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Re: Incorrect parallel computation


On 08/04/2012 09:17, Yasha Gindikin wrote:
> Dear colleagues,
>
> would you please advise me on why the following simple code executes incorrectly when parallelized?
>
> a = Subsets[Range[1, 5], {3}];
> L = Length@a;
> b = Table[0, {5}];
> c = Table[0, {5}];
> SetSharedVariable[c];
> Do[b[[a[[l, k]]]] += 1, {l, L}, {k, 3}];
> ParallelDo[c[[a[[l, k]]]] += 1, {l, L}, {k, 3}];
> b===c
>
> False
>
> Thank you!
> Best regards,
> Yasha Gindikin.
>

I think this illustrates the pitfalls of parallel computing - which is 
why I have never used this feature for serous code!


Notice the "Possible issues" section of the help for SetSharedVariable. 
You probably thought your update was atomic, but I doubt if

c[[a[[l, k]]]] += 1

qualifies as an atomic update!

Every now and again I toy with the idea of using parallel constructs - 
so thanks for reminding me of why it is not a good idea, and saving me 
much heartache!

David Bailey
http://www.dbaileyconsultancy.co.uk



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