MPI_GRAPH_CREATE, MPI_Graph_create

Purpose

Creates a new communicator containing graph topology information.

C synopsis

#include <mpi.h>
MPI_Graph_create(MPI_Comm comm_old,int nnodes, int *index,
    int *edges,int reorder,MPI_Comm *comm_graph);

C++ synopsis

#include mpi.h
MPI::Graphcomm MPI::Intracomm::Create_graph(int nnodes, const int index[],
                                            const int edges[], bool reorder) const;

FORTRAN synopsis

include 'mpif.h' or use mpi
MPI_GRAPH_CREATE(INTEGER COMM_OLD,INTEGER NNODES,INTEGER INDEX(*),
    INTEGER EDGES(*),INTEGER REORDER,INTEGER COMM_GRAPH,
    INTEGER IERROR)

Parameters

comm_old
     is the input communicator (handle) (IN)

nnodes
     is an integer specifying the number of nodes in the graph (IN)

index
     is an array of integers describing node degrees (IN)

edges
     is an array of integers describing graph edges (IN)

reorder
     if true, ranking may be reordered (logical) (IN)

comm_graph
     is the communicator with the graph topology added (handle) (OUT)

IERROR
     is the FORTRAN return code. It is always the last argument.

Description

This subroutine creates a new communicator containing graph topology
information provided by nnodes, index, edges, and reorder. MPI_GRAPH_CREATE
returns the handle for this new communicator in comm_graph.

If there are more tasks in comm_old than there are in nnodes, some tasks
are returned with a value of MPI_COMM_NULL for comm_graph.

Notes

The following example shows how to define the arguments nnodes, index, and
edges. Suppose there are four tasks (0, 1, 2, 3) with the following
adjacency matrix:
 Task      Neighbors
 0         1, 3
 1         0
 2         3
 3         0, 2

Then the input arguments are:
 Argument         Input
 nnodes           4
 index            2, 3, 4, 6
 edges            1, 3, 0, 3, 0, 2

Thus, in C, index[0] is the degree of node 0, and index[i]-index[i-1] is
the degree of node i, i=1, ..., nnodes-1. The list of neighbors of node 0
is stored in edges[j], for 0 >= j >= index[0]-1 and the list of neighbors
of node i, i > 0, is stored in edges[j], index[i-1] >= j >= index[i]-1.

In FORTRAN, index(1) is the degree of node 0, and index(i+1)- index(i) is
the degree of node i, i=1, ..., nnodes-1. The list of neighbors of node 0
is stored in edges(j), for 1 >= j >= index(1) and the list of neighbors of
node i, i > 0, is stored in edges(j), index(i)+1 >= j >= index(i+1).

Because node 0 indicates that node 1 is a neighbor, node 1 must indicate
that node 0 is its neighbor. For any edge A>B, the edge B>A must also be
specified.

Errors

MPI not initialized

MPI already finalized

Invalid communicator

Invalid communicator type
     must be intracommunicator

Invalid nnodes
     nnodes < 0 or nnodes > groupsize

Invalid node degree
     (index[i]-index[i-1]) < 0

Invalid neighbor
     edges[i] < 0 or edges[i]>=nnodes

Asymmetric graph

Conflicting collective operations on communicator

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