In OpenMP how can we run in parallel multiple code blocks where each block contains omp single and omp for loops?

Question

In C++ Openmp how could someone run in parallel multiple code blocks where each block contains omp single and omp for loops? More precisely, I have 3 functions:

block1();
block2();
block3();

I want each of these 3 functions to run in parallel. However I do not want each one of these functions to be assigned a single thread. If I wanted each one of them to use a single thread I could enclose them in three "#pragma omp single nowait" followed by a "#pragma barrier" at the end. Instead each one of these three functions may look something like this:

#pragma omp single
{
  //some code here
}

#pragma omp for nowait
for(std::size_t i=0;i<numloops;i++)
{
  //some code here
}

Notice in the above code that I need an omp single region to be executed before each parallel for loop. If I did not have this constraint I could have simply added a "nowait" to the "omp single". Instead because I have the "omp single" without a "nowait" I do not want block2() to have to wait for the "omp single" region in block1() to complete. Nor do I want block3() to have to wait for the "omp single" region in block2() to complete. Any ideas? Thanks

Laci · Accepted Answer · 2021-10-15 18:50:55Z

2

The best solution is using tasks. Run each block() in different tasks, so they run parallel:

#pragma omp parallel
#pragma omp single nowait
{
#pragma omp task
    block1();
#pragma omp task
    block2();
#pragma omp task
    block3();
}

In block() you can set some code, which is executed before the for loop and you can use taskloop to distribute work among the available threads.

void block1()
{
   //single thread code here
  {
     //.... this code runs before the loop and independent of block2 and block3
  }

  #pragma omp taskloop
  for(std::size_t i=0;i<numloops;i++)
  {
    //some code here - this is distributed among the remaining threads
  }
    
}

answered Oct 15, 2021 at 18:50

Laci

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