mpi.c

#include <stdio.h>
#include <assert.h>
#include <math.h>
#include <mpi.h>

const double PI = 3.1415926535897932;
const long STEP_NUM = 1070596096;
const double STEP_LENGTH = 1.0 / 1070596096;

int main (int argc, char* argv[])
{
  double pi, sum = 0.0;

  MPI_Init (&argc, &argv);

  int rank, size;
  // get process ID
  MPI_Comm_rank (MPI_COMM_WORLD, &rank);

  // get processes number
  MPI_Comm_size (MPI_COMM_WORLD, &size);

  // synchronize all processes and get the begin time
  MPI_Barrier(MPI_COMM_WORLD);

  if(rank == 0)
    printf("\nStart calculating with %d processes...\n", size);

  double startTime = MPI_Wtime();

  double result = 0.0;
  double x;
  // each process will caculate a part of the sum
  for (int i = rank * STEP_NUM / size; i < rank * STEP_NUM / size + STEP_NUM / size; i ++)
  {
    x = (i + 0.5) * STEP_LENGTH;
    result += 1.0 / (1.0 + x * x);
  }

  // sum up all results
  MPI_Reduce(&result, &sum, 1, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);

  // synchronize all processes and get the end time
  MPI_Barrier(MPI_COMM_WORLD);
  double endTime = MPI_Wtime();

  // caculate and print PI
  if (rank == 0)
  {
    pi = STEP_LENGTH * sum * 4;
    printf("PI = %.16lf with error %.16lf\nTime elapsed : %lf seconds.\n\n", pi, fabs(pi - PI), (endTime - startTime));
    assert(fabs(pi - PI) <= 0.001);
  }

  MPI_Finalize();

  return 0;
}