Tree Topology

Trees and Work
/*                                                              
int MPI_Graph_create(MPI_Comm oldcomm, int nnodes, int *index, int *edges, int reorder, MPI_Comm *graphcomm),
where:
oldcomm - the initial communicator,
nnodes - the number of the graph vertices,
index - An index into the list of edges, giving the arcs proceeding from  each vertex,
edges - the sequential list of the graph arcs,
reorder - the flag for pointing out  if the process ranks can be reodered,
cartcomm - the created communicator with the graph type topology.                                                               
*/
#include<mpi.h>
#include<stdio.h>
/* A 7 node binary tree*/
int main(int argc, char *argv[]) {
    MPI_Status status;
    int rank,i,j,tag_starter=34;
    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD,&rank);
    int reorder=1;   /* Can the processes be reordered, may result in improved performance.*/
    int index[] = { 2,5,8,9,10,11,12};
    int edges[] = { 1,2,0,3,4,0,5,6,1,1,2,2};
    int nneighbors, send_rank=-1,receive_ranks[]={-1,-1};
    int tags[7][7];
    for (i=0;i<7;i++) {
        for (j=0;j<7;j++) {
            (i<j)?tags[i][j]=tag_starter++:0;
        }
    }
    int * intptr=&nneighbors;
    int the_neighbors[10];
    MPI_Comm MPI_Tree_World;
    MPI_Graph_create(MPI_COMM_WORLD, 7, index, edges, reorder,&MPI_Tree_World);
    /*int MPI_Graph_neighbors_count(MPI_Comm comm,int rank, int *nneighbors); */
    MPI_Graph_neighbors_count(MPI_Tree_World,rank, intptr);
    /*int MPI_Graph_neighbors (MPI_Comm comm,int rank,int mneighbors, int *neighbors); */
    MPI_Graph_neighbors (MPI_Tree_World,rank,nneighbors, the_neighbors);
    printf(" %i has %i neighbors.\n",rank,nneighbors);
    j=0;
    for (i=0;i<nneighbors;i++) {
        if (the_neighbors[i]<rank) {
            send_rank=the_neighbors[i];
        } else {
            receive_ranks[j++]=the_neighbors[i];
        }
    }
    if (send_rank!=-1) printf("      the to send neighbor of %i is %i .\n",rank,send_rank);
    for (i=0;i<2;i++) {
        if (receive_ranks[i]!=-1) printf ("   a receive neighbor of %i is %i .\n",rank, receive_ranks[i]);
    }

    MPI_Finalize();
}
 

Reading index[] and edges[]

The index is the "rank's index" in the array of edges.

the ranks:   0     1,    2 3  4  5  6

index[] = {  2,    5,    8,9,10,11,12};

edges[] = {1,2,0,3,4,0,5,6,1, 1, 2, 2};