A simple for-loop with openMP in C

Question

I have a for loop in the following code.

int min = -1;
int pos;
int array[100];

for(i = 0; i < 100; i++){
  if(array[i] < min || min == -1){
    min = array[i];
    pos = i;
  }
}

I think that the following code is a correct implementation with openMP but it is too slow.

int min = -1;
int pos;
int array[100];

#pragma omp parallel for default(none) shared(array, min) 
for(i = 0; i < 100; i++){
#pragma omp critical
  {
    if(array[i] < min || min == -1){
      min = array[i];
      pos = i;
    }
  }
}

I think that could be data hazards if i put the critical section inside the condition instead of outside. There is a smart way to implement it? Some suggestions?

If the whole body of the loop is a critical section, the parallel pragma is completely useless. — user3185968
– user3185968, Commented Mar 31, 2015 at 13:10
Of course it's worse than the serial implementation. It is serial, but adds thread creation and synchronisation overhead. Do you really think creating a new thread is faster than reading 100 ints? You might start to see speed up if you properly parallelized the loop and used it on a much larger array. — user3185968
– user3185968, Commented Mar 31, 2015 at 14:05

user3185968 · Accepted Answer · 2015-03-31 14:34:13Z

2

I've coded up a small parallel search function. I've only tested that it compiles, but I believe the principle is sound:

#include <stddef.h>
#define MINDIVIDE 1024

int parallelminsearch(int const *array, size_t size)
{
  int minimum;
  if (size < MINDIVIDE)
    {
      minimum = array[0];
      for (size_t i = 1; i < size; i++)
        {
          if (array[i] < minimum)
          minimum = array[i];
        }
      return minimum;
    }
  int pmin[2];
  #pragma omp parallel for
  for (size_t i = 0; i < 2; i++)
    {
      pmin[i] = parallelminsearch(&array[i*size/2], (size+1)/2);
    }
  minimum = (pmin[0] < pmin[1])?pmin[0]:pmin[1];
  return minimum;
}

answered Mar 31, 2015 at 14:34

user3185968

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