source: CIVL/examples/mpi-omp/mpi-omp-pie-calculation.c@ bb03188

/****************************************************************************
              C-DAC Tech Workshop : HeGaPa-2012
                     July 16-20,2012

  Example 2          :   Mpi-Omp_PI_Calculation.c
  
  Objective          : Write an MPI-OpenMP Program to compute numerical
                       integration of PI value
  
  Input              : The number of intervals.
  
  Output             : The calculated value of PI with Abs value.

  Created         : MAY-2012    
****************************************************************************/


#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include "mpi.h"
#include <omp.h>

#define INTERVALS 6 //Originally 1000


double 
func(double x)
{
  return (4.0 / (1.0 + x * x));
}

/* Main Program */

int 
main(int argc, char *argv[])
{
  int             NoIntervals, interval, ScatterSize, Noofthreads;
  int             MyRank, Numprocs,threadid,start,end;
  int             iproc, Root = 0, valid;
  int             Source, Source_tag;
  int             Destination, Destination_tag;
  double          PI25DT = 3.141592653589793238462643;
  double          mypi, pi, h, totalsum, x,partialsum,finalsum;
  MPI_Status      status;

  /* ....MPI Initialisation.... */
  MPI_Init(&argc, &argv);
  MPI_Comm_size(MPI_COMM_WORLD, &Numprocs);
  MPI_Comm_rank(MPI_COMM_WORLD, &MyRank);

  if (MyRank == Root) {
    printf("Enter The NoIntervals Of Intervals: ");
    /*               scanf("%d", &NoIntervals); */
    NoIntervals=INTERVALS;
  }
  MPI_Bcast(&NoIntervals, 1, MPI_INT, Root, MPI_COMM_WORLD);

  if (NoIntervals <= 0) {
    if (MyRank == Root)
      printf("Invalid Value For Number Of Intervals .....\n");
    MPI_Finalize();
    exit(-1);
  }
  /* Checking For Equal Division Of Intervals Across The Processors */

  if (NoIntervals % Numprocs != 0) {
    if (MyRank == Root)
      printf("NoofIntervals Cannot Be Evenly Distributed \n");
    MPI_Finalize();
    exit(0);
  }
  if (MyRank == Root)
    ScatterSize = NoIntervals / Numprocs;


  MPI_Bcast(&ScatterSize, 1, MPI_INT, Root, MPI_COMM_WORLD);

  /*printf("the ScatterSize is %d",ScatterSize);*/
  h = 1.0 / (double) NoIntervals;
  totalsum = 0.0;
  finalsum = 0.0;

  /* OpenMP Parallel For Directive  And Critical Section */

  partialsum=0.0;

  start=MyRank * ScatterSize + 1 ;
  end=MyRank * ScatterSize + ScatterSize; 

  omp_set_num_threads(4);
/*
Originally was ...
#pragma omp parallel for private(x,interval) 
*/
#pragma omp parallel for private(x,interval) reduction(+ : partialsum)
  for (interval = start; interval <= end; interval=interval+1)
    {
      x = h * ((double) interval - 0.5);
/*
Originally had ...
                #pragma omp critical
replaced with reduction
*/
      partialsum = partialsum + func(x);
      /*printf("the threadid Myrank and partial sum for each threadis %d %d %f\n",omp_get_thread_num(),MyRank,partialsum); */
    }

  mypi = partialsum * h;
  /* printf("the each process totalsum and mypi value for each rank is %f %f %d\n",totalsum,mypi,MyRank); */
  /* ....Collect The Areas Calculated In P0,P1.... */
  MPI_Reduce(&mypi, &pi, 1, MPI_DOUBLE, MPI_SUM, Root, MPI_COMM_WORLD);

  
  if (MyRank == Root) {
    printf("\nPi Is Approximately %.16f, Error Is %.16f \n",
           pi, fabs(pi - PI25DT));
  }
  /* MPI_Termination */

  MPI_Finalize();

}