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 rank, size, grank, zero, two
 12:   PetscReal globalrank

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

 16:   PetscCallMPIA(MPI_Init(ierr))
 17: #if defined(PETSC_HAVE_ELEMENTAL)
 18:   PetscCallA(PetscElementalInitializePackage(ierr))
 19: #endif
 20: !     We can now change the communicator universe for PETSc

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

 27: !     Every PETSc routine should begin with the PetscInitialize()
 28: !     routine.
 29:   PetscCallA(PetscInitializeNoArguments(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. Sleep so that IO from different ranks
 39: !     don't get mixed up. Note this is not an ideal solution
 40:   PetscCallMPIA(MPI_Comm_rank(MPI_COMM_WORLD, grank, ierr))
 41:   globalrank = grank
 42:   PetscCallA(PetscSleep(globalrank, ierr))
 43:   if (rank == 0) write (6, 100) size, rank
 44: 100 format('No of Procs = ', i4, ' rank = ', i4)

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

 52:   PetscCallA(PetscFinalize(ierr))
 53:   PetscCallMPIA(MPI_Comm_free(PETSC_COMM_WORLD, ierr))
 54: #if defined(PETSC_HAVE_ELEMENTAL)
 55:   PetscCallA(PetscElementalFinalizePackage(ierr))
 56: #endif

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

 60:   PetscCallMPIA(MPI_Finalize(ierr))
 61: end

 63: !/*TEST
 64: !
 65: !   test:
 66: !      nsize: 5
 67: !      filter: sort -b
 68: !      filter_output: sort -b
 69: !      requires: !cuda !saws
 70: !
 71: !TEST*/