Re: [OMPI users] Wrong values when reading file with MPI IO

[Edgar Gabriel]

What version of Open MPI did you execute your test with?
Edgar

On 4/7/2016 1:54 PM, david.froger...@mailoo.org wrote:

Hello,

Here is a simple `C` program reading a file in parallel with `MPI IO`:

      #include <stdio.h>
      #include <stdlib.h>

      #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
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