Multi-core example with OpenMP slower than single core?

Steve Nuchia
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October 11, 2008 10:31 PM PDT
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There is a lot of overhead in OpenMP.  It will run slower unless the loop takes a significant fraction of a second single-threaded.  Tens of milliseconds, minimum.
Also, if the task if memory bandwidth bound rather than compute or cache bound, it will run no faster parallelized, regardless of the API used.  At least on most single-socket hardware.  You have to know your system architecture here.

 

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