c - Calculate the histogram with OpenMP

Question

I want to parallelize this code getting the best performance. "histogram" stores number of appareances of a certain colour (there are 10 different colours, so the size of histogram is 10). "img" is an array which stores a certain image information. In each index of img is stored a colour (int value, range 0..9). This is the code:

for( i=0; i<N1; i++ ){
  for( j=0; j<N2; j++ ){
    histogram[ img[i][j] ]  = histogram[ img[i][j] ] + 1;
  }
}

I tried this but the performance is so bad (worse than serial execution):

#pragma omp parallel for schedule(static, N1/nthreads) private(i,j)
for(i=0; i<N1; i++){
  for(j=0; j<N2; j++)
  {
    #pragma omp atomic
    histogram[img[i][j]]++;
  }
}

Any suggestions? Thank you.

Answer 1

With OpenMP 4.5, you can simply use a array reduction in C:

int histogram[BINS] = {0};
#pragma omp parallel for reduction(+:hist)
for(i=0; i<N1; i++) {
   for(j=0; j<N2; j++) {    
       histogram[img[i][j]]++;
   }
}