[MITgcm-support] building with MPI on a dual-core mac

[Brian Rose]

Hello MITgcmers,

This is not really a question, but I just wanted to report on my  
recent experience with setting up MITgcm with MPI support on a new  
Apple iMac with Intel Core 2 Duo processor.  Though it's a single  
machine, I got interested in building the model with MPI to take  
advantage of the dual core processor.  I didn't find much information  
about this particular setup in the email archives, but I successfully  
muddled through it on my own.  I imagine this information is obvious  
to some of you, but it certainly wasn't to me, so I figured it would  
be useful contribution to this list.

My system:
iMac w/ 2.66 GHz Intel Core 2 Duo
OS X 10.5.7
gfortran 4.4.0 (installed via fink)
mpich2-1.1

I found many ways to install MPI that *didn't* work, and one way that  
*did* work.  There is a version of OpenMPI pre-installed with OS X  
10.5, but without a Fortran compiler so it's pretty much useless for  
our purposes.

What worked for me was to download the source code for the latest  
stable release of MPICH2, and build it with gfortran using the same  
compiler flags as I need to compile MITgcm.  Specifically, I ran the  
mpich2 configure script with the following flags
F90=gfortran F77=gfortran F90FLAGS='-fsecond-underscore' FFLAGS='- 
fsecond-underscore'
and then proceeded to build mpich2 following the stock instructions  
that came with the distribution.
(without the '-fsecond-underscore' flags, I got linker errors when  
building MITgcm)

MITgcm now compiles and runs without complaint, using the optfile  
attached below.

To run the model on two cores, I did the following:
- edit SIZE.h with nPx = 2
- follow the usual build sequence, using the '-mpi' option for genmake2
- before running the model, launch the 'mpd' daemon (that was  
installed along with mpich2) by typing 'mpd &' on the command line
- to run the model:  'mpiexec -n 2 ./mitgcmuv'

And it works!  The limited testing I have done shows a speed-up factor  
of about 1.6 over a single-processor build.

Hope this information, and/or the optfile below, is useful to somebody.

Cheers
Brian



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Brian Rose
Ph.D. Candidate, Climate Physics and Chemistry
Department of Earth, Atmospheric and Planetary Sciences
Massachusetts Institute of Technology
office: 54-1419          phone: 617-253-9345
email:  brose at mit.edu
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