CODEWITHSUNDEEP
mpibenchmarking
Rylan Yancey
Rylan Yancey

Reputation: 53

MPI_Wtime() confusion - does not appear correct

I'm using MPI_Wtime() to measure the speed of a parallel application.

Running the application on 4 cores completes in 0.000061 (takes around 30 seconds)

Running on 50 cores, 0.000308. (instantaneous)

Multiplying the workload 10x, still on 50 cores, the time is 0.000752. (around a 2 minutes irl)

int main(int argc, char* argv[]) {

    ofstream file;
    file.open("primes.txt");
    file.close();

    MPI_Init(&argc, &argv);
    MPI_Status status;

    int rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    if (rank == 0)
        t1 = MPI_Wtime();

    int size;
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    if (rank == 0)
        Parent parent(size);
    else
        Child child(size, rank);

    if (rank == 0) {
        t2 = MPI_Wtime();
    }

    MPI_Finalize();

    if (rank == 0) 
        printf("Runtime = %f\n", t2 - t1);

}

Parent contains a loop to manage children.

These numbers do not make any sense. What am I doing wrong?

MPI_Wtick() is 1e-9

Upvotes: 0

Views: 266

Answers (1)

Rylan Yancey
Rylan Yancey

Reputation: 53

Thanks to @Giles Gouaillardet and @Victor Eijkhout for answering.

After moving t1 and t2 to be local and adding MPI_Barrier before each recording of the time, I was able to get an answer that made sense.

Running the code on 4 cores gave a result of 20.277840, which sounds correct.

Before, this same test gave a result of 0.000061, which does not make any sense at all.

Thank you.

Upvotes: 1

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