[Python-Dev] extremely slow exit for program having huge (45G) dict (python 2.5.2) (original) (raw)

Steve Holden steve at holdenweb.com
Sat Dec 20 14:14:49 CET 2008

Andrew MacIntyre wrote:

Mike Coleman wrote:

I have a program that creates a huge (45GB) defaultdict. (The keys are short strings, the values are short lists of pairs (string, int).) Nothing but possibly the strings and ints is shared.

The program takes around 10 minutes to run, but longer than 20 minutes to exit (I gave up at that point). That is, after executing the final statement (a print), it is apparently spending a huge amount of time cleaning up before exiting. I haven't installed any exit handlers or anything like that, all files are already closed and stdout/stderr flushed, and there's nothing special going on. I have done 'gc.disable()' for performance (which is hideous without it)--I have no reason to think there are any loops. Currently I am working around this by doing an os.exit(), which is immediate, but this seems like a bit of hack. Is this something that needs fixing, or that has already been fixed? You don't mention the platform, but... This behaviour was not unknown in the distant past, with much smaller datasets. Most of the problems then related to the platform malloc() doing funny things as stuff was free()ed, like coalescing free space. [I once sat and watched a Python script run in something like 30 seconds and then take nearly 10 minutes to terminate, as you describe (Python 2.1/Solaris 2.5/Ultrasparc E3500)... and that was only a couple of hundred MB of memory - the Solaris 2.5 malloc() had some undesirable properties from Python's point of view] PyMalloc effectively removed this as an issue for most cases and platform malloc()s have also become considerably more sophisticated since then, but I wonder whether the sheer size of your dataset is unmasking related issues. Note that in Python 2.5 PyMalloc does free() unused arenas as a surplus accumulates (2.3 & 2.4 never free()ed arenas). Your platform malloc() might have odd behaviour with 45GB of arenas returned to it piecemeal. This is something that could be checked with a small C program. Calling os.exit() circumvents the free()ing of the arenas. Also consider that, with the exception of small integers (-1..256), no interning of integers is done. If your data contains large quantities of integers with non-unique values (that aren't in the small integer range) you may find it useful to do your own interning. It's a pity a simplistic approach that redefines all space reclamation activities as null functions won't work. I hate to think of all the cycles that are being wasted reclaiming space just because a program has terminated, when in fact an os.exit() call would work just as well from the user's point of view.

Unfortunately there are doubtless programs out there that do rely on actions being taken at shutdown.

Maybe os.exit() could be more widely advertised, though ...

regards Steve

Steve Holden +1 571 484 6266 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/

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