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, zero, two

 14: !     We must call MPI_Init() first, making us, not PETSc, responsible
 15: !     for MPI

 17:   PetscCallMPIA(MPI_Init(ierr))

 19: !     We can now change the communicator universe for PETSc

 21:   zero = 0
 22:   two = 2
 23:   PetscCallMPIA(MPI_Comm_rank(MPI_COMM_WORLD, rank, ierr))
 24:   PetscCallMPIA(MPI_Comm_split(MPI_COMM_WORLD, mod(rank, two), zero, PETSC_COMM_WORLD, ierr))

 26: !     Every PETSc routine should begin with the PetscInitialize()
 27: !     routine.

 29:   PetscCallA(PetscInitialize(ierr))

 31: !     The following MPI calls return the number of processes being used
 32: !     and the rank of this process in the group.

 34:   PetscCallMPIA(MPI_Comm_size(PETSC_COMM_WORLD, size, ierr))
 35:   PetscCallMPIA(MPI_Comm_rank(PETSC_COMM_WORLD, rank, ierr))

 37: !     Here we would like to print only one message that represents all
 38: !     the processes in the group.
 39:   if (rank == 0) write (6, 100) size, rank
 40: 100 format('No of Procs = ', i4, ' rank = ', i4)

 42: !     Always call PetscFinalize() before exiting a program.  This
 43: !     routine - finalizes the PETSc libraries as well as MPI - provides
 44: !     summary and diagnostic information if certain runtime options are
 45: !     chosen (e.g., -log_view).  See PetscFinalize() manpage for more
 46: !     information.

 48:   PetscCallA(PetscFinalize(ierr))
 49:   PetscCallMPIA(MPI_Comm_free(PETSC_COMM_WORLD, ierr))

 51: !     Since we initialized MPI, we must call MPI_Finalize()

 53:   PetscCallMPIA(MPI_Finalize(ierr))
 54: end

 56: !
 57: !/*TEST
 58: !
 59: !   test:
 60: !
 61: !TEST*/