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Subject: Re: [OMPI users] Performance: MPICH2 vs OpenMPI
From: Eugene Loh (Eugene.Loh_at_[hidden])
Date: 2008-10-09 11:53:52


Brian Dobbins wrote:

On Thu, Oct 9, 2008 at 10:13 AM, Jeff Squyres <jsquyres@cisco.com> wrote:
On Oct 9, 2008, at 8:06 AM, Sangamesh B wrote:
OpenMPI : 120m 6s
MPICH2 :  67m 44s

That seems to indicate that something else is going on -- with -np 1, there should be no MPI communication, right?
Wow.  Yes.  Ditto.
I'd be more inclined to double-check how the Gromacs app is being compiled in the first place
E.g.,

mpicc -show
Someone correct me if I'm wrong, but if MPICH2 embeds optimization flags in the 'mpicc' command and OpenMPI does not, then if he's not specifying any optimization flags in the compilation of Gromacs, MPICH2 will pass its embedded ones on to the Gromacs compile and be faster.
Yes, I have one established example of this.  I built MPICH2 with CFLAGS=-O2.  I compiled a non-MPI code with "mpicc" (no flags) and got optimized performance with MPICH2 but non-optimized performance with OMPI.  About 3x difference in performance for my particular test case.  Not a single bit of MPI in the test code.
I'm rusty on my GCC, too, though - does it default to an O2 level, or does it default to no optimizations?
When I tried it, default gcc seemed to be no optimization.  In my MPICH2 "mpicc" (with optimization built in) I had to specify "mpicc -O0" explicitly to turn optimization back off again.