multiprocessing: child process race to answer (forgot to Cc: the list)

William Ray Wing wrw at mac.com
Sun Nov 3 04:07:42 CET 2013


On Nov 2, 2013, at 11:44 AM, Sherard Hall <smhall05 at gmail.com> wrote:

> Thank you for the response. Processing time is very important so I suspect having to write to disk will take more time than letting the other processes complete without finding the answer. So I did some profiling one process finds the answer in about 250ms, but since I can't stop the other processes, it takes about 800ms before I can use the answer.  Do you recommend a global variable flag? Any other suggestions?
> 
> On Nov 2, 2013 8:17 AM, "William Ray Wing" <wrw at mac.com> wrote:
> On Nov 2, 2013, at 1:03 AM, smhall05 <smhall05 at gmail.com> wrote:
> 
> > On Friday, November 1, 2013 10:52:40 PM UTC-4, MRAB wrote:
> >> On 02/11/2013 02:35, smhall05 wrote:
> >>
> >>> I am using a basic multiprocessing snippet I found:
> >>>
> >>> #-----------------------------------------------------
> >>> from multiprocessing import Pool
> >>>
> >>> def  f(x):
> >>>     return x*x
> >>>
> >>> if __name__ == '__main__':
> >>>     pool = Pool(processes=4)              # start 4 worker processes
> >>>     result = pool.apply_async(f, [10])    # evaluate "f(10)" asynchronously
> >>>     print result.get(timeout=1)
> >>>     print pool.map(f, range(10))          # prints "[0, 1, 4,..., 81]"
> >>> #---------------------------------------------------------
> >>>
> >>> I am using this code to have each process go off and solve the same problem, just with different inputs to the problem. I need to be able to kill all processes once 1 of n processes has come up with the solution. There will only be one answer.
> >>>
> >>> I have tried:
> >>>
> >>> sys.exit(0) #this causes the program to hang
> >>> pool.close()
> >>> pool.terminate
> >>>
> >>
> >> Did you actually mean "pool.terminate", or is that a typo for
> >>
> >> "pool.terminate()"?
> >>
> >>> These still allow further processing before the program terminates. What else can I try? I am not able to share the exact code at this time. I can provide more detail if I am unclear. Thank you
> >>>
> >
> > I am not sure to be honest, however it turns out that I can't use pool.terminate() because pool is defined in main and not accessible under my def in which I check for the correct answer.
> > --
> > https://mail.python.org/mailman/listinfo/python-list
> 
> So, the simplest solution to that situation is to have whichever subprocess that finds the correct answer set a flag which the calling process can check.  Depending on your OS, that flag can be anything from setting a lock to something as simple as creating a file which the calling process periodically wakes up and looks for, maybe just a file in which the subprocess has written the answer.
> 
> Bill
> 
> -- 
> https://mail.python.org/mailman/listinfo/python-list

Well, the multiprocessing library provides listeners and clients that wrap BSD style sockets and allow you to send (push) arbitrary python objects to a listener, i.e., the master.  There might be something better that was OS specific, but this will keep it pure python.  I've not tested it, but there is a simple example here on Stackoverflow: 

http://stackoverflow.com/questions/6920858/interprocess-communication-in-python

-Bill
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