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Wed, Sep 11, 03:02
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rSCINTROPAR Introduction to parallelism
pi.cc
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#include <chrono>
#include <cmath>
#include <cstdio>
#if defined(_OPENMP)
#include <omp.h>
#endif
using clk = std::chrono::high_resolution_clock;
using second = std::chrono::duration<double>;
using time_point = std::chrono::time_point<clk>;
inline int digit(double x, int n) {
return std::trunc(x * std::pow(10., n)) -
std::trunc(x * std::pow(10., n - 1)) * 10.;
}
inline double f(double a) { return (4. / (1. + a * a)); }
const int n = 10000000;
int main(int /* argc */, char ** /* argv */) {
int i;
double dx, x, sum, lsum, pi;
#if defined(_OPENMP)
int num_threads = omp_get_max_threads();
#endif
#if defined(_OPENMP)
auto omp_t1 = omp_get_wtime();
#endif
auto t1 = clk::now();
sum = 0.;
#pragma omp parallel shared(sum) private(x, lsum)
{
/* calculate pi = integral [0..1] 4 / (1 + x**2) dx */
dx = 1. / n;
lsum = 0.0;
#pragma omp for
for (i = 1; i <= n; i++) {
x = (1. * i - 0.5) * dx;
lsum = lsum + f(x);
}
#pragma omp critical
sum += lsum;
}
pi = dx * sum;
#if defined(_OPENMP)
auto omp_elapsed = omp_get_wtime() - omp_t1;
#endif
second elapsed = clk::now() - t1;
std::printf("computed pi = %.16g\n", pi);
#if defined(_OPENMP)
std::printf("wall clock time (omp_get_wtime) = %.4gs on %d threads\n",
omp_elapsed, num_threads);
#endif
std::printf("wall clock time (chrono) = %.4gs\n", elapsed.count());
for (int d = 1; d <= 15; ++d) {
std::printf("%d", digit(pi, d));
}
return 0;
}
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