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

  8:    Note that this example is not checking the error codes from the MPI calls before and after PETSc is initialized.
  9:    This is because the PETSc macro CHKERRQ() will not work in those circumstances. You should also check the 
 10:    MPI codes in this situation with your own code.
 11: T*/
 12:  #include <petscsys.h>

 14: int main(int argc, char *argv[])
 15: {
 17:   PetscMPIInt    rank, size;

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

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

 26:   /*
 27:     Every PETSc routine should begin with the PetscInitialize() routine.
 28:     argc, argv - These command line arguments are taken to extract the options
 29:                  supplied to PETSc and options supplied to MPI.
 30:     help       - When PETSc executable is invoked with the option -help,
 31:                  it prints the various options that can be applied at
 32:                  runtime.  The user can use the "help" variable place
 33:                  additional help messages in this printout.
 34:   */
 35:   PetscInitialize(&argc, &argv, (char*) 0, help);if (ierr) return ierr;

 37:   /*
 38:      The following MPI calls return the number of processes
 39:      being used and the rank of this process in the group.
 40:    */
 41:   MPI_Comm_size(PETSC_COMM_WORLD,&size);
 42:   MPI_Comm_rank(PETSC_COMM_WORLD,&rank);

 44:   /*
 45:      Here we would like to print only one message that represents
 46:      all the processes in the group.  We use PetscPrintf() with the
 47:      communicator PETSC_COMM_WORLD.  Thus, only one message is
 48:      printed representng PETSC_COMM_WORLD, i.e., all the processors.
 49:   */
 50:   PetscPrintf(PETSC_COMM_WORLD,"Number of processors = %d, rank = %d\n", size, rank);

 52:   /*
 53:      Always call PetscFinalize() before exiting a program.  This routine
 54:        - finalizes the PETSc libraries as well as MPI
 55:        - provides summary and diagnostic information if certain runtime
 56:          options are chosen (e.g., -log_view).  See PetscFinalize()
 57:      manpage for more information.
 58:   */
 59:   PetscFinalize();if (ierr) return ierr;

 61:   MPI_Comm_free(&PETSC_COMM_WORLD);if (ierr) return ierr;
 62:   /* Since we initialized MPI, we must call MPI_Finalize() */
 63:   MPI_Finalize();
 64:   return ierr;
 65: }


 68: /*TEST

 70:    test:
 71:       nsize: 5
 72:       args: -options_left no
 73:       filter: sort -b | grep -v saws_port_auto_selectcd
 74:       filter_output: sort -b

 76: TEST*/