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threadpi.cpp
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Neil Gershenfeld authoredNeil Gershenfeld authored
threadpi.cpp 1.22 KiB
//
// threadpi.cpp
// Neil Gershenfeld 3/1/20
// calculation of pi by a C++ thread sum
// pi = 3.14159265358979323846
//
#include <iostream>
#include <chrono>
#include <thread>
#include <vector>
unsigned int npts = 2e7;
unsigned int nthreads = std::thread::hardware_concurrency();
std::vector<double> results(nthreads);
void sum(int index) {
unsigned int start = npts*index+1;
unsigned int end = npts*(index+1)+1;
double result = 0;
for (unsigned int i = start; i < end; ++i)
result += 0.5/((i-0.75)*(i-0.25));
results[index] = result;
}
int main(void) {
double pi = 0;
std::thread threads[nthreads];
auto tstart = std::chrono::high_resolution_clock::now();
for (int i = 0; i < nthreads; ++i)
threads[i] = std::thread(sum,i);
for (int i = 0; i < nthreads; ++i) {
threads[i].join();
pi += results[i];
}
auto tend = std::chrono::high_resolution_clock::now();
auto dt = std::chrono::duration_cast<std::chrono::microseconds>(tend-tstart).count();
auto mflops = npts*nthreads*5.0/dt;
std::cout << "npts: " << npts << " nthreads: " << nthreads << " pi: " << pi << '\n';
std::cout << "time: " << 1e-6*dt << " estimated MFlops: " << mflops << '\n';
return 0;
}