How can I achieve reduction using OpenMP Tasks?

Question

I have this OpenMP code that performs a simple reduction:

for(k = 0; k < m; k++) 
{
      #pragma omp parallel for private(i) reduction(+:mysum) schedule(static) 
      for (i = 0; i < m; i++)
      {
           mysum += a[i][k] * a[i][k];
      }
}

I want to create a code equivalent to this one, but using OpenMP Tasks. Here is what I tried by following this article:

for(k = 0; k < m; k++) 
{
    #pragma omp parallel reduction(+:mysum)
    {
         #pragma omp single 
         {
                  for (i = 0; i < m; i++) 
                  {
                        #pragma omp task private(i) shared(k)
                        {
                                partialSum += a[i][k] * a[i][k];
                        }
                   }
         }

         #pragma omp taskwait
         mysum += partialSum;
     }
 }

The variable partialSum is declared as threadprivate and it's also a global variable:

int partialSum = 0;
#pragma omp threadprivate(partialSum)

a is a simple array of ints (m x m).

The problem is that when I run the code above (the one with tasks) multiple times, I get different results.

Do you have an idea on what should I change to make this work?

Thank you respectfully

Answer 1

private variables are uninitialized (at least not initialized by their outside value). i should be firstprivate.

If you just get rid of private(i) shared(k) everything is correct by default. k comes from outside of the parallel section and thus is implicitly shared in the parallel section. This also makes it implicitly shared in the task generating construct. Right now i is also shared/shared. If you define it locally instead, (for (int i...), it becomes implicitly private to the parallel section and thus implicitly firstprivate in the task generating construct.

You should also add

#pragma omp atomic
mysum += partialSum;

On the other hand, you don't necessarily need the taskwait (see this answer)

Note that the talk uses firstprivate correctly.