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

[Constantinos Evangelinos]

On Sunday 19 July 2009 1:47:19 pm Klymak Jody wrote:

> Thanks for the more-knowledgeable explanation.  OS X allows you to
> turn the multithreading off if you desire.  Any idea if that makes
> things go faster if you do not oversubscribe the cores?

Only if the OS scheduler is not smart enough to avoid oversubscribing a core 
(with hyperthreading) even when the other core is idle. Ie. if all virtual 
cores are seen as equivalent, then there is a potential for slowdown of a 2 
process run (it used to happen on RedHat Linux when hyperthreaded Pentium4s 
first came out). But these days I doubt it and if you see a speedup from 
using hyperthreading and 4 processes continue doing so.  

> I'm afraid I have to run genmake2 with -ignoretime because of a
> missing library.   I suppose I should track that down. I'm using
> gfortran, gcc, and openMPI, so I'm surprised the timing code isn't in
> there.  I will also have to admit that I'm using a 4 year old version
> of the gcm so maybe the timing calls have changed to be more flexible.

Migrating to a newer version of the model is more important for bug fixing 
reasons. There is an issue with the timing calls when using the double 
underscore f2c name mangling/argument passing convention but it should not be 
there with gfortran. 

Constantinos 
-- 
Dr. Constantinos Evangelinos
Department of Earth, Atmospheric and Planetary Sciences
Massachusetts Institute of Technology