Spawning a bunch of tasks at once, recursively, in OpenMP

Question

What I'm trying to do is spawn N tasks at once, by recursively dividing the iteration space with the help of tasks, in order to spawn the 'real' tasks quicker.

I can do this linearly with a loop, like so:

for (int i = 0; i < N; i+=bx)
#pragma omp task firstprivate(i)
    task_work(i);

Below is what I've got so far in my recursive version.

void rec_spawn(int start, int end, int cz)
{
    // If the iteration space lower than the chunksize 'cz', spawn the tasks 
    // linearly. Otherwise divide the iteration space in two, and call this 
    // function recursively.
    if (end - start <= cz)
    {
        for (int ii=start_outer; ii < end_outer; ii+=bx)
            #pragma omp task firstprivate(ii)
            task_work(ii)
    }
    else
    {
        // first half
        #pragma omp task firstprivate(start, end, cz)
        rec_spawn(start, start + ((end - start) / 2), cz);

        // second half
        #pragma omp task firstprivate(start, end, cz)
        rec_spawn(start + ((end - start) / 2), end, cz);
    }
     #pragma omp taskwait
}

This version is much slower, which it shouldn't be, and I suspect it is due to the #pragma omp taskwait. I want to be able to do something similarly but without the taskwait, however when I try to remove it the code segfaults.

When i try to debug the program all information I can collect is:

Program terminated with signal 11, Segmentation fault.
#0  0x00000000004057de in gomp_barrier_handle_tasks (state=<optimized out>) at ../../../gcc-4.9.0/libgomp/task.c:715
715 ../../../gcc-4.9.0/libgomp/task.c: No such file or directory.
(gdb) bt
#0  0x00000000004057de in gomp_barrier_handle_tasks (state=<optimized out>) at ../../../gcc-4.9.0/libgomp/task.c:715
#1  0x0000000000409518 in gomp_team_barrier_wait_end (bar=0x11a2874, state=0) at ../../../gcc-4.9.0/libgomp/config/linux/bar.c:94
#2  0x0000000000401d24 in main._omp_fn.0 () at src/heat-omp-rec.c:86
#3  0x000000000040705e in gomp_barrier_init (count=<optimized out>, bar=<optimized out>) at ../../../gcc-4.9.0/libgomp/config/linux/bar.h:59
#4  gomp_new_team (nthreads=4201738) at ../../../gcc-4.9.0/libgomp/team.c:166
#5  0x00007fff7d208d50 in ?? ()
#6  0x00007f7080cb69c0 in ?? ()
#7  0x0000000000000000 in ?? ()

So my question is, why is the taskwait required here (it is not required by the actual work inside task_work()), and how can the recursive spawning be rewritten to not use it.

You seem to be doing a taskwait at each level, whereas I think what you want is to create all the tasks, then wait for all of them. I think you could achieve that by having a separate "spawn" function that just invoked rec_spawn and then had a barrier. — Jim Cownie, Jul 03 '14 at 15:58

Spawning a bunch of tasks at once, recursively, in OpenMP

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