All OpenMP Tasks running on the same thread

c++, multithreading, openmp, task

Solution

Actual Problem:

You are using Visual Studio 2013.

Visual Studio has never supported OMP versions beyond 2.0 (see here).

OMP Tasks are a feature of OMP 3.0 (see spec).

Ergo, using VS at all means no OMP tasks for you.

If OMP Tasks are an essential requirement, use a different compiler. If OMP is not an essential requirement, you should consider an alternative parallel task handling library. Visual Studio includes the MS Concurrency Runtime, and the Parallel Patterns Library built on top of it. I have recently moved from OMP to PPL due to the fact I'm using VS for work; it isn't quite a drop-in replacement but it is quite capable.

My second attempt at solving this, again preserved for historical reasons:

So, the problem is almost certainly that you're defining your `omp task`s outside of a `omp parallel` region.

Here's a contrived example:

void work()
{
    #pragma omp parallel
    {
        #pragma omp single nowait
        for (int i = 0; i < 5; i++)
        {
            #pragma omp task untied
            {
                std::cout << 
                    "starting task " << i << 
                    " on thread " << omp_get_thread_num() << "\n";

                sleep(1);
            }
        }
    }
}

If you omit the `parallel` declaration, the job runs serially:

starting task 0 on thread 0
starting task 1 on thread 0
starting task 2 on thread 0
starting task 3 on thread 0
starting task 4 on thread 0

But if you leave it in:

starting task starting task 3 on thread 1
starting task 0 on thread 3
2 on thread 0
starting task 1 on thread 2
starting task 4 on thread 2

Success, complete with authentic misuse of shared output resources.

(for reference, if you omit the `single` declaration, each thread will run the loop, resulting in 20 tasks being run on my 4 cpu VM).

Original answer included below for completeness, but no longer relevant!

In every case, your `omp task` is a single, simple thing. It probably runs and completes immediately:

#pragma omp task shared(p), untied
cout << omp_get_thread_num();

#pragma omp task shared(p), untied
cout << omp_get_thread_num();

#pragma omp task shared(p), untied
cout << omp_get_thread_num();

#pragma omp task shared(p), untied
cout << omp_get_thread_num();

Because you never start one long-running task before firing off the next task, everything will probably run on the first allocated thread.

Perhaps you meant to do something like this?

if (a > 0 && b > 0 && p[a] == -1 && p[b] == -1)
{
    #pragma omp task shared(p), untied
    {
        cout << omp_get_thread_num();
        p[a] = parallelOMP(a);
    }

    #pragma omp task shared(p), untied
    {
        cout << omp_get_thread_num();
        p[b] = parallelOMP(b);
    }

    #pragma omp taskwait

    alpha = p[a];
    beta = p[b];
}

Problem

Original source