.. _mpi_dist_graph_create: MPI_Dist_graph_create ===================== .. include_body :ref:`MPI_Dist_graph_create` |mdash| Makes a new communicator to which topology information has been attached. SYNTAX ------ C Syntax ^^^^^^^^ .. code-block:: c #include 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 ^^^^^^^^^^^^^^ .. code-block:: fortran USE MPI ! or the older form: 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 Fortran 2008 Syntax ^^^^^^^^^^^^^^^^^^^ .. code-block:: fortran USE mpi_f08 MPI_Dist_Graph_create(comm_old, n, sources, degrees, destinations, weights, info, reorder, comm_dist_graph, ierror) TYPE(MPI_Comm), INTENT(IN) :: comm_old INTEGER, INTENT(IN) :: n, sources(n), degrees(n), destinations(*) INTEGER, INTENT(IN) :: weights(*) TYPE(MPI_Info), INTENT(IN) :: info LOGICAL, INTENT(IN) :: reorder TYPE(MPI_Comm), INTENT(OUT) :: comm_dist_graph INTEGER, OPTIONAL, INTENT(OUT) :: ierror 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 specifies 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). * ``info``: 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 distributed graph topology added (handle). * ``ierror``: Fortran only: Error status (integer). DESCRIPTION ----------- :ref:`MPI_Dist_graph_create` creates a new communicator *comm_dist_graph* with distributed 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 specified 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 specifies *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 specification) are allowed. The call to :ref:`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 specified 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 specific 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 specified 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 section 2.5.4. ERRORS ------ .. include:: ./ERRORS.rst .. seealso:: * :ref:`MPI_Dist_graph_create_adjacent` * :ref:`MPI_Dist_graph_neighbors` * :ref:`MPI_Dist_graph_neighbors_count`