simple Thread question

Christopher T King squirrel at WPI.EDU
Tue Aug 3 22:05:40 CEST 2004

On 3 Aug 2004, adeger wrote:

> Having trouble with my first forays into threads.  Basically, the
> threads don't seem to be working in parallel (or you might say are
> blocking).  I've boiled my problems to the following short code block
> and ensuing output.  Seems like the output should be all interleaved
> and of course it's not.  Running Python 2.2 from ActiveState on
> Windows XP (also doesn't work on Windows 2000).

The Python interpreter isn't too thread-friendly.  Because it's not 
re-entrant, it has to make use of a Global Interpreter Lock in order to 
keep internal structures from getting mangled.  This lock only allows one 
thread to access the interpreter at a time, and switches threads every 
hundred or so bytecodes.  The likely cause of your problem is that your 
loops don't reach this switching threshold -- try using xrange(100) or 

The GIL is released during blocking I/O (or other) operations, and C 
extensions can release the lock if they plan on doing lots of non-Python 
processing.  Because of the former property, another thing you can try is 
inserting a time.sleep(.1) inside of each loop -- being a blocking I/O 
operation, this should cause the GIL to be released, and your threads will 
switch each time through the loop.

Aside from the performance loss on parallel-processing machines, there is 
usually no reason to worry about the GIL in Python code: so long as you 
make proper use of the thread synchronization routines, everything will 
work as you intend it to.

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