() should not be called directly. PyMemRawMalloc() 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. I don't like the idea of adding of third layer of allocation APIs. The dichotomy between PyObjectMalloc and PyMemMalloc is already a bit gratuitous (i.e. not motivated by any actual real-world concern, as far as I can tell).

The only reason for the small object allocator to exist is because operating system allocators generally aren't optimised for frequent allocation and deallocation of small objects. You can gain a lot of speed from handling those inside the application. As the allocations grow in size, though, the application level allocator just becomes useless overhead, so it's better to delegate those operations directly to the OS.

However, it's still desirable to be able to monitor those direct allocations in debug mode, thus it makes sense to have a GIL protected direct allocation API as well. You could try to hide the existence of the latter behaviour and treat it as a private API, but why? For custom allocators, it's useful to be able to ensure you can bypass CPython's small object allocator, rather than having to rely on it being bypassed for allocations above a certain size.

As for the debug functions you added: PyMemGetRawAllocators(), PyMemSetRawAllocators(), PyMemGetAllocators(), PyMemSetAllocators(), PyMemSetupDebugHooks(), PyObjectGetArenaAllocators(), PyObjectSetArenaAllocators(). Well, do we need all 7 of them? Can't you try to make that 2 or 3?

Faux simplicity that is achieved only by failing to model a complex problem domain correctly is a bad idea (if we were satisfied with that, we could stick with the status quo).

The only question mark in my mind is over the GIL-free raw allocation APIs. I think it makes sense to at least conditionally define those as macros so an embedding application can redirect just the allocations made by the CPython runtime (rather than having to redefine malloc/realloc/free when building Python), but I don't believe the case has been adequately made for making the raw APIs configurable at runtime. Dropping that aspect would at least eliminate the PyMem_(Get|Set)RawAllocators() APIs.

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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