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

  3: #include <petscsys.h>

  5: int main(int argc, char *argv[])
  6: {
  7:   PetscMPIInt rank, size;

  9:   /* We must call MPI_Init() first, making us, not PETSc, responsible for MPI */
 10:   PetscCallMPI(MPI_Init(&argc, &argv));
 11: #if defined(PETSC_HAVE_ELEMENTAL)
 12:   PetscCall(PetscElementalInitializePackage());
 13: #endif
 14:   /* We can now change the communicator universe for PETSc */
 15:   PetscCallMPI(MPI_Comm_rank(MPI_COMM_WORLD, &rank));
 16:   PetscCallMPI(MPI_Comm_split(MPI_COMM_WORLD, rank % 2, 0, &PETSC_COMM_WORLD));

 18:   /*
 19:     Every PETSc routine should begin with the PetscInitialize() routine.
 20:     argc, argv - These command line arguments are taken to extract the options
 21:                  supplied to PETSc and options supplied to MPI.
 22:     help       - When PETSc executable is invoked with the option -help,
 23:                  it prints the various options that can be applied at
 24:                  runtime.  The user can use the "help" variable place
 25:                  additional help messages in this printout.
 26:   */
 27:   PetscFunctionBeginUser;
 28:   PetscCall(PetscInitialize(&argc, &argv, NULL, help));

 30:   /*
 31:      The following MPI calls return the number of processes
 32:      being used and the rank of this process in the group.
 33:    */
 34:   PetscCallMPI(MPI_Comm_size(PETSC_COMM_WORLD, &size));
 35:   PetscCallMPI(MPI_Comm_rank(PETSC_COMM_WORLD, &rank));

 37:   /*
 38:      Here we would like to print only one message that represents
 39:      all the processes in the group.  We use PetscPrintf() with the
 40:      communicator PETSC_COMM_WORLD.  Thus, only one message is
 41:      printed representng PETSC_COMM_WORLD, i.e., all the processors.
 42:   */
 43:   PetscCall(PetscPrintf(PETSC_COMM_WORLD, "Number of processors = %d, rank = %d\n", size, rank));

 45:   /*
 46:      Always call PetscFinalize() before exiting a program.  This routine
 47:        - finalizes the PETSc libraries as well as MPI
 48:        - provides summary and diagnostic information if certain runtime
 49:          options are chosen (e.g., -log_view).  See PetscFinalize()
 50:      manpage for more information.
 51:   */
 52:   PetscCall(PetscFinalize());
 53:   PetscCallMPI(MPI_Comm_free(&PETSC_COMM_WORLD));
 54: #if defined(PETSC_HAVE_ELEMENTAL)
 55:   PetscCall(PetscElementalFinalizePackage());
 56: #endif
 57:   /* Since we initialized MPI, we must call MPI_Finalize() */
 58:   PetscCallMPI(MPI_Finalize());
 59:   return 0;
 60: }

 62: /*TEST

 64:    test:
 65:       nsize: 5
 66:       args: -options_left no
 67:       filter: sort -b | grep -v saws_port_auto_selectcd
 68:       filter_output: sort -b

 70: TEST*/