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From: Joel Eaves (joel_at_[hidden])
Date: 2005-08-04 14:04:11


Hi group. I posted a general MPI question a while ago to the mpi
newsgroup but didn't get a response. I need to figure this out so I
thought I would try it on you.

I have written a piece of code that fills a 2D array sequentially so
that I can keep track of which elements are being dropped in the
message passing. I use the type_vector datatype to generate a datatype
for passing the columns. In C, I can see that the scatter operation
passes the first matrix to process 0 correctly but that the second
matrix to process 1 is screwed up because the elements are set
backwards by two. In other words, the second matrix begins with the
lucky 13th element instead of the 15th like it should. There is
overlap -- the same elements appear in both of the scattered matrices.
The C++ code goes over like a lead baloon. The operation is clearly
asking for data outside of the range for the filled matrix and so the
values of the scattered matrix are all screwed up. I am using the LAM
MPI v. 7.1.1 and Mac OS 10.3.8
with gcc v. 3.3. I got similar results using MPICH-2 on Linux.
Here's a piece of code written in C.

#include <mpi.h>
#include <iostream>

int main(int argc,char* argv[]){
MPI_Init(&argc,&argv);
int my_rank = MPI::COMM_WORLD.Get_rank(),n_global = 10,n_procs =
MPI::COMM_WORLD.Get_size(),
d=3,n_local = n_global/n_procs,i,k,root=0;
double A_global[n_global][d],A_local[n_local][d];
MPI_Datatype scatter;
MPI_Type_vector(n_local,1,d,MPI_DOUBLE,&scatter);
MPI_Type_commit(&scatter);
if(my_rank==root){
for(i=0;i<n_global;i++)
for(k=0;k<d;k++)
A_global[i][k] = i*d+k;
for(k=0;k<d;k++)
MPI_Scatter(&(A_global[0][k]),1,scatter,&(A_local[0][k]),1,scatter,root,MPI_COMM_WORLD);

for(i=0;i<n_local;i++){
for(k=0;k<d;k++)
cout << A_local[i][k] << "\t";
cout << endl;

}

MPI_Finalize();
return 0;
}

In C++, the code is
#include <mpi.h>
#include <iostream>

int main(int argc,char* argv[]){
MPI::Init();
int my_rank = MPI::COMM_WORLD.Get_rank(),n_global = 10,n_procs =
MPI::COMM_WORLD.Get_size(),
d=3,n_local = n_global/n_procs,i,k,root=0;
double A_global[n_global][d],A_local[n_local][d];
MPI::Datatype scatter(MPI::DOUBLE);
scatter.Create_vector(n_local,1,d);
scatter.Commit();
if(my_rank==root){
for(i=0;i<n_global;i++)
for(k=0;k<d;k++)
A_global[i][k] = i*d+k;
for(k=0;k<d;k++)
MPI::COMM_WORLD.Scatter(&(A_global[0][k]),1,scatter,&(A_local[0][k]),1,scatter,root);

for(i=0;i<n_local;i++){
for(k=0;k<d;k++)
cout << A_local[i][k] << "\t";
cout << endl;

}

MPI::Finalize();
return 0;
}

I'm running the process (after a lamboot) with the command

mpirun -np 2 scatter.out

and compiling with the command

mpic++ Scatter.cpp -o scatter.out

Can anyone help out with this? I don't
understand why the commands for C++ are returning erroneous results
that are *different* than they are from the C program.

Thanks,

Joel