[MITgcm-support] mpi with 4 processors

[Holly Dail]

Hi Angela -

Your problem, especially the 2-D one, may simply be too small to benefit from more than 2 processors (i.e. the added communication costs of using 4 isn't compensating for the faster computation).  Have you found that the nSx=6 is helpful for your machine?  I'm currently running on 24 processors (Pleiades) with the following SIZE.h:

     &           sNx = 20,
     &           sNy = 27,
     &           OLx =   3,
     &           OLy =   3,
     &           nSx =   1,
     &           nSy =   1,
     &           nPx =   6,
     &           nPy =   4,
     &           Nx  = sNx*nSx*nPx,
     &           Ny  = sNy*nSy*nPy,
     &           Nr  =  23)

Holly

On Nov 1, 2010, at Nov 1 , 1:44 PM, Angela Marie Zalucha wrote:

> Hi,
> 
> I am trying to figure out how to use the MIT GCM with multiple processors. I found the examples for how to modify SIZE.h for 2 processors.  In 2D I use:
> 
> sNx =   1,
> sNy =  32,
> nSx =   1,
> nSy =   1,
> nPx =   1,
> nPy =   2,
> 
> In 3D I use:
> 
> sNx =  16,
> sNy =  32,
> nSx =   6,
> nSy =   1,
> nPx =   2,
> nPy =   1,
> 
> Indeed, there is a significant improvement in speed over 1 processor.  I have up to 4 processors available to me, but the configurations I have tried with 4 processors don't run any faster than with 2.  Is there any optimum configuration I can use with 4 processors?  Or 3 processors if 4 is not possible?
> 
> It might be that I'm not understanding how to scale things for more processors, so any example with n > 2 would be helpful.
> 
> I use gfortran for the compiler and mpich2 for mpi.  I am running on computers at Southwest Research Institute in Boulder, CO.  I am using 30 vertical levels.
> 
>  Angela
> 
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> Angela Zalucha, Ph.D.
> Postdoctoral Associate
> Department of Earth, Atmospheric,
>  and Planetary Sciences
> Massachusetts Institute of Technology
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