omp max reduction with storage of index

问题

Using c++ openmp 3.1 I implemented a max reduction which stores the maximum value of integer variable (score) of an vector of objects (s). But I also want to store the vector index to acces the (s) object with the maximum score. My current unsuccesfull implementation looks like this:

//s is a vector of sol objects which contain apart from other variables an  integer     score    variable s[].score            
int bestscore = 0;
int bestant = 0;                
#pragma omp parallel shared(bestant)
{//start parallel session
    #pragma omp for    nowait reduction(max : bestscore)
    for (int ant = 0; ant<maxsols; ++ant) // for all ants
    {
        //procedures on s[ant] object which update the  int s[ant].score
        if (s[ant].score > bestscore)
        {
            //find the object with the highest score
            bestscore = s[ant].score;
            bestant = ant;//i also want know which ant has the highest score
        }
    }
}

The code compiles and runs. the maximum bestscore is found but bestant gets a random index. The ant linked to the fastest thread to finish gets stored in bestant. bestscore start with a value of 0 so in most cases s[ant].score will have a higher score and bestscore and bestant are updated. I think I need a reduction operator for bestant like "on update of bestscore".

回答1:

Try this

int bestscore = 0;
int bestant = 0;
#pragma omp parallel
{
    int bestscore_private = 0;
    int bestant_private = 0;
    #pragma omp for nowait
    for (int ant = 0; ant<maxsols; ++ant) {         
        if (s[ant].score > bestscore_private) {
            bestscore_private = s[ant].score;
            bestant_private = ant;
        }
    }
    #pragma omp critical 
    {
        if(bestscore_private>bestscore) {
            bestscore = bestscore_private;
            bestant = besant_private;
        }
    }
}

回答2:

Two observations:

I think you only need to compare bestscore_private to bestscore when its value is changed, this way the number of comparisons is reduced.

Also, at least in todays omp you can utilize a critical section inside the if conditional, this way comparisons between bestscore_private a bestscore would be carried out in parallel and the infrequent (hope so) update to bestscore will be carried out in a critical manner.

int bestscore = 0; int bestant = 0; #pragma omp parallel {

  int bestscore_private = 0;
  int bestant_private = 0;
  #pragma omp for nowait
  for (int ant = 0; ant<maxsols; ++ant) {         
      if (s[ant].score > bestscore_private) {
          bestscore_private = s[ant].score;
          bestant_private = ant;   
          if(bestscore_private>bestscore){   
            #pragma omp critical 
            {

            bestscore = bestscore_private;
            bestant = besant_private;
            }
          }
      }
  }

}

来源：https://stackoverflow.com/questions/24782038/omp-max-reduction-with-storage-of-index

标签

c++

reduction

openmp