5.6.3.1 Example 1: Three-Group ``Pipeline"

Figure 5.1: Three-group pipeline.

Groups 0 and 1 communicate. Groups 1 and 2 communicate. Therefore, group 0 requires one inter-communicator, group 1 requires two inter-communicators, and group 2 requires 1 inter-communicator.

   main(int argc, char **argv)
   {
     MPI_Comm   myComm;       /* intra-communicator of local sub-group */
     MPI_Comm   myFirstComm;  /* inter-communicator */
     MPI_Comm   mySecondComm; /* second inter-communicator (group 1 only) */
     int membershipKey;
     int rank;

     MPI_Init(&argc, &argv);
     MPI_Comm_rank(MPI_COMM_WORLD, &rank);

     /* User code must generate membershipKey in the range [0, 1, 2] */
     membershipKey = rank % 3;

     /* Build intra-communicator for local sub-group */
     MPI_Comm_split(MPI_COMM_WORLD, membershipKey, rank, &myComm);

     /* Build inter-communicators.  Tags are hard-coded. */
     if (membershipKey == 0)
     {                     /* Group 0 communicates with group 1. */
       MPI_Intercomm_create( myComm, 0, MPI_COMM_WORLD, 1,
                            1, &myFirstComm);
     }
     else if (membershipKey == 1)
     {              /* Group 1 communicates with groups 0 and 2. */
       MPI_Intercomm_create( myComm, 0, MPI_COMM_WORLD, 0,
                            1, &myFirstComm);
       MPI_Intercomm_create( myComm, 0, MPI_COMM_WORLD, 2,
                            12, &mySecondComm);
     }
     else if (membershipKey == 2)
     {                     /* Group 2 communicates with group 1. */
       MPI_Intercomm_create( myComm, 0, MPI_COMM_WORLD, 1,
                            12, &myFirstComm);
     }

     /* Do work ... */

     switch(membershipKey)  /* free communicators appropriately */
     {
     case 1:
        MPI_Comm_free(&mySecondComm);
     case 0:
     case 2:
        MPI_Comm_free(&myFirstComm);
        break;
     }

     MPI_Finalize();
   }

MPI-Standard for MARMOT