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pi_gather.cc
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Wed, May 8, 06:21

pi_gather.cc
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/*
This exercise is taken from the class Parallel Programming Workshop (MPI,
OpenMP and Advanced Topics) at HLRS given by Rolf Rabenseifner
*/
#include <chrono>
#include <cstdio>
#include <cmath>
#include <mpi.h>
#include <vector>
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, pi;
int psize, prank;
MPI_Init(NULL, NULL);
MPI_Comm_size(MPI_COMM_WORLD, &psize);
MPI_Comm_rank(MPI_COMM_WORLD, &prank);
auto mpi_t1 = MPI_Wtime();
auto t1 = clk::now();
int nlocal = n / psize;
int istart = 1 + nlocal * prank;
int iend = nlocal * (prank + 1);
/* calculate pi = integral [0..1] 4 / (1 + x**2) dx */
dx = 1. / n;
sum = 0.0;
for (i = istart; i <= iend; i++) {
x = (1. * i - 0.5) * dx;
sum = sum + f(x);
}
if(prank == 0) {
std::vector<double> sums(psize);
MPI_Gather(&sum, 1, MPI_DOUBLE, sums.data(), 1, MPI_DOUBLE, 0, MPI_COMM_WORLD);
sum = 0.;
for(auto s : sums) {
sum += s;
}
} else {
MPI_Gather(&sum, 1, MPI_DOUBLE, NULL, 0, MPI_DOUBLE, 0, MPI_COMM_WORLD);
}
MPI_Bcast(&sum, 1, MPI_DOUBLE, 0, MPI_COMM_WORLD);
pi = dx * sum;
auto mpi_elapsed = MPI_Wtime() - mpi_t1;
second elapsed = clk::now() - t1;
if(prank == 0) {
std::printf("computed pi = %.16g\n", pi);
std::printf("wall clock time (mpi_wtime) = %.4gs with %d process\n", mpi_elapsed, psize);
std::printf("wall clock time (chrono) = %.4gs\n", elapsed.count());
for(int d = 1; d <= 15; ++d) {
std::printf("%d", digit(pi, d));
}
}
MPI_Finalize();
return 0;
}

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