SYNTAX

C Syntax

#include <mpi.h>
int MPI_Dist_graph_create(MPI_Comm comm_old, int n, const int sources[],
        const int degrees[], const int destinations[], const int weights[],
        MPI_Info info, int reorder, MPI_Comm *comm_dist_graph)

Fortran Syntax

INCLUDE 'mpif.h'
MPI_DIST_GRAPH_CREATE(COMM_OLD, N, SOURCES, DEGREES, DESTINATIONS, WEIGHTS,
                INFO, REORDER, COMM_DIST_GRAPH, IERROR)
        INTEGER COMM_OLD, N, SOURCES(*), DEGRES(*), WEIGHTS(*), INFO
        INTEGER COMM_DIST_GRAPH, IERROR
        LOGICAL   REORDER

INPUT PARAMETERS

comm_old

Input communicator without topology (handle).

n

Number of source nodes for which this process specifies edges (non-negative integer).

sources

Array containing the n source nodes for which this process species edges (array of non-negative integers).

degrees

Array specifying the number of destinations for each source node in the source node array (array of non-negative integers).

destinations

Destination nodes for the source nodes in the source node array (array of non-negative integers).

weights

Weights for source to destination edges (array of non-negative integers).

Hints on optimization and interpretation of weights (handle).

reorder

Ranking may be reordered (true) or not (false) (logical).

OUTPUT PARAMETERS

comm_dist_graph

Communicator with distibuted graph topology added (handle).

IERROR

Fortran only: Error status (integer).

DESCRIPTION

MPI_Dist_graph_create creates a new communicator comm_dist_graph with distrubuted graph topology and returns a handle to the new communicator. The number of processes in comm_dist_graph is identical to the number of processes in comm_old. Concretely, each process calls the constructor with a set of directed (source,destination) communication edges as described below. Every process passes an array of n source nodes in the sources array. For each source node, a non-negative number of destination nodes is specied in the degrees array. The destination nodes are stored in the corresponding consecutive segment of the destinations array. More precisely, if the i-th node in sources is s, this species degrees[i] edges (s,d) with d of the j-th such edge stored in destinations[degrees[0]+...+degrees[i-1]+j]. The weight of this edge is stored in weights[degrees[0]+...+degrees[i-1]+j]. Both the sources and the destinations arrays may contain the same node more than once, and the order in which nodes are listed as destinations or sources is not signicant. Similarly, different processes may specify edges with the same source and destination nodes. Source and destination nodes must be process ranks of comm_old. Different processes may specify different numbers of source and destination nodes, as well as different source to destination edges. This allows a fully distributed specification of the communication graph. Isolated processes (i.e., processes with no outgoing or incoming edges, that is, processes that do not occur as source or destination node in the graph specication) are allowed. The call to MPI_Dist_graph_create is collective.

If reorder = false, all processes will have the same rank in comm_dist_graph as in comm_old. If reorder = true then the MPI library is free to remap to other processes (of comm_old) in order to improve communication on the edges of the communication graph. The weight associated with each edge is a hint to the MPI library about the amount or intensity of communication on that edge, and may be used to compute a

WEIGHTS

Weights are specied as non-negative integers and can be used to influence the process remapping strategy and other internal MPI optimizations. For instance, approximate count arguments of later communication calls along specic edges could be used as their edge weights. Multiplicity of edges can likewise indicate more intense communication between pairs of processes. However, the exact meaning of edge weights is not specied by the MPI standard and is left to the implementation. An application can supply the special value MPI_UNWEIGHTED for the weight array to indicate that all edges have the same (effectively no) weight. It is erroneous to supply MPI_UNWEIGHTED for some but not all processes of comm_old. If the graph is weighted but n = 0, then MPI_WEIGHTS_EMPTY or any arbitrary array may be passed to weights. Note that MPI_UNWEIGHTED and MPI_WEIGHTS_EMPTY are not special weight values; rather they are special values for the total array argument. In Fortran, MPI_UNWEIGHTED and MPI_WEIGHTS_EMPTY are objects like MPI_BOTTOM (not usable for initialization or assignment). See MPI-3 § 2.5.4.