OpenMP for PI calculation test

openmp_pi.cpp

#include <iostream>
#include <omp.h>
#include <ctime>

using namespace std;

#define N 3000000000L

int main() {
    omp_set_num_threads(8);
    auto c = time(NULL);
    double pi = 0;

    omp_set_num_threads(8);
#pragma omp parallel for reduction(+: pi)
    for (long i = 0; i < N; ++i) {
        pi += 1 / (double) (i * 2 + 1) * (i & 1 ? -1 : 1);
    }
    cout << "Parallel 8 cores:" << (time(NULL) - c) << endl;

    pi = 0;
    c = time(NULL);
    omp_set_num_threads(4);
#pragma omp parallel for reduction(+: pi)
    for (long i = 0; i < N; ++i) {
        pi += 1 / (double) (i * 2 + 1) * (i & 1 ? -1 : 1);
    }
    cout << "Parallel 4 cores:" << (time(NULL) - c) << endl;

    pi = 0;
    c = time(NULL);
    omp_set_num_threads(2);
#pragma omp parallel for reduction(+: pi)
    for (long i = 0; i < N; ++i) {
        pi += 1 / (double) (i * 2 + 1) * (i & 1 ? -1 : 1);
    }
    cout << "Parallel 2 cores:" << (time(NULL) - c) << endl;

    pi = 0;
    c = time(NULL);
    for (long i = 0; i < N; ++i) {
        pi += 1 / (double) (i * 2 + 1) * (i & 1 ? -1 : 1);
    }
    cout << "Serial:  " << (time(NULL) - c) << endl;
    return 0;
}

output on HP DV6 7002tx
I7 3612QM

Parallel 8 cores:4
Parallel 4 cores:8
Parallel 2 cores:17
Serial: 32