[MITgcm-support] MPI rules to save compute time?

Mon Nov 14 11:10:08 EST 2011

Consider number of grid points per process, "including" overlap regions.
The narrower "run1" has larger overlap regions, hence more computations
per process, possibly impacting processor cache too.

Dimitris Menemenlis

On Nov 14, 2011, at 7:47 AM, Chun-Yan Zhou wrote:

Dear all,
 I use verification/exp4 to test the MPI.
 The the Nx=80, Ny=42 and Nr=8.
 I use differnt nPx and Npy to run the same iteration number and the data files are same.
 &PARM04
 usingCartesianGrid=.TRUE.,
 delX=80*5.e3,
 delY=42*5.e3,
 delR= 8*562.5,
 &
           sNx   sNy  nPx nPy processes    time cost
run0    80      42   1       1          1             20mins
run1    80      21   1       2          2             20mins
run2     40     42   2       1          2             12mins
run3     40      21  2        2          4            10mins

comparing run0 and run1,  nPy=2 didn't speed up the computation. But nPx=2 case is much faster than NPx=1(run0 VS. run2). Since the grid spaces are the same in the X and Y direction, what causes the compute time difference?  Any idea about this? Any rules to assign the nPx and nPy values in order to save time?

Thanks in advance!
chunyan

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