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

[Martin Losch]

Hi Jean-Michel,

global_vec_sum also sums over all times and precessors, as far as I  
can see, it jus does it for a vector of values, instead of a scalar  
(as global_sum does).

Martin

On 17 Jan 2008, at 17:15, Jean-Michel Campin wrote:

> 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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