Q: multiprocessing.Queue size limitations or bug...

ryles rylesny at gmail.com
Thu Aug 27 08:56:49 CEST 2009


On Aug 26, 4:56 am, Michael Riedel <mrie... at inova-semiconductors.de>
wrote:
> Sorry for being not more specific but I'm not absolutely certain whether
> I encountered a bug or did anything wrong:
>
> The (stupid) code below results in a stall forever or not at 'p0.join()'
> depending on the value of TROUBLE_MAKER.
>
> Any help, thoughts, comments?
>
> Thank you for your time.
>
> Michael
>
> # ----------------------------------------------------------------------
>
> from multiprocessing import Process, Queue
>
> # bit vector size
> BVS=8
>
> #
> TROUBLE_MAKER=12  # for greater values p0.join() is never satisfied...
>
> def evaluate(q, id, start=0, stop=2**BVS):
>
>     cmin = {0: []}
>
>     for mask0 in range(start, stop):
>         for mask1 in range(0, 2**BVS):
>             for mask2 in range(mask1, TROUBLE_MAKER):
>                 cmin[0].append((mask0, mask1, mask2))
>
>     print 'process %d finished (dict layout: %d/%d)...' % (id,
> len(cmin), len(cmin[0]))
>     q.put(cmin.copy())
>     q.close()
>
> if __name__ == '__main__':
>
>     q0 = Queue()
>     q1 = Queue()
>     q2 = Queue()
>     q3 = Queue()
>
>     part = 2**BVS/4
>     p0 = Process(target=evaluate, args=(q0, 0, 0*part, 1*part),
> name='worker_0')
>     p1 = Process(target=evaluate, args=(q1, 1, 1*part, 2*part),
> name='worker_1')
>     p2 = Process(target=evaluate, args=(q2, 2, 2*part, 3*part),
> name='worker_2')
>     p3 = Process(target=evaluate, args=(q3, 3, 3*part, 4*part),
> name='worker_3')
>     p0.start()
>     print 'process 0 started...'
>     p1.start()
>     print 'process 1 started...'
>     p2.start()
>     print 'process 2 started...'
>     p3.start()
>     print 'process 3 started...'
>     # main process stalls at p0.join() for bigger TROUBLE_MAKER
>     p0.join()
>     p1.join()
>     p2.join()
>     p3.join()
>     res0 = q0.get()
>     res1 = q1.get()
>     res2 = q2.get()
>     res3 = q3.get()
>     print 'results fetched...'
>
> # ----------------------------------------------------------------------
>
> --

There is a warning related to this in the documentation:

http://docs.python.org/library/multiprocessing.html#pipes-and-queues

Basically, you should reverse the order of the get() and join() calls.

multiprocessing does a pretty nice job of abstracting away the low-
level details of IPC, but there are still some gotchas. As you've
noticed, your program will deadlock when there is a large enough
amount of data being put into the queue. This is related to a hidden
thread that exists inside each of your child processes. The thread is
responsible for taking your queue items from an internal buffer and
then writing them into a pipe that your parent process will read from
when get() is called. The pipe mechanism is what allows the two
processes to pass information, and is supported directly by the
Operating System. However, the pipe has a limited capacity, and when
it is full, the writer thread is stuck waiting for the reader to read
enough from the pipe so that it can finish its write. The problem is
that your parent process (reader) is not actually calling get() to
drain the pipe. Instead it's stuck in join() waiting for the writer to
complete.



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