[MITgcm-support] zonal or meridional means on multiple processors

[Samar Khatiwala]

Hi Martin

My memory is somewhat hazy, but I recall doing this for zonal means  
by locally SUMMING the variable in x and putting
that sum in a (local) array of length number of local y points. Then  
I believe I did an MPI_Gather (or perhaps MPI_Gatherv)
to collect those local arrays on root. Then its a simple matter to  
computing zonal means from this information. A bit of a hack
but it seemed to work.

Another possibility (which I haven't tried) would be to create a new  
communicator between a group of processors with
MPI_Comm_create ( MPI_Comm comm, MPI_Group group, MPI_Comm *comm_out )
and then using the allreduce operation on that comm.

Samar

On Jan 17, 2008, at 7:57 AM, Martin Losch wrote:

> Hi there,
>
> in a multiprocessor set up I would like to compute a zonal (in i- 
> direction) or meridional (in j-direction) mean for an online  
> diagnostic. Is there a convenient way of doing this in the MITgcm  
> or will I have to get out my MPI handbook (not that I know how to  
> use it)?
>
> In ebm_zonalmean I see an example with global_sum, but I am not  
> sure that this will work, as global_sum contains an MPI_Allreduce  
> which should (in my view of the world) sum over all processors and  
> not only over the zonal ones.
>
> Martin
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