1: !
  2: !     This introductory example illustrates running PETSc on a subset
  3: !     of processes
  4: !
  5: ! -----------------------------------------------------------------------
  6: #include <petsc/finclude/petscsys.h>
  7: program main
  8:   use petscsys
  9:   implicit none

 11:   PetscErrorCode ierr
 12:   PetscMPIInt rank, size
 13:   PetscMPIInt :: zero = 0, two = 2

 15: ! We must call MPI_Init() first, making us, not PETSc, responsible
 16: ! for MPI
 17:   PetscCallMPIA(MPI_Init(ierr))

 19: ! We can now change the communicator universe for PETSc
 20:   PetscCallMPIA(MPI_Comm_rank(MPI_COMM_WORLD, rank, ierr))
 21:   PetscCallMPIA(MPI_Comm_split(MPI_COMM_WORLD, mod(rank, two), zero, PETSC_COMM_WORLD, ierr))

 23: ! Every PETSc routine should begin with the PetscInitialize()
 24: ! routine.
 25:   PetscCallA(PetscInitialize(ierr))

 27: ! The following MPI calls return the number of processes being used
 28: ! and the rank of this process in the group.
 29:   PetscCallMPIA(MPI_Comm_size(PETSC_COMM_WORLD, size, ierr))
 30:   PetscCallMPIA(MPI_Comm_rank(PETSC_COMM_WORLD, rank, ierr))

 32: ! Here we would like to print only one message that represents all
 33: ! the processes in the group.
 34:   if (rank == 0) write (6, 100) size, rank
 35: 100 format('No of Procs = ', i4, ' rank = ', i4)

 37: ! Always call PetscFinalize() before exiting a program.  This
 38: ! routine - finalizes the PETSc libraries as well as MPI - provides
 39: ! summary and diagnostic information if certain runtime options are
 40: ! chosen (e.g., -log_view).  See PetscFinalize() manpage for more
 41: ! information.
 42:   PetscCallA(PetscFinalize(ierr))
 43:   PetscCallMPIA(MPI_Comm_free(PETSC_COMM_WORLD, ierr))

 45: ! Since we initialized MPI, we must call MPI_Finalize()
 46:   PetscCallMPIA(MPI_Finalize(ierr))
 47: end

 49: !
 50: !/*TEST
 51: !
 52: !   test:
 53: !
 54: !TEST*/