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Old 2011-08-28, 19:27   #1
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fivemack's Avatar
Feb 2006
Cambridge, England

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Default How do I get comprehensible MPI behaviour

I'm trying to work out what the best grid-size to use for running MPI jobs on my 48-core machine is. In particular, I'd quite like to run a big msieve job on half of the machine while the other half runs lasieve4I14e.

Just running jobs with different settings isn't getting me numbers I believe. For a 1109891-cycle matrix a 6x4 grid is 18% faster than a 4x6 grid, and a 3x8 grid is faster than either whilst an 8x3 grid is 34% slower than 3x8; for a 4012661-cycle matrix the 6x4 grid is 11% slower than 4x6. I suppose I should rerun the four grid sizes for the small matrix three times each and see if (as I fear) I've got enormous between-repeat variability ... but I've no good idea how to reduce between-repeat variability. Possibly running them on a perfectly idle machine would be sensible, but that's not how I want to be using them in practice.
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