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

[Klymak Jody]

Hi Brian,

On 18-Jul-09, at 7:32 PM, Brian Rose wrote:

> But in general, when would you expect to see a performance boost  
> from increasing the number of processes beyond the number of  
> hardware cores?  Currently during my 2-core, 2-process runs, each  
> process is already occupying about 99% of its respective CPU.

Ooops, my mistake - I forgot that "Core 2 Duo" just means 2 cores.   
Sneaky Intel marketing. Of course there is no purpose to  
oversubscribing a core.

However, on the new Nehalem Xeon chips there is some advantage to  
having more processes than cores, though its not huge.  Some very  
rough tests with a run that has no written output on a 2x4 core Xeon:

8  Processes on one machine:       54 minutes
16 processes on one machine:      40 minutes

However, if you have two machines, connected by gigabit ethernet:

8+8 proc spread on two machines: 29 minutes

This was for a grid 10x150x1600 with the non-hydrostatic code turned  
on, using openMPI.

So for the new Nehalem chips my crude tests seem to imply that there  
is an advantage to oversubscribing if your MPI overhead is small.

Cheers,  Jody