MPI_Send and MPI_Recv question.

Question

So I am trying to figure out the communication overhead of sending and receiving information between processors using MPI in C code.

I need to pass a buffer in both the send and receive, but all I want to do is to time how long it takes to do n communications between two processors.

Here is the my entire code:

main(int argc, char** argv){

int n;
int rank;
int time;
int i;
MPI_Status status;

MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);

if(rank == 0){
    n = atoi(argv[1]);
    printf("Size of data set = %d\n", n);
}

MPI_Bcast(&n, 1, MPI_INT, 0, MPI_COMM_WORLD);

for(i = 0; i < n; i++){
    if(rank == 0){
        MPI_Send(&n, 1, MPI_INT, 1, 0, MPI_COMM_WORLD);
    }

    else{
        MPI_Recv(&n, 1, MPI_INT, 0, 1, MPI_COMM_WORLD, &status);
    }
}

MPI_Barrier(MPI_COMM_WORLD);

time = clock();
printf("Time: %d\n", time);

MPI_Finalize();

}

I did some testing and I have found that it works the way I want it to when I take out the for loop. So what is wrong with the for loop that causes either an infinite loop or seg fault?

Answer 1

Have you tried the streams benchmark ?

Answer 2

I believe the code would hang unless you used exactly two MPI processes.