MPI Point to Point Communication to Collective Communication - c++

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.