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
 10:   PetscErrorCode ierr
 11:   PetscMPIInt, parameter :: zero = 0, two = 2
 12:   PetscMPIInt rank, size, grank
 13:   PetscReal globalrank

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

 17:   PetscCallMPIA(MPI_Init(ierr))
 18: #if defined(PETSC_HAVE_ELEMENTAL)
 19:   PetscCallA(PetscElementalInitializePackage(ierr))
 20: #endif
 21: ! We can now change the communicator universe for PETSc
 22:   PetscCallMPIA(MPI_Comm_rank(MPI_COMM_WORLD, rank, ierr))
 23:   PetscCallMPIA(MPI_Comm_split(MPI_COMM_WORLD, mod(rank, two), zero, PETSC_COMM_WORLD, ierr))

 25: ! Every PETSc routine should begin with the PetscInitialize()
 26: ! routine.
 27:   PetscCallA(PetscInitializeNoArguments(ierr))

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

 34: ! Here we would like to print only one message that represents all
 35: ! the processes in the group. Sleep so that IO from different ranks
 36: ! don't get mixed up. Note this is not an ideal solution
 37:   PetscCallMPIA(MPI_Comm_rank(MPI_COMM_WORLD, grank, ierr))
 38:   globalrank = grank
 39:   PetscCallA(PetscSleep(globalrank, ierr))
 40:   if (rank == 0) write (6, 100) size, rank
 41: 100 format('No of Procs = ', i4, ' rank = ', i4)

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

 49:   PetscCallA(PetscFinalize(ierr))
 50:   PetscCallMPIA(MPI_Comm_free(PETSC_COMM_WORLD, ierr))
 51: #if defined(PETSC_HAVE_ELEMENTAL)
 52:   PetscCallA(PetscElementalFinalizePackage(ierr))
 53: #endif

 55: ! Since we initialized MPI, we must call MPI_Finalize()
 56:   PetscCallMPIA(MPI_Finalize(ierr))
 57: end

 59: !/*TEST
 60: !
 61: !   test:
 62: !      nsize: 5
 63: !      filter: sort -b
 64: !      filter_output: sort -b
 65: !      requires: !cuda !saws
 66: !
 67: !TEST*/