DeinoMPI

The Great and Terrible implementation of MPI-2

MPI_Dims_create

Creates a division of processors in a cartesian grid

int MPI_Dims_create(
  int nnodes,
  int ndims,
  int *dims
);

Parameters

nnodes: [in] number of nodes in a grid (integer)
ndims: [in] number of cartesian dimensions (integer)
dims: [in/out] integer array of size ndims specifying the number of nodes in each dimension. A value of 0 indicates that MPI_Dims_create should fill in a suitable value.

Remarks

For cartesian topologies, the function MPI_DIMS_CREATE helps the user select a balanced distribution of processes per coordinate direction, depending on the number of processes in the group to be balanced and optional constraints that can be specified by the user. One use is to partition all the processes (the size of MPI_COMM_WORLD's group) into an n-dimensional topology.

The entries in the array dims are set to describe a cartesian grid with ndims dimensions and a total of nnodes nodes. The dimensions are set to be as close to each other as possible, using an appropriate divisibility algorithm. The caller may further constrain the operation of this routine by specifying elements of array dims. If dims[i] is set to a positive number, the routine will not modify the number of nodes in dimension i; only those entries where dims[i] = 0 are modified by the call.

Negative input values of dims[i] are erroneous. An error will occur if nnodes is not a multiple of .

For dims[i] set by the call, dims[i] will be ordered in non-increasing order. Array dims is suitable for use as input to routine MPI_CART_CREATE. MPI_DIMS_CREATE is local.

Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as mallocor other non-MPICH runtime routines that are themselves not interrupt-safe.

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierris an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGERin Fortran.

Errors

All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

MPI_SUCCESS: No error; MPI routine completed successfully.

Example Code

The following sample code illustrates MPI_Dims_create.

#include "mpi.h"
#include <stdio.h>

int prodof( int, const int[] );
/*
* Test edge cases of Dims_create
*/
int prodof( int ndims, const int dims[] )
{
    int i, prod=1;
    for (i=0; i<ndims; i++)
        prod *= dims[i];
    return prod;
}

int main( int argc, char *argv[] )
{
    int errs = 0;
    int dims[4], nnodes;

    MPI_Init( &argc, &argv );
    /* 2 dimensional tests */
    for (nnodes=1; nnodes <= 32; nnodes = nnodes * 2) {
        dims[0] = 0;
        dims[1] = nnodes;

        MPI_Dims_create( nnodes, 2, dims );
        if (prodof(2, dims) != nnodes) {
            errs++;
            printf( "Dims_create returned the wrong decomposition. " );
            printf( "Is [%d x %d], should be 1 x %d\n", dims[0], dims[1], nnodes );
            fflush(stdout);
        }

        /* Try calling Dims_create with nothing to do (all dimensions specified) */
        dims[0] = 1;
        dims[1] = nnodes;
        MPI_Dims_create( nnodes, 2, dims );
        if (prodof(2, dims) != nnodes) {
            errs++;
            printf( "Dims_create returned the wrong decomposition (all given). " );
            printf( "Is [%d x %d], should be 1 x %d\n", dims[0], dims[1], nnodes );
            fflush(stdout);
        }
    }

    /* 4 dimensional tests */
    for (nnodes=4; nnodes <= 32; nnodes = nnodes * 2) {
        dims[0] = 0;
        dims[1] = nnodes/2;
        dims[2] = 0;
        dims[3] = 2;

        MPI_Dims_create( nnodes, 4, dims );
        if (prodof(4, dims) != nnodes) {
            errs++;
            printf( "Dims_create returned the wrong decomposition. " );
            printf( "Is [%d x %d x %d x %d], should be 1 x %d x 1 x 2\n", dims[0], dims[1], dims[2], dims[3], nnodes/2 );
            fflush(stdout);
        }

        /* Try calling Dims_create with nothing to do (all dimensions specified) */
        dims[0] = 1;
        dims[1] = nnodes/2;
        dims[2] = 1;
        dims[3] = 2;
        MPI_Dims_create( nnodes, 2, dims );
        if (prodof(4, dims) != nnodes) {
            errs++;
            printf( "Dims_create returned the wrong decomposition (all given). " );
            printf( "Is [%d x %d x %d x %d], should be 1 x %d x 1 x 2\n", dims[0], dims[1], dims[2], dims[3], nnodes/2 );
            fflush(stdout);
        }
    }

    MPI_Finalize();
    return errs;
}

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