OpenMP循环并行化

Question

I'm learning OpenMP and have some problems: Parallel program slower than serial, i'm confusing (1 thread vs 2 threads) My code:

#include <iostream>
#include <omp.h>
using namespace std;

int main()
{       
    int threadsNumber=1;
    int S=0;

    cout << "Enter number of threads:\n";
    cin >> threadsNumber;

    double start, end, calculationTime;
    omp_set_num_threads(threadsNumber);
    start = omp_get_wtime();

    #pragma omp parallel for reduction(+: S)
    for(int i=1;i<1000;i++) {
        S+= 10;
    }
    #pragma omp end parallel

    end = omp_get_wtime();

    calculationTime = end - start;

    cout << "Время выполнения: " << calculationTime << "\n";
    cout<<"S = "<< S <<"\n";

    return 0;
}

Results: 1 thread: 2.59876e-05 2 threads: 0.000102043

Where my mistake? Thank you!

Answer 1

As JF Sebastian pointed out in a comment, you don't get much benefit from parallelization because your loop with 1000 iterations is rather quick. That means the overhead it takes to create a 2nd thread is larger than what you save due to parallelization. When you increase the number of loop iterations and thus give the threads more to do, the benefit of multi-threading becomes more apparent.