Kevin Cazabon wrote:
A couple years ago I did some work on enabling multi-threading in the
PIL core library, with the help of Fredrik and others. We successfully
implemented it by releasing the GIL prior to starting many of the larger
routines/functions in the C library, and re-acquiring it before handing
back to the wrappers. I can't remember exactly which functions we did
this to in total, but it definitely included the Image.resize code and
so forth. On my dual-processor machine, I saw almost double performance
on multi-threaded resizing operations - proving it worked. However, we
never touched the pixel access sections.
You could look at the resize code in the C module and use the same
method to try it with the functions you use most. The trick is just
figuring out the right places to release and recapture the GIL.
The fact that the OP uses pixel access pretty much implies that most of
the processing time in his program is spent on the Python level, which
means that the GIL is the culprit here. Releasing and reacquiring the
lock would most likely take a lot more time than just fetching the pixel.
To fully get around the GIL for cases like this, you have to use
processing or similar libraries. For some cases, the simple
thread/subprocess approach outlined here:
http://effbot.org/zone/wide-finder.htm
can work (see the addenda section for examples using the processing and
pprocess libraries).
Depending on the access/crunching ratio, psyco might also help:
http://psyco.sourceforge.net/
</F>
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