MPI_Dist_graph_create(3) | Open MPI | MPI_Dist_graph_create(3) |

# 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

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) INTEGERCOMM_OLD, N, SOURCES(*), DEGRES(*), WEIGHTS(*), INFOINTEGERCOMM_DIST_GRAPH, IERRORLOGICALREORDER

# 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_oldINTEGER, INTENT(IN) ::n,sources(n),degrees(n),destinations(*)INTEGER, INTENT(IN) ::weights(*)TYPE(MPI_Info), INTENT(IN) ::infoLOGICAL, INTENT(IN) :: reorder TYPE(MPI_Comm), INTENT(OUT) ::comm_dist_graphINTEGER, 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 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.

# SEE ALSO

MPI_Dist_graph_create_adjacent MPI_Dist_graph_neighbors MPI_Dist_graph_neighbors_count

November 24, 2021 | 4.1.2 |