Question

I’m having trouble adjusting my thinking to suit OpenMP’s way of doing things.

Roughly, what I want is:

for(int i=0; i<50; i++)
{
   doStuff();
   thread t;
   t.start(callback(i)); //each time around the loop create a thread to execute callback
}

I think I know how this would be done in c++11, but I need to be able to accomplish something similar with OpenMP.

Answer 1

The closest thing to what you want are OpenMP tasks, available in OpenMP v3.0 and later compliant compilers. It goes like:

#pragma omp parallel
{
    #pragma omp single
    for (int i = 0; i < 50; i++)
    {
        doStuff();
        #pragma omp task
        callback(i);
    }
}

This code will make the loop execute in one thread only and it will create 50 OpenMP tasks that will call callback() with different parameters. Then it will wait for all tasks to finish before exiting the parallel region. Tasks will be picked (possibly at random) by idle threads to be executed. OpenMP imposes an implicit barrier at the end of each parallel region since its fork-join execution model mandates that only the main thread runs outside of parallel regions.

Here is a sample program (ompt.cpp):

#include <stdio.h>
#include <unistd.h>
#include <omp.h>

void callback (int i)
{
   printf("[%02d] Task stated with thread %d\n", i, omp_get_thread_num());
   sleep(1);
   printf("[%02d] Task finished\n", i);
}

int main (void)
{
   #pragma omp parallel
   {
      #pragma omp single
      for (int i = 0; i < 10; i++)
      {
         #pragma omp task
         callback(i);
         printf("Task %d created\n", i);
      }
   }
   printf("Parallel region ended\n");

   return 0;
}

Compilation and execution:

$ g++ -fopenmp -o ompt.x ompt.cpp
$ OMP_NUM_THREADS=4 ./ompt.x
Task 0 created
Task 1 created
Task 2 created
[01] Task stated with thread 3
[02] Task stated with thread 2
Task 3 created
Task 4 created
Task 5 created
Task 6 created
Task 7 created
[00] Task stated with thread 1
Task 8 created
Task 9 created
[03] Task stated with thread 0
[01] Task finished
[02] Task finished
[05] Task stated with thread 2
[04] Task stated with thread 3
[00] Task finished
[06] Task stated with thread 1
[03] Task finished
[07] Task stated with thread 0
[05] Task finished
[08] Task stated with thread 2
[04] Task finished
[09] Task stated with thread 3
[06] Task finished
[07] Task finished
[08] Task finished
[09] Task finished
Parallel region ended

Note that tasks are not executed in the same order they were created in.

GCC does not support OpenMP 3.0 in versions older than 4.4. Unrecognised OpenMP directives are silently ignored and the resulting executable will that code section in serial:

$ g++-4.3 -fopenmp -o ompt.x ompt.cpp
$ OMP_NUM_THREADS=4 ./ompt.x
[00] Task stated with thread 3
[00] Task finished
Task 0 created
[01] Task stated with thread 3
[01] Task finished
Task 1 created
[02] Task stated with thread 3
[02] Task finished
Task 2 created
[03] Task stated with thread 3
[03] Task finished
Task 3 created
[04] Task stated with thread 3
[04] Task finished
Task 4 created
[05] Task stated with thread 3
[05] Task finished
Task 5 created
[06] Task stated with thread 3
[06] Task finished
Task 6 created
[07] Task stated with thread 3
[07] Task finished
Task 7 created
[08] Task stated with thread 3
[08] Task finished
Task 8 created
[09] Task stated with thread 3
[09] Task finished
Task 9 created
Parallel region ended