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

/*T
   Concepts: introduction to PETSc;
   Concepts: process^subset set PETSC_COMM_WORLD
   Processors: 2

   Note that this example is not checking the error codes from the MPI calls with CHKERRQ() before PETSc is initialized
   and after PETSc is finalized. This is because the PETSc macro CHKERRQ() will not work in those circumstances. 
T*/
 #include <petscsys.h>

int main(int argc, char *argv[])
{
  PetscErrorCode ierr;
  PetscMPIInt    rank, size;

  /* We must call MPI_Init() first, making us, not PETSc, responsible for MPI */
  ierr = MPI_Init(&argc, &argv);if (ierr) return ierr;

  /* We can now change the communicator universe for PETSc */
  ierr = MPI_Comm_rank(MPI_COMM_WORLD, &rank);if (ierr) return ierr;
  ierr = MPI_Comm_split(MPI_COMM_WORLD, rank%2, 0, &PETSC_COMM_WORLD);if (ierr) return ierr;

  /*
    Every PETSc routine should begin with the PetscInitialize() routine.
    argc, argv - These command line arguments are taken to extract the options
                 supplied to PETSc and options supplied to MPI.
    help       - When PETSc executable is invoked with the option -help,
                 it prints the various options that can be applied at
                 runtime.  The user can use the "help" variable place
                 additional help messages in this printout.
  */
  ierr = PetscInitialize(&argc, &argv, (char*) 0, help);if (ierr) return ierr;

  /*
     The following MPI calls return the number of processes
     being used and the rank of this process in the group.
   */
  ierr = MPI_Comm_size(PETSC_COMM_WORLD,&size);CHKERRQ(ierr);
  ierr = MPI_Comm_rank(PETSC_COMM_WORLD,&rank);CHKERRQ(ierr);

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

  /*
     Always call PetscFinalize() before exiting a program.  This routine
       - finalizes the PETSc libraries as well as MPI
       - provides summary and diagnostic information if certain runtime
         options are chosen (e.g., -log_view).  See PetscFinalize()
     manpage for more information.
  */
  ierr = PetscFinalize();if (ierr) return ierr;

  ierr = MPI_Comm_free(&PETSC_COMM_WORLD);if (ierr) return ierr;
  /* Since we initialized MPI, we must call MPI_Finalize() */
  ierr = MPI_Finalize();
  return ierr;
}


/*TEST

   test:
      nsize: 5
      args: -options_left no
      filter: sort -b | grep -v saws_port_auto_selectcd
      filter_output: sort -b

TEST*/