when does the GIL really block?

Craig Allen callen314 at gmail.com
Fri Aug 1 03:27:36 CEST 2008

I have followed the GIL debate in python for some time.  I don't want
to get into the regular debate about if it should be gotten rid of
(though I am curious about the status of that for Python 3)...
personally I think I can do multi-threaded programming well, but I
also see the benefits of a multiprocess approach. I'm not so
egotistical that I don't realize perhaps my mt programming has not
been "right" (though it worked and was debuggable) or more likely that
doing it right I have avoided even trying some things people want mt
programming to do... i.e. to do mt programming right you start to use
queues a lot, inter-thread asynchronous, non-blocking, communication,
which is essentially the multi-process approach, using IPC (except
that the threads can see the same memory when, in your special case,
you know that's ok. Given something like a reader-writer lock, this
can have benefits... but again, whatever.

My question is that given this problem, years ago before I started
writing in python I wrote some short programs in python which could,
in fact, busy both my CPUs.  In retrospect I assume I did not have
code in my run function that causes a GIL lock... so I have done this

I start two threads... I use gkrellm to watch my processors (dual
processor machine).  If I merely print a number... both CPUS are
getting 90% simultaneous loads. If I increment a counter and print it
too, the same, and if I create a small list and sort it, the same. I
did not expect this... I expected to see one processor pegged at
around 100%, which should sometimes switch to the other processor.
Granted, the same program in C/C++ would peg both processors at
100%... but given that the overhead in the interpreter cannot explain
the extra usage, I assume the code in my thread's run functions is
actually executing non-serially.

I assume this is because what I am doing does not require the GIL to
be locked for a significant part of the time my code is running...
what code could I put in my run function to see the behavior I
expected?  What code could I put there to take advantage of the
possibility that really the GIL is not locked enough to cause actual
serialization of the threads...  anyone care to explain?

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