1: static char help[] = "Introductory example that illustrates running PETSc on a subset of processes.\n\n";

  3: /*T
  4:    Concepts: introduction to PETSc;
  5:    Concepts: process^subset set PETSC_COMM_WORLD
  6:    Processors: 2
  7: T*/
  8:  #include <petscsys.h>

 10: int main(int argc, char *argv[])
 11: {
 13:   PetscMPIInt    rank, size;

 15:   /* We must call MPI_Init() first, making us, not PETSc, responsible for MPI */
 16:   MPI_Init(&argc, &argv);

 18:   /* We can now change the communicator universe for PETSc */
 19:   MPI_Comm_rank(MPI_COMM_WORLD, &rank);
 20:   MPI_Comm_split(MPI_COMM_WORLD, rank%2, 0, &PETSC_COMM_WORLD);

 22:   /*
 23:     Every PETSc routine should begin with the PetscInitialize() routine.
 24:     argc, argv - These command line arguments are taken to extract the options
 25:                  supplied to PETSc and options supplied to MPI.
 26:     help       - When PETSc executable is invoked with the option -help,
 27:                  it prints the various options that can be applied at
 28:                  runtime.  The user can use the "help" variable place
 29:                  additional help messages in this printout.
 30:   */
 31:   PetscInitialize(&argc, &argv, (char*) 0, help);

 33:   /*
 34:      The following MPI calls return the number of processes
 35:      being used and the rank of this process in the group.
 36:    */
 37:   MPI_Comm_size(PETSC_COMM_WORLD,&size);
 38:   MPI_Comm_rank(PETSC_COMM_WORLD,&rank);

 40:   /*
 41:      Here we would like to print only one message that represents
 42:      all the processes in the group.  We use PetscPrintf() with the
 43:      communicator PETSC_COMM_WORLD.  Thus, only one message is
 44:      printed representng PETSC_COMM_WORLD, i.e., all the processors.
 45:   */
 46:   PetscPrintf(PETSC_COMM_WORLD,"Number of processors = %d, rank = %d\n", size, rank);

 48:   /*
 49:      Always call PetscFinalize() before exiting a program.  This routine
 50:        - finalizes the PETSc libraries as well as MPI
 51:        - provides summary and diagnostic information if certain runtime
 52:          options are chosen (e.g., -log_summary).  See PetscFinalize()
 53:      manpage for more information.
 54:   */
 55:   PetscFinalize();

 57:   /* Since we initialized MPI, we must call MPI_Finalize() */
 58:   MPI_Finalize();
 59:   return 0;
 60: }