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

[Jean-Michel Campin]

Hi Martin,

There is a pkg Ed wrote that do regridding (pkg/regrid)
that use a global sum for a vector:
> eesupp/src/global_vec_sum.F
This should do it.

Jean-Michel

On Thu, Jan 17, 2008 at 04:53:12PM +0100, Martin Losch wrote:
> Hi Samar,
> thanks, but it does mean that I'll have to use MPI - routines  
> directly, something I usually don't do (I know it's embarrassing,  
> that I don't even know the basic). Do you have by any chance the code  
> snippet with MPI_Gather in the MITgcm context?
> 
> Martin
> On 17 Jan 2008, at 15:12, Samar Khatiwala wrote:
> 
> >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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> >
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