[MPI] Solution 7 : Game

This material is part of the course Practical Introduction to Parallel Programming and meant for educational purposes only.

Task

Implement a game in which two groups of processes compete with each other by drawing random numbers.

The game proceeds in \(N\) rounds in which the two groups of processes generate a random number. The group with the largest number wins that round.

To generate a number per group, all processes in that group generate a random number independently. The largest number across all processes in the group is compared with the random number generated in a similar way by the other group.

The process with rank zero (within MPI_COMM_WORLD) should keep track of the number of wins for each group.

Hints

Use the provided function next_random to generate the stream of random numbers.

Test the program with 2, 3, 4, 5, 6, 7 and 8 processes. Do the results match your expectations?

Note that this exercise is not so much about performance but about correctness.

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Question 1: Game

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


/* ---------------------------------------------------------------------
 * next_random()
 *
 * Returns a uniform random number in the range [0,1].
 * ------------------------------------------------------------------ */


double                    next_random

  ( void )

{
  static int  initialized = 0;
  int         next;

  if ( ! initialized )
  {
    unsigned int  seed = (unsigned int) time ( NULL );
    int           my_rank;
    int           flag;

    MPI_Initialized ( &flag );

    if ( flag )
    {
      MPI_Comm_rank ( MPI_COMM_WORLD, &my_rank );

      seed += (unsigned int) (my_rank * 347);
    }

    srand ( seed );

    initialized = 1;
  }

  next = rand ();

  return ((double) next / (double) RAND_MAX);
}


/* ---------------------------------------------------------------------
 * main()
 * ------------------------------------------------------------------ */


int                       main

  ( int                     argc,
    char**                  argv )

{
  const int  N    = 100;
  const int  root = 0;
  const int  tag  = 1;

  MPI_Comm   comm;
  double     number;
  double     max_num;
  double     recv_num;
  int        proc_count;
  int        my_rank;
  int        color;
  int        wins[2];
  int        iiter;


  MPI_Init      ( &argc, &argv );
  MPI_Comm_size ( MPI_COMM_WORLD, &proc_count );
  MPI_Comm_rank ( MPI_COMM_WORLD, &my_rank );

  if ( proc_count < 2 )
  {
    fprintf ( stderr, "At least two processes are required.\n" );
    return 1;
  }

  wins[0] = 0;
  wins[1] = 0;
  color   = my_rank % 2;

  MPI_Comm_split ( MPI_COMM_WORLD, color, my_rank, &comm );

  if ( my_rank == 0 )
  {
    printf ( "Starting game ...\n" );
  }

  for ( iiter = 0; iiter < N; iiter++ )
  {
    number = next_random ();

    /* Determine the largest random number in this group. */

    MPI_Reduce ( &number, &max_num, 1, MPI_DOUBLE,
                 MPI_MAX, root, comm );

    /* Determine which group wins this round. */

    if      ( my_rank == 0 )
    {
      MPI_Recv ( &recv_num, 1, MPI_DOUBLE, 1, tag,
                 MPI_COMM_WORLD, MPI_STATUS_IGNORE );

      if      ( recv_num > max_num )
      {
        wins[1]++;
      }
      else if ( recv_num < max_num )
      {
        wins[0]++;
      }
    }
    else if ( my_rank == 1 )
    {
      MPI_Send ( &max_num, 1, MPI_DOUBLE, 0, tag,
                 MPI_COMM_WORLD );
    }
  }

  if ( my_rank == 0 )
  {
    printf ( "Score after %d rounds:\n\n", N );
    printf ( "  group 1 : %d wins\n",   wins[0] );
    printf ( "  group 2 : %d wins\n\n", wins[1] );
  }

  MPI_Comm_free ( &comm );
  MPI_Finalize  ();

  return 0;
}

Question 2: Number of MPI processes

Specify the number of MPI processes (1-64) that should be used for running your program (mpirun -np <nproc>)

Question 3: Command Line Argument

Specify the command line arguments that should be passed to your program when it is run. Arguments must be given in quotes and separated by commas. Leave this filed empty if you do not want to specify command line arguments.

Example: "1","int","arg=2" is interpreted as three arguments 1, int, and arg=2.

Author(s)	Erik-Jan Lingen, Matthias Möller
Deadline	No deadline
Submission limit	No limitation

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