MPI Hands-On; Collective Communications I

1. Broadcasting an integer value to all of the MPI processes, A program code10.c that reads an integer value from the terminal and distributes the value to all of the MPI processes.
   • Each process should print out its rank and the value it received. Values should be read until a negative integer is given as input.
   • You may find it helpful to include a fflush( stdout) to the code; after the printf calls in your program. Without this, output may not appear when you expect it.

       1 #include <stdio.h>
       2 #include "mpi.h"
       3 
       4 int main( int argc, char **argv )
       5 {
       6   int rank, value;
       7   MPI_Init( &argc, &argv );
       8 
       9   MPI_Comm_rank( MPI_COMM_WORLD, &rank );
      10   do {
      11     if (rank == 0) 
      12       scanf( "%d", &value );
      13 
      14     MPI_Bcast( &value, 1, MPI_INT, 0, MPI_COMM_WORLD );
      15         
      16     printf( "Process %d got %d\n", rank, value );
      17     fflush(stdout);
      18         
      19   } while (value >= 0);
      20 
      21   MPI_Finalize( );
      22   return 0;
      23 }
   

2. Broadcasting the name of the master process, A program code11.c that first broadcasts the name of the master process then each nodes send hello messages to master node.

       1 #include <stdio.h>
       2 #include <string.h>
       3 #include <mpi.h>
       4 
       5 #define TRUE 1
       6 #define FALSE 0
       7 #define MASTER_RANK 0
       8 
       9 int main( int argc, char *argv[] )
      10 {
      11   int count, pool_size, my_rank, my_name_length, i_am_the_master = FALSE;
      12   char my_name[BUFSIZ/2], master_name[BUFSIZ/2], send_buffer[2*BUFSIZ],
      13     recv_buffer[2*BUFSIZ];
      14   MPI_Status status;
      15 
      16   MPI_Init(&argc, &argv);
      17   MPI_Comm_size(MPI_COMM_WORLD, &pool_size);
      18   MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
      19   MPI_Get_processor_name(my_name, &my_name_length);
      20 
      21   if (my_rank == MASTER_RANK) {
      22     i_am_the_master = TRUE;
      23     strcpy (master_name, my_name);
      24   }
      25 
      26   MPI_Bcast(master_name, BUFSIZ, MPI_CHAR, MASTER_RANK, MPI_COMM_WORLD);
      27 
      28   sprintf(send_buffer, "hello %s, greetings from %s, rank = %d",
      29 	  master_name, my_name, my_rank);
      30   MPI_Send (send_buffer, strlen(send_buffer) + 1, MPI_CHAR,
      31 	    MASTER_RANK, 0, MPI_COMM_WORLD);
      32 
      33   if (i_am_the_master) {
      34     for (count = 1; count <= pool_size; count++) {
      35       MPI_Recv (recv_buffer, BUFSIZ, MPI_CHAR, MPI_ANY_SOURCE, MPI_ANY_TAG,
      36 		MPI_COMM_WORLD, &status);
      37       printf ("%s\n", recv_buffer);
      38     }
      39   }
      40 
      41   MPI_Finalize();
      42 }
   

3. Computation of PI number with collective communications. This example code12.c evaluates $\pi$ by numerically evaluating the integral
   $$\int_0^1\frac{1}{1+x^2}dx=\frac{\pi}{4}$$
   This code computes PI (with a very simple method) but does not use MPI_Send and MPI_Recv. Instead, it uses collective operations to send data to and from all of the running processes.
   • The routine MPI_Bcast sends data from one process to all others.
   • The routine MPI_Reduce combines data from all processes (by adding them in this case), and returning the result to a single process.

       1 #include <stdio.h>
       2 #include "mpi.h"
       3 #include <math.h>
       4 
       5 int main( int argc, char *argv[] )
       6 {
       7   int done = 0, n, myid, numprocs, i, rc;
       8   double PI25DT = 3.141592653589793238462643;
       9   double mypi, pi, h, sum, x, a;
      10 
      11   MPI_Init(&argc,&argv);
      12   MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
      13   MPI_Comm_rank(MPI_COMM_WORLD,&myid);
      14   while (!done)
      15   {
      16     if (myid == 0) {
      17         printf("Enter the number of intervals: (0 quits) ");
      18         scanf("%d",&n);
      19     }
      20     MPI_Bcast(&n, 1, MPI_INT, 0, MPI_COMM_WORLD);
      21     if (n == 0) break;
      22   
      23     h   = 1.0 / (double) n;
      24     sum = 0.0;
      25     for (i = myid + 1; i <= n; i += numprocs) {
      26         x = h * ((double)i - 0.5);
      27         sum += 4.0 / (1.0 + x*x);
      28     }
      29     mypi = h * sum;
      30     
      31     MPI_Reduce(&mypi, &pi, 1, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);
      32     
      33     if (myid == 0)
      34         printf("pi is approximately %.16f, Error is %.16f\n",
      35                pi, fabs(pi - PI25DT));
      36   }
      37   MPI_Finalize();
      38 }