MPI_Graph_create(3) Open MPI MPI_Graph_create(3)
NAME
MPI_Graph_create - Makes a new communicator to which topology informa-
tion has been attached.
SYNTAX
C Syntax
#include <mpi.h>
int MPI_Graph_create(MPI_Comm comm_old, int nnodes, const int index[],
const int edges[], int reorder, MPI_Comm *comm_graph)
Fortran Syntax
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_GRAPH_CREATE(COMM_OLD, NNODES, INDEX, EDGES, REORDER,
COMM_GRAPH, IERROR)
INTEGER COMM_OLD, NNODES, INDEX(*), EDGES(*)
INTEGER COMM_GRAPH, IERROR
LOGICAL REORDER
Fortran 2008 Syntax
USE mpi_f08
MPI_Graph_create(comm_old, nnodes, index, edges, reorder, comm_graph,
ierror)
TYPE(MPI_Comm), INTENT(IN) :: comm_old
INTEGER, INTENT(IN) :: nnodes, index(nnodes), edges(*)
LOGICAL, INTENT(IN) :: reorder
TYPE(MPI_Comm), INTENT(OUT) :: comm_graph
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
C++ Syntax
#include <mpi.h>
Graphcomm Intracomm::Create_graph(int nnodes, const int index[],
const int edges[], bool reorder) const
INPUT PARAMETERS
comm_old Input communicator without topology (handle).
nnodes Number of nodes in graph (integer).
index Array of integers describing node degrees (see below).
edges Array of integers describing graph edges (see below).
reorder Ranking may be reordered (true) or not (false) (logical).
OUTPUT PARAMETERS
comm_graph
Communicator with graph topology added (handle).
IERROR Fortran only: Error status (integer).
DESCRIPTION
MPI_Graph_create returns a handle to a new communicator to which the
graph topology information is attached. If reorder = false then the
rank of each process in the new group is identical to its rank in the
old group. Otherwise, the function may reorder the processes. If the
size, nnodes, of the graph is smaller than the size of the group of
comm_old, then some processes are returned MPI_COMM_NULL, in analogy to
MPI_Cart_create and MPI_Comm_split. The call is erroneous if it speci-
fies a graph that is larger than the group size of the input communica-
tor.
The three parameters nnodes, index, and edges define the graph struc-
ture. nnodes is the number of nodes of the graph. The nodes are num-
bered from 0 to nnodes-1. The ith entry of array index stores the total
number of neighbors of the first i graph nodes. The lists of neighbors
of nodes 0, 1, ..., nnodes-1 are stored in consecutive locations in ar-
ray edges. The array edges is a flattened representation of the edge
lists. The total number of entries in index is nnodes and the total
number of entries in edges is equal to the number of graph edges.
The definitions of the arguments nnodes, index, and edges are illus-
trated with the following simple example.
Example: Assume there are four processes 0, 1, 2, 3 with the following
adjacency matrix:
Process Neighbors
0 1, 3
1 0
2 3
3 0, 2
Then, the input arguments are:
nnodes = 4
index = 2, 3, 4, 6
edges = 1, 3, 0, 3, 0, 2
Thus, in C, index[0] is the degree of node zero, and index[i] - in-
dex[i-1] is the degree of node i, i=1, . . . , nnodes-1; the list of
neighbors of node zero is stored in edges[j], for 0 <= j <= index[0] -
1 and the list of neighbors of node i, i > 0 , is stored in edges[j],
index[i-1] <= j <= index[i] - 1.
In Fortran, index(1) is the degree of node zero, and index(i+1) - in-
dex(i) is the degree of node i, i=1, . . . , nnodes-1; the list of
neighbors of node zero is stored in edges(j), for 1 <= j <= index(1)
and the list of neighbors of node i, i > 0, is stored in edges(j), in-
dex(i) + 1 <= j <= index(i + 1).
ERRORS
Almost all MPI routines return an error value; C routines as the value
of the function and Fortran routines in the last argument. C++ func-
tions 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.
SEE ALSO
MPI_Graph_get
MPI_Graphdims_get
4.1.2 Nov 24, 2021 MPI_Graph_create(3)
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