MPI 点对点通信到集体通信

Question

I am learning MPI and I am trying to convert my MPI program from Point to Point Communication to MPI Collectives ..

Below is a fragment of my code for Matrix Multiplication using MPI Point to Point communication ...

int i;
    if(rank == 0) {
        for(i = 1; i < size; i++){
            MPI_Send(&rows, 1, MPI_INT, i, 0, MPI_COMM_WORLD);
            MPI_Send(&columns, 1, MPI_INT, i, 0, MPI_COMM_WORLD);
        }
    } else {
        MPI_Recv(&rows, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, &status);
        MPI_Recv(&columns, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, &status);
    }   

    int local_block_size = rows / size;
    int process, column_pivot;

    if(rank == 0) {
        for(i = 1; i < size; i++){
            MPI_Send((matrix_1D_mapped + (i * (local_block_size * rows))), (local_block_size * rows), MPI_DOUBLE, i, 0, MPI_COMM_WORLD);
            MPI_Send((rhs + (i * local_block_size)), local_block_size, MPI_DOUBLE, i, 0, MPI_COMM_WORLD);
        }
        for(i = 0; i < local_block_size * rows; i++){
            matrix_local_block[i] = matrix_1D_mapped[i];
        }
        for(i = 0; i < local_block_size; i++){
            rhs_local_block[i] = rhs[i];
        }
    } else {
        MPI_Recv(matrix_local_block, local_block_size * rows, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD, &status);
        MPI_Recv(rhs_local_block, local_block_size, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD, &status);
    }

I am thinking about replacing MPI_Send with MPI_Bcast ... will that be the correct approach ?

Answer 1

For the first communication that data sent to all receivers is in fact identical, thus MPI_Bcast is the correct approach. The second communication distributes different chunks of a larger array to the recipients, this is done as a collective with MPI_Scatter . Note that scatter includes the root rank in the communication, so you can omit the manual local copy.