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MPI_Dims_create.3.rst

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MPI_Dims_create

:ref:`MPI_Dims_create` |mdash| Creates a division of processors in a Cartesian grid.

SYNTAX

C Syntax

#include <mpi.h>

int MPI_Dims_create(int nnodes, int ndims, int dims[])

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_DIMS_CREATE(NNODES, NDIMS, DIMS, IERROR)
     INTEGER NNODES, NDIMS, DIMS(*), IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Dims_create(nnodes, ndims, dims, ierror)
     INTEGER, INTENT(IN) :: nnodes, ndims
     INTEGER, INTENT(INOUT) :: dims(ndims)
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS

  • nnodes: Number of nodes in a grid (integer).
  • ndims: Number of Cartesian dimensions (integer).

IN/OUT PARAMETER

  • dims: Integer array of size ndims specifying the number of nodes in each dimension.

OUTPUT PARAMETER

  • ierror: Fortran only: Error status (integer).

DESCRIPTION

For Cartesian topologies, the function :ref:`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 ((pi) over (i, dims[i] != 0)) dims[i].

For dims[i] set by the call, dims[i] will be ordered in nonincreasing order. Array dims is suitable for use as input to routine :ref:`MPI_Cart_create`. :ref:`MPI_Dims_create` is local.

Example:

dims
before                                       dims
call         function call           on return
-----------------------------------------------------
(0,0)        MPI_Dims_create(6, 2, dims)     (3,2)
(0,0)        MPI_Dims_create(7, 2, dims)     (7,1)
(0,3,0)      MPI_Dims_create(6, 3, dims)     (2,3,1)
(0,3,0)      MPI_Dims_create(7, 3, dims)     erroneous call
------------------------------------------------------

ERRORS