Why OpenMP version is slower?

Question

I am experimenting with OpenMP. I wrote some code to check its performance. On a 4-core single Intel CPU with Kubuntu 11.04, the following program compiled with OpenMP is around

Answer 1

The problem is that the variable k is considered to be a shared variable, so it has to be synced between the threads. A possible solution to avoid this is:

#include 
#include 

using namespace std;

int main ()
{
  long double i=0;

#pragma omp parallel for reduction(+:i)
  for(int t=1; t<30000000; t++){       
    long double k=0.7;
    for(int n=1; n<16; n++){
      i=i+pow(k,n);
    }
  }

  cout << i<<"\t";
  return 0;
}

Following the hint of Martin Beckett in the comment below, instead of declaring k inside the loop, you can also declare k const and outside the loop.

Otherwise, ejd is correct - the problem here does not seem bad parallelization, but bad optimization when the code is parallelized. Remember that the OpenMP implementation of gcc is pretty young and far from optimal.