Calculating Fibonacci numbers

Overview

Teaching: 10 min
Exercises: 5 min

Questions

• How to recurse in parallel

Objectives

• Use general parallel tasks

The last technique we will look at is the task structure in OpenMP. In this case, you can use the task keyword within a parallel section to define sets of tasks that get queued up to run within the threads of the parallel section. They could run in any order. If you need to have results from the tasks before you can continue, you can use the taskwait keyword to tell OpenMP to pause your program until the tasks are done. This would look like the following:

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

int fib(int n) {
   int i, j;
   if (n<2)
      return n;
   else {
      #pragma omp task shared(i)
      i=fib(n-1);
      #pragma omp task shared(j)
      j=fib(n-2);
      #pragma omp taskwait
      return i+j;
   }
}

int main(int argc, char **argv){
   int n, result;
   char *a = argv[1];
   n = atoi(a);
   
   #pragma omp parallel
   {
      #pragma omp single
      result = fib(n);
   }
   printf("Result is %d\n", result);
}

In the main function, you need to start the parallel section so that all of the threads are launched. Since we only want the parent call to fib done once, we need to use the single keyword.

Task gotchas

There are a few gotchas to be aware of. While the intention is that tasks will run in parallel, there is nothing in the specification that guarantees this behavior. You may need to check how your particular environment works.

Tasks?

How do tasks work with different number of threads?

Key Points

• OpenMP can manage general parallel tasks