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threadpi.cpp

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  • 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;
       }