openmp program elapsed time not scaling with increased threads

Posted by Griff on Stack Overflow See other posts from Stack Overflow or by Griff
Published on 2012-12-15T04:59:48Z Indexed on 2012/12/15 5:03 UTC
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loops

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parallel-processing

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fortran

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openmp

I've got this openmp fortran program doing an embarrassingly parallel problem - do loop over 512^3 elements. See output below. Why would there be such strange behavior in the elapsed time as a function of threads? I thought it would peak at a sweet spot then slowly degrade. This clearly isn't happening. Perhaps I misunderstand something about openmp.

Threads, omp_get_wtime
1, 103.76298500015400
2, 65.346454000100493
4, 45.923643999965861
7, 38.074195000110194
8, 36.968765000114217
9, 39.45981499995105
10,40.753379000118002
12,39.577559999888763
14,37.909950000001118

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