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Subject: [OMPI users] difference between single and double precision
From: Mathieu Gontier (mathieu.gontier_at_[hidden])
Date: 2010-12-03 03:02:14

  Dear OpenMPI users

I am dealing with an arithmetic problem. In fact, I have two variants of
my code: one in single precision, one in double precision. When I
compare the two executable built with MPICH, one can observed an
expected difference of performance: 115.7-sec in single precision
against 178.68-sec in double precision (+54%).

The thing is, when I use OpenMPI, the difference is really bigger:
238.5-sec in single precision against 403.19-sec double precision (+69%).

Our experiences have already shown OpenMPI is less efficient than MPICH
on Ethernet with a small number of processes. This explain the
differences between the first set of results with MPICH and the second
set with OpenMPI. (But if someone have more information about that or
even a solution, I am of course interested.)
But, using OpenMPI increases the difference between the two arithmetic.
Is it the accentuation of the OpenMPI+Ethernet loss of performance, is
it another issue into OpenMPI or is there any option a can use?

Thank you for your help.

Mathieu Gontier