Question

I have code that looks something like this:

for(i=0; i<max;i++){
   for(j=0; j<max2;j++)
     //stuff
}

for(i=0; i<max;i++){
   for(j=0; j<max2;j++)
     //other stuff
}

for(i=0; i<max;i++){
   for(j=0; j<max2;j++)
     //final stuff
}

I want to parallelize this using OpenMP. What’s the best method? I tried doing the #pragma omp parallel private(i) at the beginning and #pragma omp for before every j loop. This is what I mean:

 #pragma omp parallel private(i)
 {
 for(i=0; i<max;i++){
   #pragma omp for
   for (j=0; j<max2;j++){
   //and so on and so forth

The thing is, this gives me no performance boost whatsoever. I suspect this is because the 3 for loops aren’t run in parallel…if I could get those 3 to run at the same time, I think I could get a performance boost. Any ideas? Thanks!

Answer 1

A quick fix is to make a iterative section and parallel this:

#pragma omp for
for (k=0;k<3;k++){
  if (k==0) do_stuff();
  if (k==1) do_other_stuff();
  if (k==2) do_other_other_stuff();
}

A better fix is to use omp sections directive. (Solution taken from here)

#pragma omp parallel sections
{
  #pragma omp section
  {
     /* Executes in thread 1 */
     do_stuff();
  } 
  #pragma omp section
  {
    /* Executes in thread 2 */ 
    do_other_stuff();   
  } 
  #pragma omp section
  {
    /* Executes in thread 3 */
    do_other_other_stuff();
  }   
}