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Re: Gfortran vs Ifort when using MPI
On 8 Jul 2008, at 21:25, FX wrote:
Depending on the system, using the right -march makes a big
difference. Try -march=native (not supported on all targets). (If
the compiler thinks that there is no SSE, -march=native can help a
lot; if I recall correctly, for x86 the settings are rather
conservative.
For a 50x-100x performance difference, I suppose there is something
*awfully* wrong going on, like wrong alignment or NaNs all over the
place. I suggest a) checking the results, b) roughly profiling the
gfortran code ("gfortran -pg" and using gprof) to get an idea where
it spends so much time. It'll be much easier than us shooting in
the dark.
One thing you might check for in your profiling is presence of, and
different handling of, denormals - that can make huge difference on x86.
Iain