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phys-743-exercises/pi/solutions-mpi/pi_gather_bcast.cc
8055887fc7b7students-2019

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pi_gather_bcast.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 <numeric>
#include <vector>
#include <mpi.h>
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;
MPI_Init(NULL, NULL);
int prank, psize;
MPI_Comm_rank(MPI_COMM_WORLD, &prank);
MPI_Comm_size(MPI_COMM_WORLD, &psize);
auto t1 = clk::now();
auto ln = n/psize + (prank < n % psize ? 1 : 0);
auto i_start = prank * ln + (prank < n % psize ? 0 : n % psize);
auto i_end = i_start + ln;
/* calculate pi = integral [0..1] 4 / (1 + x**2) dx */
dx = 1. / n;
double lsum = 0.0;
for (i = i_start; i < i_end; i++) {
x = 1. * i * dx;
lsum = lsum + f(x);
}
std::vector<double> partial_sums;
if (prank == 0) {
partial_sums.resize(psize);
}
MPI_Gather(&lsum, 1, MPI_DOUBLE, partial_sums.data(), 1, MPI_DOUBLE, 0, MPI_COMM_WORLD);
if (prank == 0) {
sum = std::accumulate(partial_sums.begin(), partial_sums.end(), 0);
}
pi = dx * sum;
MPI_Bcast(&sum, 1, MPI_DOUBLE, 0, MPI_COMM_WORLD);
second elapsed = clk::now() - t1;
if (prank == 0) {
std::printf("computed pi = %.16g\n", pi);
std::printf("wall clock time (chrono) = %.4gs\n", elapsed.count());
std::printf("n mpi process = %.d\n", psize);
for(int d = 1; d <= 15; ++d) {
std::printf("%d", digit(pi, d));
}
}
MPI_Finalize();
return 0;
}
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