cpi.c

并行计算的例子程序 环境需要lam/Linux 可从lam-mpi.org找到

/*
 * Copyright (c) 2001-2002 The Trustees of Indiana University.  
 *                         All rights reserved.
 * Copyright (c) 1998-2001 University of Notre Dame. 
 *                         All rights reserved.
 * Copyright (c) 1994-1998 The Ohio State University.  
 *                         All rights reserved.
 * 
 * This file is part of the LAM/MPI software package.  For license
 * information, see the LICENSE file in the top level directory of the
 * LAM/MPI source distribution.
 * 
 * $HEADER$
 *
 * $Id: cpi.c,v 1.4 2002/11/23 04:06:58 jsquyres Exp $
 *
 * Portions taken from the MPICH distribution example cpi.c.
 *
 * Example program to calculate the value of pi by integrating f(x) =
 * 4 / (1 + x^2).
 */

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


/* Constant for how many values we'll estimate */

#define NUM_ITERS 1000


/* Prototype the function that we'll use below. */

static double f(double);


int
main(int argc, char *argv[])
{
  int iter, rank, size, i;
  double PI25DT = 3.141592653589793238462643;
  double mypi, pi, h, sum, x;
  double startwtime = 0.0, endwtime;
  int namelen;
  char processor_name[MPI_MAX_PROCESSOR_NAME];

  /* Normal MPI startup */

  MPI_Init(&argc, &argv);
  MPI_Comm_size(MPI_COMM_WORLD, &size);
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  MPI_Get_processor_name(processor_name, &namelen);

  printf("Process %d of %d on %s\n", rank, size, processor_name);

  /* Do approximations for 1 to 100 points */

  for (iter = 2; iter < NUM_ITERS; ++iter) {
    h = 1.0 / (double) iter;
    sum = 0.0;
    
    /* A slightly better approach starts from large i and works back */

    if (rank == 0)
      startwtime = MPI_Wtime();

    for (i = rank + 1; i <= iter; i += size) {
      x = h * ((double) i - 0.5);
      sum += f(x);
    }
    mypi = h * sum;
    
    MPI_Reduce(&mypi, &pi, 1, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);
    
    if (rank == 0) {
      printf("%d points: pi is approximately %.16f, error = %.16f\n",
	     iter, pi, fabs(pi - PI25DT));
      endwtime = MPI_Wtime();
      printf("wall clock time = %f\n", endwtime - startwtime);
      fflush(stdout);
    }
  }

  /* All done */

  MPI_Finalize();
  return 0;
}


static double
f(double a)
{
  return (4.0 / (1.0 + a * a));
}