MPI_Finalize() doesn't finalize

c, linux, mpi

Solution

Well, sure. Look at what's going on here:

        if (rank == 0) {
            printf("Enter the number of intervals: (0 quits) \n");
            scanf("%d",&n);
        }
        if (n == 0) break;

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

If you enter '0' on rank 0, then rank 0 breaks, but all the other ranks have whatever they had in n last time around (or, in the case of the first iteration, whatever random stuff was in that memory location). So ranks 1-(N-1) enter the `MPI_Bcast`, patiently waiting to participate in a broadcast to find out what `n` is, and meanwhile rank 0 has exited the loop and is sitting at `MPI_Finalize` wondering where everyone else is.

You just need to flip those lines:

        MPI_Bcast(&n, 1, MPI_INT, 0, MPI_COMM_WORLD);
        if (n == 0) break;

Now everyone decides whether or not to break out of the loop at the same time.

Problem

``` #include <stdio.h> #include <math.h> #include <mpi.h> int main(int argc, char *argv[]) { int i, done = 0, n; double PI25DT = 3.141592653589793238462643; double pi, tmp, h, sum, x; int numprocs, rank; MPI_Status status; MPI_Init(&argc, &argv); MPI_Comm_size(MPI_COMM_WORLD, &numprocs); MPI_Comm_rank(MPI_COMM_WORLD, &rank); if (numprocs < 1) printf("numprocs = %d, should be run with numprocs > 1\n", numprocs); else { while (!done) { if (rank == 0) { printf("Enter the number of intervals: (0 quits) \n"); scanf("%d",&n); } if (n == 0) break; MPI_Bcast(&n, 1, MPI_INT, 0, MPI_COMM_WORLD); h = 1.0 / (double) n; sum = 0.0; for (i = 1 + rank; i <= n; i+=numprocs) { x = h * ((double)i - 0.5); sum += 4.0 / (1.0 + x*x); } MPI_Reduce(&sum, &tmp, 1, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD); if (rank == 0) { pi = h * tmp; printf("pi is approximately %.16f, Error is %.16f\n", pi, fabs(pi - PI25DT)); } } } MPI_Finalize(); } ``` I run this program with number of processes = 10 and after typing 0 in scanf("%d", &n); the program doesn't seem to finalize and I have to close it with Ctrl+C. 10 total processes killed (some possibly by mpirun during cleanup) mpirun: clean termination accomplished Appears in console after killing process.

Original source