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 with CHKERRQ() before PETSc is initialized
  9:    and after PETSc is finalized. This is because the PETSc macro CHKERRQ() will not work in those circumstances.
 10: T*/
 11: #include <petscsys.h>

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

 18:   /* We must call MPI_Init() first, making us, not PETSc, responsible for MPI */
 19:   MPI_Init(&argc, &argv);if (ierr) return ierr;
 20: #if defined(PETSC_HAVE_ELEMENTAL)
 21:   PetscElementalInitializePackage();if (ierr) return ierr;
 22: #endif
 23:   /* We can now change the communicator universe for PETSc */
 24:   MPI_Comm_rank(MPI_COMM_WORLD, &rank);if (ierr) return ierr;
 25:   MPI_Comm_split(MPI_COMM_WORLD, rank%2, 0, &PETSC_COMM_WORLD);if (ierr) return ierr;

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

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

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

 53:   /*
 54:      Always call PetscFinalize() before exiting a program.  This routine
 55:        - finalizes the PETSc libraries as well as MPI
 56:        - provides summary and diagnostic information if certain runtime
 57:          options are chosen (e.g., -log_view).  See PetscFinalize()
 58:      manpage for more information.
 59:   */
 60:   PetscFinalize();if (ierr) return ierr;
 61:   MPI_Comm_free(&PETSC_COMM_WORLD);if (ierr) return ierr;
 62: #if defined(PETSC_HAVE_ELEMENTAL)
 63:   PetscElementalFinalizePackage();if (ierr) return ierr;
 64: #endif
 65:   /* Since we initialized MPI, we must call MPI_Finalize() */
 66:   MPI_Finalize();
 67:   return ierr;
 68: }

 70: /*TEST

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

 78: TEST*/