CODEWITHSUNDEEP
copenmp
user2320239
user2320239

Reputation: 1038

OpenMP average of an array

I'm trying to learn OpenMP for a program I'm writing. For part of it I'm trying to implement a function to find the average of a large array. Here is my code:

double mean(double* mean_array){
    double mean = 0;

    omp_set_num_threads( 4 );
    #pragma omp parallel for reduction(+:mean)



    for (int i=0; i<aSize; i++){
        mean = mean + mean_array[i];

    }

    printf("hello %d\n", omp_get_thread_num());



    mean = mean/aSize;

    return mean;

}

However if I run the code it runs slower than the sequential version. Also for the print statement I get:

hello 0
hello 0

Which doesn't make much sense to me, shouldn't there be 4 hellos?

Any help would be appreciated.

Upvotes: 1

Views: 2827

Answers (1)

simpel01
simpel01

Reputation: 1782

First, the reason why you are not seeing 4 "hello"s, is because the only part of the program which is executed in parallel is the so called parallel region enclosed within an #pragma omp parallel. In your code that is the loop body (since the omp parallel directive is attached to the for statement), the printf is in the sequential part of the program.

rewriting the code as follows would do the trick:

    double mean = 0;
    #pragma omp parallel num_threads(4)
    {
      #pragma omp for reduction(+:mean)
      for (int i=0; i<aSize; i++) {
         mean += mean_array[i];
      }
      mean /= aSize;
      printf("hello %d\n", omp_get_thread_num());
    }

Second, the fact your program runs slower than the sequential version, it can depend on multiple factors. First of all, you need to make sure the array is large enough so that the overhead of creating those threads (which usually happens when the parallel region is created) is negligible. Also, for small arrays you may be running into "cache false sharing" issues in which threads are competing for the same cache line causing performance degradation.

Upvotes: 2

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