MPI_Comm_split not working with MPI_Bcast

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With following code I am splitting 4 processes in column groups, then broadcasting in same column from diagonal (0,3). Process 0 broadcasts to 2. And 3 should broadcast to 1. But it is not working as expected. Can some one see whats wrong ?

0 1
2 3

    #include <stdio.h>
    #include <stdlib.h>
    #include <iostream>
    #include <mpi.h>
    #include <mpi.h>
    using namespace std;
    
    int main(int argc, char **argv){
        MPI_Comm col_comm,row_comm;
        int myrank, size, even, value=0;
        int localRank=0;
        MPI_Init (&argc, &argv);
        MPI_Comm_rank (MPI_COMM_WORLD, &myrank);
        MPI_Comm_size (MPI_COMM_WORLD, &size);      
        MPI_Comm_split(MPI_COMM_WORLD, myrank%2, myrank, &col_comm); 
        MPI_Comm_rank (col_comm, &localRank);
            
        if(myrank%3==0){
            value = myrank*5+1;
            MPI_Bcast(&value, 1, MPI_INT, localRank, col_comm);
        }
        
        printf("Rank=%d | LocalRank=%d | Got broadcast value of %d\n", myrank, localRank, value);
        MPI_Finalize();
        return 0;
    }

The Output

ubuntu@root:~/matrixmult$ mpirun comtest -np 4
Rank=0 | LocalRank=0 | Got broadcast value of 1
Rank=1 | LocalRank=0 | Got broadcast value of 0
Rank=2 | LocalRank=1 | Got broadcast value of 0
Rank=3 | LocalRank=1 | Got broadcast value of 16
openmpi mpi parallel-processing c c++
1 Answers

MPI_Bcast

Broadcasts a message from the process with rank "root" to all other processes of the communicator

is a collective communication routine, hence it should be called by all the processes in a given communicator. Therefore, you need to remove the following condition if(myrank%3==0) and then you need to adapt the root process accordingly, instead of using localRank.

In your current code, only the processes with myrank 0 and 3 called the MPI_Bcast (both belonging to different communicators). So process 0 calls

MPI_Bcast(&value, 1, MPI_INT, localRank, col_comm);

which basically means that it broadcasted the value to itself. The same happens with process 3. Hence, the output:

Rank=0 | LocalRank=0 | Got broadcast value of 1
Rank=1 | LocalRank=0 | Got broadcast value of 0
Rank=2 | LocalRank=1 | Got broadcast value of 0
Rank=3 | LocalRank=1 | Got broadcast value of 16

Rank=0 and Rank=3 communicated with themselves, while the other processes where not part of the MPI_Bcast. Hence the value of 0 for both of them.

Try the following:

int main(int argc, char **argv){
    MPI_Comm col_comm,row_comm;
    int myrank, size, even, value=0;
    int localRank=0;
    MPI_Init (&argc, &argv);
    MPI_Comm_rank (MPI_COMM_WORLD, &myrank);
    MPI_Comm_size (MPI_COMM_WORLD, &size);      
    MPI_Comm_split(MPI_COMM_WORLD, myrank%2, myrank, &col_comm); 
    MPI_Comm_rank (col_comm, &localRank);
     
    if(myrank == 0 || myrank == 3)    
       value = myrank*5+1;
    
    MPI_Bcast(&value, 1, MPI_INT, myrank%2 != 0, col_comm);
    
    printf("Rank=%d | LocalRank=%d | Got broadcast value of %d\n", myrank, localRank, value);
    MPI_Finalize();
    return 0;
}

Process 0 broadcasts to 2. And 3 should broadcast to 1.

Output:

Rank=0 | LocalRank=0 | Got broadcast value of 1
Rank=1 | LocalRank=0 | Got broadcast value of 16
Rank=2 | LocalRank=1 | Got broadcast value of 1
Rank=3 | LocalRank=1 | Got broadcast value of 16
user_13668710
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