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MPI_Graph_neighbors.3.rst

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MPI_Graph_neighbors

MPI_Graph_neighbors Returns the neighbors of a node associated with a graph topology.

SYNTAX

C Syntax

#include <mpi.h>

int MPI_Graph_neighbors(MPI_Comm comm, int rank, int maxneighbors,
 int neighbors[])

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_GRAPH_NEIGHBORS(COMM, RANK, MAXNEIGHBORS, NEIGHBORS, IERROR)
 INTEGER COMM, RANK, MAXNEIGHBORS, NEIGHBORS(*), IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Graph_neighbors(comm, rank, maxneighbors, neighbors, ierror)
 TYPE(MPI_Comm), INTENT(IN) :: comm
 INTEGER, INTENT(IN) :: rank, maxneighbors
 INTEGER, INTENT(OUT) :: neighbors(maxneighbors)
 INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS

  • comm: Communicator with graph topology (handle).
  • rank: Rank of process in group of comm (integer).
  • maxneighbors: Size of array neighbors (integer).

OUTPUT PARAMETERS

  • neighbors: Ranks of processes that are neighbors to specified process (array of integers).
  • ierror: Fortran only: Error status (integer).

DESCRIPTION

Example: Suppose that comm is a communicator with a shuffle-exchange topology. The group has 2n members. Each process is labeled by a(1), ..., a(n) with a(i) E{0,1}, and has three neighbors: exchange (a(1), ..., a(n) = a(1), ..., a(n-1), a(n) (a = 1 - a), shuffle (a(1), ..., a(n)) = a(2), ..., a(n), a(1), and unshuffle (a(1), ..., a(n)) = a(n), a(1), ..., a(n-1). The graph adjacency list is illustrated below for n=3.

exchange        shuffle     unshuffle

node neighbors(1) neighbors(2) neighbors(3) 0(000) 1 0 0 1(001) 0 2 4 2(010) 3 4 1 3(011) 2 6 5 4(100) 5 1 2 5(101) 4 3 6 6(110) 7 5 3 7(111) 6 7 7

Suppose that the communicator comm has this topology associated with it. The following code fragment cycles through the three types of neighbors and performs an appropriate permutation for each.

!  assume: each process has stored a real number A.
!  extract neighborhood information
CALL MPI_COMM_RANK(comm, myrank, ierr)
CALL MPI_GRAPH_NEIGHBORS(comm, myrank, 3, neighbors, ierr)
!  perform exchange permutation
CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(1), 0, &
                          neighbors(1), 0, comm, status, ierr)
!  perform shuffle permutation
CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(2), 0, &
                          neighbors(3), 0, comm, status, ierr)
!  perform unshuffle permutation
CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(3), 0, &
                          neighbors(2), 0, comm, status, ierr)

ERRORS

* MPI_Graph_neighbors_count