OpenMP program is slower than sequential one

Question

When I try the following code

double start = omp_get_wtime();

long i;

#pragma omp parallel for
    for (i = 0; i <= 1000000000; i++) {
        double x = rand();
    }

    double end = omp_get_wtime();

    printf("%f\n", end - start);

Execution time is about 168 seconds, while the sequential version only spends 20 seconds.

I'm still a newbie in parallel programming. How could I get a parallel version that's faster that the sequential one?

Answer 1

The random number generator rand(3) uses global state variables (hidden in the (g)libc implementation). Access to them from multiple threads leads to cache issues and also is not thread safe. You should use the rand_r(3) call with seed parameter private to the thread:

long i;
unsigned seed;

#pragma omp parallel private(seed)
{
    // Initialise the random number generator with different seed in each thread
    // The following constants are chosen arbitrarily... use something more sensible
    seed = 25234 + 17*omp_get_thread_num();
    #pragma omp for
    for (i = 0; i <= 1000000000; i++) {
       double x = rand_r(&seed);
    }
}

Note that this will produce different stream of random numbers when executed in parallel than when executed in serial. I would also recommend erand48(3) as a better (pseudo-)random number source.