Hello,
Here is a simple `C` program reading a file in parallel with `MPI IO`:
#include
#include
#include "mpi.h"
#define N 10
main( int argc, char **argv )
{
int rank, size;
MPI_Init(&argc, &argv);
MPI_Comm_rank( MPI_COMM_WORLD, &rank );
MPI_Comm_size( MPI_COMM_WORLD, &size );
int i0 = N * rank / size;
int i1 = N * (rank+1) / size;
printf("rank: %d, i0: %d, i1: %d\n", rank, i0, i1);
int i;
double* data = malloc( (i1-i0)*sizeof(double) );
for (i = 0 ; i < i1-i0 ; i++)
data[i] = 123.;
MPI_File f;
MPI_File_open(MPI_COMM_WORLD, "data.bin", MPI_MODE_RDONLY,
MPI_INFO_NULL, &f);
MPI_File_set_view(f, i0, MPI_DOUBLE, MPI_DOUBLE, "native",
MPI_INFO_NULL);
MPI_Status status;
MPI_File_read(f, data, i1-i0, MPI_DOUBLE, &status);
int count;
MPI_Get_count(&status, MPI_DOUBLE, &count);
printf("rank %d, %d value read\n", rank, count);
for (i = 0 ; i < i1-i0 ; i++) {
printf("rank: %d index: %d value: %.2f\n", rank, i,
data[i]);
}
MPI_File_close(&f);
MPI_Finalize();
free(data);
return 0;
}
With one processus:
./read_mpi_io
Values read are correct:
rank: 0, i0: 0, i1: 10
rank 0, 10 value read
rank: 0 index: 0 value: 0.00
rank: 0 index: 1 value: 1.00
rank: 0 index: 2 value: 2.00
rank: 0 index: 3 value: 3.00
rank: 0 index: 4 value: 4.00
rank: 0 index: 5 value: 5.00
rank: 0 index: 6 value: 6.00
rank: 0 index: 7 value: 7.00
rank: 0 index: 8 value: 8.00
rank: 0 index: 9 value: 9.00
But with two processus:
mpirun -n 2 ./read_mpi_io
I get wrong values (zeros):
rank: 0, i0: 0, i1: 5
rank: 1, i0: 5, i1: 10
rank 0, 5 value read
rank: 0 index: 0 value: 0.00
rank 1, 5 value read
rank: 1 index: 0 value: 0.00
rank: 0 index: 1 value: 1.00
rank: 0 index: 2 value: 2.00
rank: 1 index: 1 value: 0.00
rank: 1 index: 2 value: 0.00
rank: 1 index: 3 value: 0.00
rank: 1 index: 4 value: 0.00
rank: 0 index: 3 value: 3.00
rank: 0 index: 4 value: 4.00
What's wrong in my C code?
Thanks,
David
