multiprocessing deadlock

paulC pclinch at gmail.com
Fri Oct 23 15:02:20 CEST 2009


On Oct 23, 3:18 am, Brian Quinlan <br... at sweetapp.com> wrote:
> My test reduction:
>
> import multiprocessing
> import queue
>
> def _process_worker(q):
>      while True:
>          try:
>              something = q.get(block=True, timeout=0.1)
>          except queue.Empty:
>              return
>          else:
>              print('Grabbed item from queue:', something)
>
> def _make_some_processes(q):
>      processes = []
>      for _ in range(10):
>          p = multiprocessing.Process(target=_process_worker, args=(q,))
>          p.start()
>          processes.append(p)
>      return processes
>
> 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
>
> #    The deadlock only occurs on Mac OS X and only when these lines
> #    are commented out:
> #    for p in processes:
> #        p.join()
>
> for i in range(100):
>      _do(i)
>
> --------------
>
> Output (on Mac OS X using the svn version of py3k):
> % ~/bin/python3.2 moprocessmoproblems.py
> Run: 0
> Grabbed item from queue: 0
> Grabbed item from queue: 1
> Grabbed item from queue: 2
> ...
> Grabbed item from queue: 29
> Run: 1
>
> At this point the script produces no additional output. If I uncomment
> the lines above then the script produces the expected output. I don't
> see any docs that would explain this problem and I don't know what the
> rule would be e.g. you just join every process that uses a queue before
>   the queue is garbage collected.
>
> Any ideas why this is happening?
>
> Cheers,
> Brian

I can't promise a definitive answer but looking at the doc.s:-

isAlive()
    Return whether the thread is alive.

    Roughly, a thread is alive from the moment the start() method
returns until its run() method terminates. The module function
enumerate() returns a list of all alive threads.

I guess that the word 'roughly' indicates that returning from the start
() call does not mean that all the threads have actually started, and
so calling join is illegal. Try calling isAlive on all the threads
before returning from _make_some_processes.

Regards, Paul C.



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