CODEWITHSUNDEEP
clinuxopenmp
Dervin Thunk
Dervin Thunk

Reputation: 20119

Improving Performance in OpenMP program

Suppose an array arr of SIZE=128Mb with values from 0 to 128Mb-1. Now suppose the following code:

#pragma omp parallel num_threads(NUM_THREADS)
{
  int me = omp_get_thread_num();
  odds_local[me] = 0;
  int count = 0;

#pragma omp for 
  for (int i = 0; i < SIZE; i++)
      if (arr[i]%2 != 0)
    count++;

  odds_local[me] = count;
}

and finally a loop that iterates over the values of odds_local[me] to get the final result. For this, if I time it and report user time in Linux I get 0.97s for both 1 thread and 2 threads. That is to say, no speedup whatsoever.

Is there anything I should be improving in this program to better the speedup?

Upvotes: 0

Views: 116

Answers (1)

Lubo Antonov
Lubo Antonov

Reputation: 2318

I ran your exact code and with 1 thread I get 390ms, with 2 I get 190ms. Your problem is not in the code. It has to be something basic. These are the things I can think of:

  • not linking with OpenMP (with g++ filename -fopenmp);
  • running on a single core machine;
  • running on a dual core, with something else occupying the other core;
  • timing something more than this loop, which is dominating the calculation.

Upvotes: 1

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