Boost Random and OpenMP

Question

I\'m getting a \"bus error\" from an OpenMP parallel section of code. I recreated a simple version of my problem below. The code essentially makes many calls to the function

Answer 1

You have two options:

• have individual random number generators for each thread and seed them differently
• use mutual exclusion

First, an example of mutual exclusion:

# pragma omp parallel for
for (int bb=0; bb<10000; bb++)
{
    for (int i=0; i<20000; i++)
    {
        // enter critical region, disallowing simulatneous access to eng
        #pragma omp critical
        {
            int a = uniform_distribution(0,20000);
        }
        // presumably some more code...
    }
    // presumably some more code...
}

Next, an example of thread-local storage with seeding:

# pragma omp parallel
{
    // declare and seed thread-specific generator
    boost::random::mt19937 eng(omp_get_thread_num());
    #pragma omp for
    for (int bb=0; bb<10000; bb++)
    {
        for (int i=0; i<20000; i++)
        {
            int a = uniform_distribution(0,20000, eng);
            // presumably some more code...
        }
        // presumably some more code...
    }
}

Both of these snippets are just illustrative, depending on your requirements (say security related vs. a game vs. modelling) you may want to pick one over the other. You will probably also want to change the exact implementation to suit your usage. For instance, how you seed the generator is important if you want it to be either repeatable or closer to truly random (whether that's possible is system specific). This applies to both solutions equally (though to get reproducibility in the mutual exclusion case is harder).

The thread-local generator may run faster while the mutual exclusion case should use less memory.

EDIT: To be clear, the mutual exclusion solutions only makes sense if the generation of the random numbers is not the bulk of the thread's work (that is // presumably some more code... in the example exists and doesn't take a trivial amount of time to complete). The critical section only needs to encompass the access to the shared variable, changing your architecture a little would allow you finer control over that (and in the thread-local storage case, could also allow you to avoid passing an eng reference around)