MPI Cartesian Topology: MPI_Neighbor_alltoall wrong data received

Question

I have an MPI Cartesian Topology and want to send every nodes rank to their neighbors with MPI_Neighbor_alltoall. I can't figure out, where the error is and i also implemented my own MPI_Neighbor_alltoall which doesn't work. I minimalised my code to an (hopefully) easy to understand code snippet.

alltoall.c

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


int main(int argc, char** argv) {


// MPI_Cart variables
MPI_Comm cart_comm;
MPI_Comm mycomm;
int ndims=2;
int periods[2]={0,0};
int coord[2];


    int dims[2]={3,3};
    int xdim = dims[0];
    int ydim = dims[1];
    int comm_size = xdim*ydim;

    // Initialize the MPI environment and pass the arguments to all the processes.
    MPI_Init(&argc, &argv);

    // Get the rank and number of the process
    int rank, size;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    // output: dimensions
    if(rank==0){
        printf("dims: [%i] [%i]\n", xdim, ydim);
    }

    // enough nodes
    if(comm_size<=size){

        // Communicator count has to match nodecount in dims
        // so we create a new Communicator with matching nodecount
        int color;
        int graphnode;
        if(rank<comm_size){
            //printf("%d<%d\n",rank,comm_size);
            color=0;
            graphnode=1;
        } else {
            //printf("%d>=%d\n",rank,comm_size);
            // not used nodes
            color=1;
            graphnode=0;
        }

        MPI_Comm_split(MPI_COMM_WORLD, color, rank, &mycomm);
        MPI_Comm_rank(mycomm, &rank);
        MPI_Comm_size(mycomm, &size);


        // ***GRAPHNODE-SECTION***
        if(graphnode){

            // Create Dimensions
            MPI_Dims_create(size, ndims, dims);

            // Create Cartesian
            MPI_Cart_create(mycomm, ndims, dims, periods, 1, &cart_comm);

            // Get the name of the processor
            char processor_name[MPI_MAX_PROCESSOR_NAME];
            int len;
            MPI_Get_processor_name(processor_name, &len);

            // Get coordinates
            MPI_Cart_coords(cart_comm, rank, ndims, coord);

            // sending
            int *sendrank = &rank;
            int recvrank[4];
            MPI_Neighbor_alltoall(sendrank , 1, MPI_INT, recvrank, 1, MPI_INT, cart_comm);


            printf("my rank: %i, received ranks: %i %i %i %i\n", rank, recvrank[0], recvrank[1], recvrank[2], recvrank[3]);



        } else {
            // *** SPARE NODES SECTION ***

        }
    } else {
        // not enough nodes reserved
        if(rank==0)
            printf("not enough nodes\n");
    }

    // Finalize the MPI environment.
    MPI_Finalize();

}

So this code creates a 3x3 cartesian topology. It finalizes, if there are not enough nodes and let the spare nodes do nothing, when there are too many nodes. Even though this should be easy, i am still doing something wrong, because the output lacks some data.

output

$ mpicc alltoall.c
$ mpirun -np 9 a.out
dims: [3] [3]
my rank: 2, received ranks: -813779952 5 0 32621
my rank: 1, received ranks: 1415889936 4 0 21
my rank: 5, received ranks: 9 8 0 32590
my rank: 3, received ranks: 9 6 -266534912 21
my rank: 7, received ranks: 9 32652 0 21
my rank: 8, received ranks: 9 32635 0 32635
my rank: 6, received ranks: 9 32520 1372057600 21
my rank: 0, received ranks: -1815116784 3 -1803923456 21
my rank: 4, received ranks: 9 7 0 21

As you can see in the output, nobody has node 1,2 as neighbor and where does the 21 come from? Rank 4 should be the only node, that has 4 neighbors, but that should be {1,3,5,7} right? I really have no idea where my mistake here is.

coordinates should look like this:

[0,0] [1,0] [2,0]
[0,1] [1,1] [2,1]
[0,2] [1,2] [2,2]

and ranks like this:

0   3   6
1   4   7
2   5   8

Answer 1

you are accessing a lot of uninitialized data (both in sendrank and recvrank)

here is a rewritten version of your test program that works for me

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


int main(int argc, char** argv) {


// MPI_Cart variables
MPI_Comm cart_comm;
MPI_Comm mycomm;
int ndims=2;
int periods[2]={0,0};
int coord[2];


    int dims[2]={3,3};
    int xdim = dims[0];
    int ydim = dims[1];
    int comm_size = xdim*ydim;

    // Initialize the MPI environment and pass the arguments to all the processes.
    MPI_Init(&argc, &argv);

    // Get the rank and number of the process
    int rank, size;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    // output: dimensions
    if(rank==0){
        printf("dims: [%i] [%i]\n", xdim, ydim);
    }

    // enough nodes
    if(comm_size<=size){

        // Communicator count has to match nodecount in dims
        // so we create a new Communicator with matching nodecount
        int color;
        int graphnode;
        if(rank<comm_size){
            //printf("%d<%d\n",rank,comm_size);
            color=0;
            graphnode=1;
        } else {
            //printf("%d>=%d\n",rank,comm_size);
            // not used nodes
            color=1;
            graphnode=0;
        }

        MPI_Comm_split(MPI_COMM_WORLD, color, rank, &mycomm);
        MPI_Comm_rank(mycomm, &rank);
        MPI_Comm_size(mycomm, &size);


        // ***GRAPHNODE-SECTION***
        if(graphnode){

            // Create Dimensions
            MPI_Dims_create(size, ndims, dims);

            // Create Cartesian
            MPI_Cart_create(mycomm, ndims, dims, periods, 1, &cart_comm);

            // Get the name of the processor
            char processor_name[MPI_MAX_PROCESSOR_NAME];
            int len;
            MPI_Get_processor_name(processor_name, &len);

            // Get coordinates
            MPI_Cart_coords(cart_comm, rank, ndims, coord);

            // sending
            int sendrank[4];
            int recvrank[4];
            int i;
            char * neighbors[4];
            for (i=0; i<4; i++) {
                 sendrank[i] = rank;
                 recvrank[i] = -1;
            }
            MPI_Neighbor_alltoall(sendrank , 1, MPI_INT, recvrank, 1, MPI_INT, cart_comm);
            for (i=0; i<4; i++) {
                if (-1 != recvrank[i]) {
                    asprintf(&neighbors[i], "%d ", recvrank[i]);
                } else {
                    neighbors[i] = "";
                }
            }

            printf("my rank: %i, received ranks: %s%s%s%s\n", rank, neighbors[0], neighbors[1], neighbors[2], neighbors[3]);


        } else {
            // *** SPARE NODES SECTION ***

        }
    } else {
        // not enough nodes reserved
        if(rank==0)
            printf("not enough nodes\n");
    }

    // Finalize the MPI environment.
    MPI_Finalize();

}