On Mar 14, 7:11 am, Ahmad Syukri bin Abdollah <syoc...@gmail.com>
wrote:
> I'm trying this on Python 3.0.1
> Consider the following code:
> """
> import multiprocessing as mp
>
> def jambu(b,i,gl):
> for n in range(100000):
> with gl[i]:
> b[i]+=2
> with gl[3-i]:
> b[3-i]-=1
>
> def main():
> b = mp.RawArray('i',4)
> gl = []
> proc = []
> for i in range(len(b)):
> gl.append(mp.Lock())
> proc.append(mp.Process(target=jambu,args=(b,i,gl)))
> for p in proc:
> p.start()
> for p in proc:
> p.join()
> print(b[:])
> print(sum(b[:]))
> main()
> """
> (Yes, I'm aware that I didn't pass the lock array as shared variable,
> but since they're not reassigned, it should be okay) The above code
> should produce an output like this:
> [100000, 100000, 100000, 100000]
> 400000
> Now, in the documentation for multiprocessing module, it says that
> multiprocessing.Array will automatically create a lock to ensure
> process-safe synchronization. So I suppose the above code can be
> simplified as follows:
> """
> import multiprocessing as mp
>
> def jambu(b,i):
> for n in range(100000):
> b[i]+=2
> b[3-i]-=1
>
> def main():
> b = mp.Array('i',4)
> proc = []
> for i in range(len(b)):
> gl.append(mp.Lock())
> proc.append(mp.Process(target=jambu,args=(b,i)))
> for p in proc:
> p.start()
> for p in proc:
> p.join()
> print(b[:])
> print(sum(b[:]))
> main()
> """
> The output of this second code isn't consistent with the first one,
> implying multiprocessing.Array (or even multiprocessing.Value; I've
> also tried with this one) isn't as atomic as I understand it should
> be. So what is the actual meaning of having "a Lock object which will
> be used to synchronize access to the value"? Is it only for getting,
> and not for assigning values?
>
> Regards,
> Ahmad Syukri
Your code hung on my machine. The call to 'main()' should be in an
'if __name__' block:
if __name__== '__main__':
main()
Is it possible you are just seeing the effects of the non-atomic
'__iadd__' operation? That is, the value is read, added, and written
at different times, between which other processes might have
intervened.
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