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Subject: Re: [OMPI users] Problems Broadcasting/Scattering Data
From: Dino Rossegger (dino.rossegger_at_[hidden])
Date: 2008-01-08 11:14:47


Thanks

jody schrieb:
> 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?
Yes I want to broadcast stat, and split it in the first dimension , so
that, for instance:
Process 1 gets stat[0][x] - stat[19][x]
Process 2 gets stat[20][x] -...

> 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.
So neither MPI_Broadcast nor scatter can handle 2 dimensional arrays?
But also if it is like that, is it normal that there are only 0 in the
array? For me that sounds more as if the data isn't transmitted at all
and not that it isn't splitted correctly. But I'll try the folding,
maybe this will help.

Thanks

> 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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