How to parallelise a code inside a while using OpenMP

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I am trying to parallelise the heat_plate algorithm but I am stuck at this bit of code inside my while:

while(1)
{
.....
.....

#pragma omp parallel shared(diff, u, w) private(i, j, my_diff)
{
  my_diff = 0.0;
  #pragma omp for 
    for (i = 1; i < M - 1; i++) 
    {
      for (j = 1; j < N - 1; j++)
      {
        if ( my_diff < fabs (w[i][j] - u[i][j]))
        {
          my_diff = fabs (w[i][j] - u[i][j]);
        }
      }
    }
    #pragma omp critical
    {
      if (diff < my_diff)
      {
        diff = my_diff;
      }
    }
}

....
....
}

Not only I can't get it to work in parallel it actually takes longer to finish

Edit:

The Program runs in parallel .

Thank you in advance for your help.

openmp parallel-processing multithreading c performance
1 Answers

In OpenMP this data dependency:

for (i = 1; i < M - 1; i++) 
  for (j = 1; j < N - 1; j++)
    if ( my_diff < fabs (w[i][j] - u[i][j]))
        my_diff = fabs (w[i][j] - u[i][j]);

is typically solved using OpenMP reduction feature, which in your case will avoid the critical region (after the parallel for) and, consequently, improve the overall performance of the parallelization. So if you apply that feature your code would look like the following:

#pragma omp parallel shared(u, w) private(i, j)
{
  #pragma omp for reduction(max:diff)
  for (i = 1; i < M - 1; i++) 
     for (j = 1; j < N - 1; j++)
        if ( diff < fabs (w[i][j] - u[i][j]))
             diff = fabs (w[i][j] - u[i][j]);
}

In turn you can merge both pragmas into one:

  #pragma omp parallel for reduction(max:diff) shared(u, w) private(i, j)
  for (i = 1; i < M - 1; i++) 
     for (j = 1; j < N - 1; j++)
        if ( diff < fabs (w[i][j] - u[i][j]))
             diff = fabs (w[i][j] - u[i][j]);
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