How to fork a large number of threads in OpenMP? - c

Question

for some reason I need to stress my processor and I want to fork a lot of threads in OpenMP. In pthreads you can easily do it using a for loop since it is forking a thread is just a function call. But in OpenMP you have to have something like this:
#pragma omp parallel sections
{
#pragma omp section
{
//section 0
}
#pragma omp section
{
//section 1
}
.... // repeat omp section for n times
}
I am just wondering if there is any easier way to fork a large number of threads in OpenMP?

Answer 1

You don't need to do anything special, almost. Just write code for a compute-intensive task and put it inside a parallel region. Then indicate what number of threads you want. In order to do that, you use omp_set_dynamic(0) to disable dynamic threads (this helps to achieve the number of threads you want, but it still won't be guaranteed), then omp_set_num_threads(NUM_THREADS) to indicate what number of threads you want.
Then each thread will clone the task you indicate in the code. Simple as that.
const int NUM_THREADS = 100;
omp_set_dynamic(0);
omp_set_num_threads(NUM_THREADS);
#pragma omp parallel
{
// How many threads did we really get? Let's write it once only.
#pragma omp single
{
cout << "using " << omp_get_num_threads() << " threads." << std::endl;
}
// write some compute-intensive code here
// (be sure to print the result at the end, so that
// the compiler doesn't throw away useless instructions)
}

Answer 2

To do what you want, you get the thread number and then do different things based on which thread you are.
// it's not guaranteed you will actually get this many threads
omp_set_num_threads( NUM_THREADS );
int actual_num_threads;
#pragma omp parallel
{
#pragma omp single
{
actual_num_threads = omp_get_num_threads();
}
int me = omp_get_thread_num();
if ( me < actual_num_threads / 2 ) {
section1();
}
else {
section2();
}
}