Ubuntu Manpage: MPI_Dist_graph_create - Makes a new communicator to which topology information has been

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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_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.

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.