MPI_Cart_Shift.Corner Neighborhood

Question

I need to create a communicator with cube topology and then select face of the cube, using the MPI_Cart_Shift implemented messaging between processes that are on the brink. For example I am process with rank 0(R0) my neighborhoods are R2, R4, R6(lower face of cube). I can find R2 and R4, but I can't understand how to find R6. My code:

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

int main(int argc, char *argv[])
{
int rank, k;
int size; 
int ndims = 3; 
int source, dest;
int up,down,right,left,up3, down3;

int edges[6][4] = {{0,1,5,4},
                {4,5,7,6},
                {2,3,1,0},
                {6,7,3,2},
                {1,3,7,5},
                {0,2,6,7}};

int t, incep=0;
char processor_name[MPI_MAX_PROCESSOR_NAME];

MPI_Comm comm, comm3d;
int dims[3]={0,0,0}, coords[3]={0,0,0},
    periods[3]={1,1,1}, reorder = 0;


MPI_Status status;


int user_edge;

MPI_Init(&argc, &argv);

MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);

MPI_Dims_create(size, ndims, dims);


  MPI_Cart_create(MPI_COMM_WORLD, ndims, dims, periods, reorder, &comm);


  MPI_Cart_coords(comm, rank, 3, coords);

 fflush(stdout);
     printf("Rank %d coordinates are %d %d %d\n", rank, coords[0], coords[1], coords[2]);
 MPI_Barrier(comm);

    int leftrank, rightrank;
    int downrank, uprank;

MPI_Comm_rank(comm, &rank);
    MPI_Cart_coords(comm, rank, 2, coords);
MPI_Cart_shift(comm, 0, -1, &downrank, &uprank); 
MPI_Sendrecv(buffer, 10, MPI_INT, downrank, 123, buffer2, 10, MPI_INT, uprank, 123, comm, &status);
MPI_Cart_shift(comm, 1, -1, &rightrank, &leftrank); 
    MPI_Sendrecv(buffer, 10, MPI_INT, leftrank, 123, buffer2, 10, MPI_INT, rightrank, 123, comm, &status);


printf("P:%d My neighbors are rightRank: %d downRank:%d leftRank:%d upRank:%d diagonal:%d diagonalX:%d\n", rank,rightrank,downrank,leftrank,uprank,diagonal,diagonalX);



MPI_Finalize();

return 0;

}

I will try add something like this MPI_Cart_shift(comm, 2, 1, &diagonal, &diagonalX); But for R0 it show me R1 and it i understand.... How can I get corner neighborhoods?

Answer 1

You can use MPI_Cart_rank to find the information you need.

int MPI_Cart_rank(MPI_Comm comm, const int coords[], int *rank)

Here comm is a communicator with Cartesian topology. coords is a integer array (the size of this array being the number of dimensions of the Cartesian topology) containing the coordinate of a process (in your case, 1,1,0 for R6). Then the output rank would return the global rank of that process you can use in subsequent communication.

BTW, MPI_Cart_coords performs the opposite direction, i.e. from rank to coordinate.