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

[Chun-Yan Zhou]

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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