plucky (3) MPI_Dist_graph_create.openmpi.3.gz
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
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
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
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 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
Almost all MPI routines return an error value; C routines as the return result of the function and
Fortran routines in the last argument.
Before the error value is returned, the current MPI error handler associated with the communication
object (e.g., communicator, window, file) is called. If no communication object is associated with the
MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error
handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after
MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler.
The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using
the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/‐
MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all
other MPI functions.
Open MPI includes three predefined error handlers that can be used:
• MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.
• MPI_ERRORS_ABORT An error handler that can be invoked on a communicator, window, file, or session. When
called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a
window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in
the corresponding window or file. If called on a session, aborts only the local process.
• MPI_ERRORS_RETURN Returns an error code to the application.
MPI applications can also implement their own error handlers by calling:
• MPI_Comm_create_errhandler then MPI_Comm_set_errhandler
• MPI_File_create_errhandler then MPI_File_set_errhandler
• MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
• MPI_Win_create_errhandler then MPI_Win_set_errhandler
Note that MPI does not guarantee that an MPI program can continue past an error.
See the MPI man page for a full list of MPI error codes.
See the Error Handling section of the MPI-3.1 standard for more information.
SEE ALSO:
• MPI_Dist_graph_create_adjacent
• MPI_Dist_graph_neighbors
• MPI_Dist_graph_neighbors_count
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Feb 17, 2025 MPI_DIST_GRAPH_CREATE(3)