[Numpy-discussion] OT: performance in C extension; OpenMP, or SSE ?
faltet at pytables.org
Sun Feb 20 07:09:37 EST 2011
A Sunday 20 February 2011 00:01:59 Sturla Molden escrigué:
> pthreads will give you better control than OpenMP, but are messy and
> painful to work with.
> With MPI you have separate processes, so everything is completely
> isolated. It's more difficult to program and debug than OpenMP code,
> but will usually perform better.
To be more specific, MPI will perform better if you don't need to share
the memory of your working set among all your processes, but in case you
need to do this, threads (pthreads, OpenMP) lets you access all parts of
the working set in memory much more easily. In fact, all the threads of
a process can access the complete working set transparently and
efficiently (although this is precisely why they are trickier to
program: you have to explicitly avoid simultaneous writing in the same
memory area), not to mention that threads are much cheaper to create
Generally speaking, if your problem is large, CPU intensive and not very
memory bounded, MPI usually leads to better results. Otherwise threads
tend to do a better job. The thing is that memory is increasingly
becoming more and more a bottleneck nowadays, so threads are here to
stay for a long, long time (which is certainly unfortunate for
programmers, specially for pthreads ones :).
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