[MITgcm-support] Parameters to parallelise a coupled run ?

[Jean-Michel Campin]

Hi Alexandre,

There was a problem in the coupler mapping interface (specially 
regarding the run-off mapp that the coupler uses) that has been
found recently and fixed (Nov 27, 2013 and Dec 02, 2013).
I would recommand to update your code (checkpoint64r contains 
these fixes).

However, this would have very little or no impact on the
how fast the model will run on 1+48+48 procs.

One (obvious) thing is that the default option in "run_cpl_test" 
is to compile with "-devel" (very slow).
Otherwise, you my want to try smaller number of procs (e.g.,
1+12+12 and 1+24+24) to check when this scaling/speed degrades.

Cheers,
Jean-Michel

On Thu, Dec 12, 2013 at 10:53:35AM +0100, Alexandre Pohl wrote:
> Hi all,
> 
> I asked you a few weeks ago about a way to merge netcdf output files. I am still learning to use the MITgcm, to hopefully be able to apply it soon to paleoclimate modelling (the Ordovician glaciation).
> 
> But I am now having problems to parallelise the code. I use the cubed-sphere exch2 grid. I started from the "cpl_aim+ocn" verification case. It runs well with 6 faces, 6 tiles and 3 processes (1 for the ocean, 1 for the atmosphere and 1 for the coupler). 
> 
> To parallelise the code, I then divided each face of the cube into 8 tiles using the SIZE.h file :
> 
>      &           sNx =  16,
>      &           sNy =   8,
>      &           OLx =   2,
>      &           OLy =   2,
>      &           nSx =   1,
>      &           nSy =   1,
>      &           nPx =  48,
>      &           nPy =   1,
>      &           Nx  = sNx*nSx*nPx,        > which gives 768 points in X
>      &           Ny  = sNy*nSy*nPy,        > which gives 8 points in Y
>      &           Nr  =  15)  [Nr=5 for the atmosphere]
> 
> It used the "adjustment.cs-32x32x1" verification files to write my own, except that I gave 1 process to each tile ("nSx=1" above), which finally makes 6*8=48 processes ("nPx=48") instead of a single process.
> 
> I added " usingMPI=.TRUE.," to my eedata files and set  "nTx=1".
> 
> I am now wondering how processes I have to use for the ocean and for the atmosphere. I use the "run_cpl_test" script to prepare, compile and execute the model. I tried "Npr=97; NpOc=48;" which means 48 processes for the ocean, 48 for the atmosphere and 1 single processor for the coupler, but the model unfortunately does not run faster with these 97 processes than with only 3.
> 
> Could you tell me if my parameters are wrong?
> Have I forgotten something?
> 
> I would be very interested if somebody could describe a set of parameters that allow to parallelise the model efficiently.
> 
> Thank you for your help,
> Alexandre
> 
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