Lab 2:  Lab exercise for MPI Groups and Communicator Management
 

The attached below is an incomplete C-code. In this lab asignment, you are asked to:
 

1. Read through the program and identify which sections of code correspond to each of the elements of the program structure listed below.
2. Insert the appropriate calls to MPI routines at the points indicated.
3. Check if there is any error in the code.
4. Compile the program.
5. Specify how many nodes you wish to use (3).
6. Run the program.'
7. Check the correctness of your work by comparing the even-ranked output with file even.data and odd-ranked with file odd.data.

 
 

There are 2 parts to the project:

1. Build 2 groups, one for even-numbered tasks (starting with task 0), one for odd-numbered tasks.
2. Do a sum reduction for each group.

The comments in the lab code will direct you towards different solutions.

The program structure is as follows:
 

• Each node determines its rank in MPI_COMM_WORLD
• Two groups/communicators are created -- one consisting of even-ranked tasks and one of odd-ranked tasks. (Lab project)
• Each task calculates a floating-point value xsin
• The sum of xsin is accumulated in task 0 for each new communicator. (Lab project)
• The results for the even-ranked tasks is written to the file even.rep; for the odd-ranked tasks to odd.rep.

 

void set_group(MPI_Comm[], MPI_Group[], int);

      main(int argc, char **argv)
{
      int ntask, rank_in_world, rank_in_newgrp;
      MPI_Comm newcomm[2];
      MPI_Group newgrp[2];
      float xsin, xsum;
      FILE *fpeven, *fpodd;

      MPI_Init(&argc, &argv);
/*
   ==================================================================
    Establish number of tasks and rank of this task in MPI_COMM_WORLD
   ==================================================================
*/
      MPI_Comm_size(MPI_COMM_WORLD,&ntask);
      MPI_Comm_rank(MPI_COMM_WORLD,&rank_in_world);
/*
   ==============================
    Call routine to set up groups
   ==============================
*/
      set_group(newcomm, newgrp, ntask);
      xsin = fabs(sin(151.*(float)(rank_in_world+10)));
/*
  ==================================================================
    Project:  Sum all values of xsin on even-numbered tasks

    First:    Learn your rank in this new group (MPI_UNDEFINED if
              you are not a member), assign to the variable
              rank_in_newgrp
    Second:   Members accumulate the sum across the group, and store
              it as xsum on the task with rank 0
  ==================================================================
*/
      /*  =====>  Check membership here */

      if (rank_in_newgrp != MPI_UNDEFINED)  {

        /*  =====>  Accumulate sum here */

        if (rank_in_newgrp == 0)  {
          fpeven = fopen("even.rep","w");
          fprintf(fpeven,"Sum of even tasks: %12.5f\n",xsum);
          fclose (fpeven);  }
      }
/*
  ==================================================================
    Project:  Sum all values of xsin on odd-numbered tasks

    First:    Learn your rank in this new group (MPI_UNDEFINED if
              you are not a member), assign to the variable
              rank_in_newgrp
    Second:   Members accumulate the sum across the group, and store
              it as xsum on the task with rank 0
  ==================================================================
*/
      /*  =====>  Check membership here */
 
      if (rank_in_newgrp != MPI_UNDEFINED)  {

        /*  =====>  Accumulate sum here */

        if (rank_in_newgrp == 0)  {
          fpodd = fopen("odd.rep","w");
          fprintf(fpodd,"Sum of odd tasks: %12.5f\n",xsum);
          fclose (fpodd);  }
      }
      MPI_Finalize();
}

 
      void set_group(MPI_Comm newcomm[], MPI_Group newgrp[], int ntask)
      {
      int list[100], ktr, i;
      MPI_Group base_grp;
/*
   ==============================================================
    Project:  Get base group from MPI_COMM_WORLD communicator
              using the variable base_grp to hold the information
   ==============================================================
*/
      /*  =====>  Put code here   */
 
/*
   ================================================================
    Project:  Build a group containing even-numbered tasks
              and create the corresponding communicator.
              Put the new group information in the variable
              newgrp[0] and the communicator in variable newcomm[0]
   ================================================================
*/
      ktr = 0;
      for(i=0 ; i < ntask ; i=i+2) {
         list[ktr] = i;
         ktr ++;
         }
 
      /*  =====>  Put code here   */
 
/*
   ================================================================
    Project:  Build a group containing odd-numbered tasks
              and create the corresponding communicator.
              Put the new group information in the variable
              newgrp[1] and the communicator in variable newcomm[1]
   ================================================================
*/
      /*  =====>  Put code here   */

   }