MPI_Graph_neighbors − Returns the neighbors of a node associated with a graph topology.
#include
<mpi.h>
int MPI_Graph_neighbors(MPI_Comm comm, int
rank, int maxneighbors,
int neighbors[]) |
USE MPI
! or the older form: INCLUDE ’mpif.h’
MPI_GRAPH_NEIGHBORS(COMM, RANK, MAXNEIGHBORS, NEIGHBORS,
IERROR)
INTEGER |
COMM, RANK, MAXNEIGHBORS, NEIGHBORS(*), IERROR |
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 |
comm |
Communicator with graph topology (handle). |
|||
rank |
Rank of process in group of comm (integer). |
maxneighbors
Size of array neighbors (integer).
neighbors |
Ranks of processes that are neighbors to specified process (array of integers). | ||
IERROR |
Fortran only: Error status (integer). |
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
neighbors(1) |
neighbors(2) |
neighbors(3) | ||||||||
1 |
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0 |
0 | |||||||||
0 |
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2 |
4 | |||||||||
3 |
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4 |
1 | |||||||||
2 |
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6 |
5 | |||||||||
5 |
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1 |
2 | |||||||||
4 |
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3 |
6 | |||||||||
7 |
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5 |
3 | |||||||||
6 |
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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.
C assume: each
process has stored a real number A.
C extract neighborhood information
CALL MPI_COMM_RANK(comm, myrank, ierr)
CALL MPI_GRAPH_NEIGHBORS(comm, myrank, 3, neighbors, ierr)
C perform exchange permutation
CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(1), 0,
+ neighbors(1), 0, comm, status, ierr)
C perform shuffle permutation
CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(2), 0,
+ neighbors(3), 0, comm, status, ierr)
C perform unshuffle permutation
CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(3), 0,
+ neighbors(2), 0, comm, status, ierr)
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI::Exception object.
Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.
MPI_Graph_neighbors_count