Python - Memory Leak

Question

I'm working on solving a memory leak in my Python application.

Here's the thing - it really only appears to happen on Windows Server 2008 (not R2) but not earlier versions of Windows, and it also doesn't look like it's happening on Linux (although I haven't done nearly as much testing on Linux).

To troubleshoot it, I set up debugging on the garbage collector:

gc.set_debug(gc.DEBUG_UNCOLLECTABLE | gc.DEBUG_INSTANCES | gc.DEBUG_OBJECTS)

Then, periodically, I log the contents of gc.garbage.

Thing is, gc.garbage is always empty, yet my memory usage goes up and up and up.

Very puzzling.

Answer 1

If there's never any garbage in gc.garbage, then I'm not sure what you're trying to do by enabling GC debugging. Sure, it'll tell you which objects are considered for cleanup, but that's not particularly interesting if you end up with no circular references that can't be cleaned up.

If your program is using more and more memory according to the OS, there can generally be four different cases at play:

1. Your application is storing more and more things, keeping references to each one so they don't get collected.
2. Your application is creating circular references between objects that can't be cleaned up by the gc module (typically because one of them has a __del__ method.)
3. Your application is freeing (and re-using) memory, but the OS doesn't want the memory re-used, so it keeps allocating new blocks of memory.
4. The leak is a real memory leak but in a C/C++ extension module your code is using.

From your description it sounds like it's unlikely to be #1 (as it would behave the same on any OS) and apparently not #2 either (since there's nothing in gc.garbage.) Considering #3, Windows (in general) has a memory allocator that's notoriously bad with fragmented allocations, but Python works around this with its obmalloc frontend for malloc(). It may still be an issue specific in Windows Server 2008 system libraries that make it look like your application is using more and more memory, though. Or it may be a case of #4, a C/C++ extension module, or a DLL used by Python or an extension module, with a memory leak.

Answer 2

In general, the first culprit for memory leaks in python is to be found in C extensions.
Do you use any of them?

Furthermore, you say the issue happens only on 2008; I would then check extensions for any incompatibility, because with Vista and 2008 there were quite a lot of small changes that caused issues on that field.
As and alternative, try to execute your application in Windows compatibility mode, choosing Windows XP - this could help solving the issue, especially if it's related to changes in the security.