Overcoming python performance penalty for multicore CPU

John Nagle nagle at animats.com
Wed Feb 3 00:02:49 CET 2010

    I know there's a performance penalty for running Python on a
multicore CPU, but how bad is it?  I've read the key paper
("www.dabeaz.com/python/GIL.pdf"), of course.  It would be adequate
if the GIL just limited Python to running on one CPU at a time,
but it's worse than that; there's excessive overhead due to
a lame locking implementation.  Running CPU-bound multithreaded
code on a dual-core CPU runs HALF AS FAST as on a single-core
CPU, according to Beasley.

    My main server application, which runs "sitetruth.com"
has both multiple processes and multiple threads in each process.
The system rates web sites, which involves reading and parsing
up to 20 pages from each domain.  Analysis of each domain is
performed in a separate process, but each process uses multiple
threads to read process several web pages simultaneously.

    Some of the threads go compute-bound for a second or two at a time as
they parse web pages.  Sometimes two threads (but never more than three)
in the same process may be parsing web pages at the same time, so
they're contending for CPU time.

    So this is nearly the worst case for the lame GIL lock logic.
Has anyone tried using "affinity" ("http://pypi.python.org/pypi/affinity")
to lock each Python process to a single CPU?  Does that help?

				John Nagle

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