圖拓撲建立和通訊

以分散式方式建立圖拓撲，以便每個節點定義其鄰居。每個節點用 MPI_Neighbor_allgather 在鄰居之間傳遞其等級。

#include <mpi.h>
#include <stdio.h>

#define nnode 4

int main()
{
    MPI_Init(NULL, NULL);

    int rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    int source = rank;
    int degree;
    int dest[nnode];
    int weight[nnode] = {1, 1, 1, 1};
    int recv[nnode] = {-1, -1, -1, -1};
    int send = rank;

    // set dest and degree.
    if (rank == 0)
    {
        dest[0] = 1;
        dest[1] = 3;
        degree = 2;
    }
    else if(rank == 1)
    {
        dest[0] = 0;
        degree = 1;
    }
    else if(rank == 2)
    {
        dest[0] = 3;
        dest[1] = 0;
        dest[2] = 1;
        degree = 3;
    }
    else if(rank == 3)
    {
        dest[0] = 0;
        dest[1] = 2;
        dest[2] = 1;
        degree = 3;
    }

    // create graph.
    MPI_Comm graph;
    MPI_Dist_graph_create(MPI_COMM_WORLD, 1, &source, &degree, dest, weight, MPI_INFO_NULL, 1, &graph);

    // send and gather rank to/from neighbors.
    MPI_Neighbor_allgather(&send, 1, MPI_INT, recv, 1, MPI_INT, graph);

    printf("Rank: %i, recv[0] = %i, recv[1] = %i, recv[2] = %i, recv[3] = %i\n", rank, recv[0], recv[1], recv[2], recv[3]);

    MPI_Finalize();
    return 0;
}