quad_mpi.c

# include <stdlib.h>
# include <stdio.h>
# include <math.h>
# include <time.h>

# include "mpi.h"

int main ( int argc, char *argv[] );
double f ( double x );
void timestamp ( void );

/******************************************************************************/

int main ( int argc, char *argv[] )

/******************************************************************************/
/*
  Purpose:

    MAIN is the main program for QUAD_MPI.

  Licensing:

    This code is distributed under the GNU LGPL license. 

  Modified:

    19 July 2010

  Author:

    John Burkardt
*/
{
  double a;
  double b;
  double error;
  double exact;
  int i;
  int master = 0;
  double my_a;
  double my_b;
  int my_id;
  int my_n;
  double my_total;
  int n;
  int p;
  int p_num;
  int source;
  MPI_Status status;
  int tag;
  int target;
  double total;
  double wtime;
  double x;

  a =  0.0;
  b = 10.0;
  n = 10000000;
  exact = 0.49936338107645674464;
/*
  Initialize MPI.
*/
  MPI_Init ( &argc, &argv );
/*
  Get this processor's ID.
*/
  MPI_Comm_rank ( MPI_COMM_WORLD, &my_id );
/*
  Get the number of processes.
*/
  MPI_Comm_size ( MPI_COMM_WORLD, &p_num );

  if ( my_id == master )
  {
/*
  We want N to be the total number of evaluations.
  If necessary, we adjust N to be divisible by the number of processes.
*/
    my_n = n / ( p_num - 1 );
    n = ( p_num - 1 ) * my_n;

    wtime = MPI_Wtime ( );

    timestamp ( );
    printf ( "\n" );
    printf ( "QUAD_MPI\n" );
    printf ( "  C/MPI version\n" );
    printf ( "  Estimate an integral of f(x) from A to B.\n" );
    printf ( "  f(x) = 50 / (pi * ( 2500 * x * x + 1 ) )\n" );
    printf ( "\n" );
    printf ( "  A = %f\n", a );
    printf ( "  B = %f\n", b );
    printf ( "  N = %d\n", n );
    printf ( "  EXACT = %24.16f\n", exact );
    printf ( "\n" );
    printf ( "  Use MPI to divide the computation among\n" );
    printf ( "  multiple processes.\n" );
  }

  source = master;
  MPI_Bcast ( &my_n, 1, MPI_INT, source, MPI_COMM_WORLD );
/*
  Process 0 assigns each process a subinterval of [A,B].
*/
  if ( my_id == master )
  {
    for ( p = 1; p <= p_num - 1; p++ )
    {
      my_a = ( ( double ) ( p_num - p     ) * a   
             + ( double ) (         p - 1 ) * b ) 
             / ( double ) ( p_num     - 1 );

      target = p;
      tag = 1;
      MPI_Send ( &my_a, 1, MPI_DOUBLE, target, tag, MPI_COMM_WORLD );

      my_b = ( ( double ) ( p_num - p - 1 ) * a   
             + ( double ) (         p     ) * b ) 
             / ( double ) ( p_num     - 1 );

      target = p;
      tag = 2;
      MPI_Send ( &my_b, 1, MPI_DOUBLE, target, tag, MPI_COMM_WORLD );
    }
    total = 0.0;
    my_total = 0.0;
  }
/*
  Processes receive MY_A, MY_B, and compute their part of the integral.
*/
  else
  {
    source = master;
    tag = 1;
    MPI_Recv ( &my_a, 1, MPI_DOUBLE, source, tag, MPI_COMM_WORLD, &status );

    source = master;
    tag = 2;
    MPI_Recv ( &my_b, 1, MPI_DOUBLE, source, tag, MPI_COMM_WORLD, &status );

    my_total = 0.0;
    for ( i = 1; i <= my_n; i++ )
    {
      x = ( ( double ) ( my_n - i     ) * my_a 
          + ( double ) (        i - 1 ) * my_b )
          / ( double ) ( my_n     - 1 );
      my_total = my_total + f ( x );
    }

    my_total = ( my_b - my_a ) * my_total / ( double ) ( my_n );

    printf ( "  Process %d contributed MY_TOTAL = %f\n", my_id, my_total );
  }
/*
  Each process sends its value to the master process.
*/
  MPI_Reduce ( &my_total, &total, 1, MPI_DOUBLE, MPI_SUM, master, MPI_COMM_WORLD );
/*
  Compute the weighted estimate.
*/
  if ( my_id == master )
  {
    error = fabs ( total - exact );
    wtime = MPI_Wtime ( ) - wtime;

    printf ( "\n" );
    printf ( "  Estimate = %24.16f\n", total );
    printf ( "  Error = %e\n\n", error );
    printf ( "  Time = %f\n\n", wtime );
  }
/*
  Terminate MPI.
*/
  MPI_Finalize ( );
/*
  Terminate.
*/
  if ( my_id == master )
  {
    printf ( "\n" );
    printf ( "QUAD_MPI:\n" );
    printf ( "  Normal end of execution.\n" );
    printf ( "\n" );
    timestamp ( );
  }

  return 0;
}
/******************************************************************************/

double f ( double x )

/******************************************************************************/
/*
  Purpose:

    F evaluates the function.
*/
{
  double pi;
  double value;

  pi = 3.141592653589793;
  value = 50.0 / ( pi * ( 2500.0 * x * x + 1.0 ) );

  return value;
}
/******************************************************************************/

void timestamp ( void )

/******************************************************************************/
/*
  Purpose:

    TIMESTAMP prints the current YMDHMS date as a time stamp.

  Example:

    31 May 2001 09:45:54 AM

  Licensing:

    This code is distributed under the GNU LGPL license. 

  Modified:

    24 September 2003

  Author:

    John Burkardt

  Parameters:

    None
*/
{
# define TIME_SIZE 40

  static char time_buffer[TIME_SIZE];
  const struct tm *tm;
  time_t now;

  now = time ( NULL );
  tm = localtime ( &now );

  strftime ( time_buffer, TIME_SIZE, "%d %B %Y %I:%M:%S %p", tm );

  printf ( "%s\n", time_buffer );

  return;
# undef TIME_SIZE
}