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**NAME**

MPI_Dist_graph_create- Makes a new communicator to which topology information has been attached.

**SYNTAX**

**C** **Syntax**

#include <mpi.h> int MPI_Dist_graph_create(MPI_Commcomm_old, intn, const intsources[], const intdegrees[], const intdestinations[], const intweights[], MPI_Info info, intreorder, 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) INTEGERCOMM_OLD,N,SOURCES(*),DEGRES(*),WEIGHTS(*),INFOINTEGERCOMM_DIST_GRAPH,IERRORLOGICALREORDER

**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 thensource 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 communicatorcomm_dist_graphwith distrubuted graph topology and returns a handle to the new communicator. The number of processes incomm_dist_graphis identical to the number of processes incomm_old. Concretely, each process calls the constructor with a set of directed (source,destination) communication edges as described below. Every process passes an array ofnsource nodes in thesourcesarray. For each source node, a non-negative number of destination nodes is specied in thedegreesarray. The destination nodes are stored in the corresponding consecutive segment of thedestinationsarray. More precisely, if the i-th node in sources is s, this speciesdegrees[i]edges(s,d) with d of the j-th such edge stored indestinations[degrees[0]+...+degrees[i-1]+j]. The weight of this edge is stored inweights[degrees[0]+...+degrees[i-1]+j]. Both thesourcesand thedestinationsarrays 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 butn= 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 ยง 2.5.4.

**ERRORS**

Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. 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_Dist_graph_create_adjacent MPI_Dist_graph_neighbors MPI_Dist_graph_neighbors_count