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Subject: Re: [OMPI users] Problems Broadcasting/Scattering Data
From: jody (jody.xha_at_[hidden])
Date: 2008-01-08 10:57:25


I'm not sure if i understad your code correctly -
i imagine you want to use the scatter command to
broadcast the contents of your 2-dimensional array stat,
is that right?

If so, then the problem is that Scatter actually gets an array of pointers
and sends these pointers trying to interpret them as doubles.

You either have to use several scatter commands or "fold" your
2D-Array into a one-dimensional array.

Hope this helps
  Jody

On Jan 8, 2008 3:54 PM, Dino Rossegger <dino.rossegger_at_[hidden]> wrote:
> Hi,
> I have a problem distributing a 2 dimensional array over 3 processes.
>
> I tried different methods to distribute the data (Broadcast, Send/Recv,
> Scatter) but all of them didn't work for me. The output of the root
> processor (0 in my case) is always okay, the output of the others are
> simple 0.
>
> The Array stat is filled with entrys from a file (I left out the
> generation of the Array Data since this is much code and it works
> (tested the whole thing in "single" mode.))
>
> Here are the important parts of the Source Code:
>
> const int ARRAYSIZE = 150;
> int main(int argc, char* argv[])
> {
> MPI_Init(&argc,&argv);
> int rank, anzprocs,recvcount,sendcnt;
> MPI_Comm_size(MPI_COMM_WORLD,&anzprocs);
> MPI_Comm_rank(MPI_COMM_WORLD,&rank);
>
> const int WORKING = ARRAYSIZE/anzprocs;
>
> double stat[ARRAYSIZE][2];
> double stathlp[WORKING][2];
>
> double stat2[WORKING][5];
> double stat3[anzprocs][ARRAYSIZE][5];
> if(rank==0)sendcnt=WORKING*2;
> MPI::COMM_WORLD.Scatter(stat,sendcnt,MPI::DOUBLE,stathlp,WORKING*2,MPI::DOUBLE,0);
>
> for(int i=0;i<WORKING;i++){
> stat2[i][0]=stathlp[i][0];
> stat2[i][1]=stathlp[i][1];
> stat2[i][2]=(stat2[i][0]*stat2[i][1]);
> stat2[i][3]=(stat2[i][0]*stat2[i][0]);
> stat2[i][4]=(stat2[i][1]*stat2[i][1]);
> }
> if (rank==0) recvcount=WORKING*5;
> MPI_Gather(&stat2, WORKING*5, MPI_DOUBLE,&stat3, recvcount, MPI_DOUBLE,
> 0, MPI_COMM_WORLD);
> if (rank==0){
> cout << stat3[0][0][0] << endl;
> cout << stat3[1][0][0] << endl;
> cout << stat3[2][0][0] << endl;
> }
> }
>
> I don't know any further since my experience with OMPI is also not too
> big. Is there anything specific I have to know about distributing 2
> Dimensional Arrays? I don't think that the error is in the MPI_Gather,
> since I did a cout of the data on all nodes and the output was the same.
>
> Thanks in advance and sorry for my bad english
> Dino
>
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