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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[])
INCLUDE ’mpif.h’
MPI_GRAPH_NEIGHBORS(COMM, RANK, MAXNEIGHBORS, NEIGHBORS, IERROR)
INTEGER COMM, RANK, MAXNEIGHBORS, NEIGHBORS(*), IERROR
#include <mpi.h>
void Graphcomm::Get_neighbors(int rank, int maxneighbors,
int neighbors[]) const
- 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
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.
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
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