OpenMP implementation of reduction

Question

I need to implement reduction operation (for each thread the value should be stored in different array entry). However, it runs slower for more threads. Any suggestions?

Answer 1

Did you try to use reduction?

double global_sum = 0.0;
#pragma omp parallel for shared(h,n,a) reduction(+:global_sum) 
for (i = 1; i < n; i++) {
    global_sum += f(a  + i* h);
}

Howerver there may be a lot of other reasons why it runs slow. For example you should not create 16 threads if you have only 2 CPU cores and so on.