multiprocessing: child process race to answer
William Ray Wing
wrw at mac.com
Sat Nov 2 08:17:08 EDT 2013
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
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