multiprocessing deadlock

Brian Quinlan brian at sweetapp.com
Sat Oct 24 07:48:38 CEST 2009


On 24 Oct 2009, at 14:10, Gabriel Genellina wrote:

> En Thu, 22 Oct 2009 23:18:32 -0300, Brian Quinlan  
> <brian at sweetapp.com> escribió:
>
> I don't like a few things in the code:
>
>> def _do(i):
>>     print('Run:', i)
>>     q = multiprocessing.Queue()
>>     for j in range(30):
>>         q.put(i*30+j)
>>     processes = _make_some_processes(q)
>>
>>     while not q.empty():
>>         pass
>
> I'd use time.sleep(0.1) or something instead of this busy wait, but  
> see below.

This isn't my actual code, it is a simplification of my code designed  
to minimally demonstrate a possible bug in multiprocessing.

>
>> #    The deadlock only occurs on Mac OS X and only when these lines
>> #    are commented out:
>> #    for p in processes:
>> #        p.join()
>
> I don't know how multiprocessing deals with it, but if you don't  
> join() a process it may become a zombie, so it's probably better to  
> always join them. In that case I'd just remove the wait for  
> q.empty() completely.

I'm actually not looking for workarounds. I want to know if this is a  
multiprocessing bug or if I am misunderstanding the multiprocessing  
docs somehow and my demonstrated usage pattern is somehow incorrect.

Cheers,
Brian

>
>> for i in range(100):
>>     _do(i)
>
> Those lines should be guarded with: if __name__ == '__main__':
>
> I don't know if fixing those things will fix your problem, but at  
> least the code will look neater...
>
> -- 
> Gabriel Genellina
>
> -- 
> http://mail.python.org/mailman/listinfo/python-list




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