1: !
  2: !
  3: ! Description: Illustrates the use of VecCreateGhost()
  4: !
  5: !
  6: !      Ghost padding is one way to handle local calculations that
  7: !      involve values from other processors. VecCreateGhostBlock() provides
  8: !      a way to create vectors with extra room at the end of the vector
  9: !      array to contain the needed ghost values from other processors,
 10: !      vector computations are otherwise unaffected.
 11: !
 12: #include <petsc/finclude/petscvec.h>
 13: program main
 14:   use petscvec
 15:   implicit none

 17:   PetscMPIInt size, rank
 18:   PetscInt, parameter :: nghost = 2, bs = 2, nlocal = bs*6
 19:   PetscInt i, rstart, rend, ifrom(2)
 20:   PetscBool flag
 21:   PetscErrorCode ierr
 22:   PetscScalar value, tarray(20)
 23:   Vec lx, gx, gxs
 24:   PetscViewer singleton

 26:   PetscCallA(PetscInitialize(ierr))
 27:   PetscCallMPIA(MPI_Comm_rank(PETSC_COMM_WORLD, rank, ierr))
 28:   PetscCallMPIA(MPI_Comm_size(PETSC_COMM_WORLD, size, ierr))

 30:   PetscCheckA(size == 2, PETSC_COMM_WORLD, PETSC_ERR_WRONG_MPI_SIZE, 'Requires 2 processors')

 32: !
 33: !     Construct a two dimensional graph connecting nlocal degrees of
 34: !     freedom per processor. From this we will generate the global
 35: !     indices of needed ghost values
 36: !
 37: !     For simplicity we generate the entire graph on each processor:
 38: !     in real application the graph would stored in parallel, but this
 39: !     example is only to demonstrate the management of ghost padding
 40: !     with VecCreateGhost().
 41: !
 42: !     In this example we consider the vector as representing
 43: !     degrees of freedom in a one dimensional grid with periodic
 44: !     boundary conditions.
 45: !
 46: !        ----Processor  1-----------  ----Processor 2 --------
 47: !         0 1 2 3 4 5 6 7 8 9 10 11    12 13 14 15 16 17 18 19 20 21 22 23
 48: !                              |--|----|---|
 49: !         |-|--------------------------------------------------------|--|
 50: !

 52:   if (rank == 0) then
 53:     ifrom = [11, 6]
 54:   else
 55:     ifrom = [0, 5]
 56:   end if

 58: !     Create the vector with two slots for ghost points. Note that both
 59: !     the local vector (lx) and the global vector (gx) share the same
 60: !     array for storing vector values.

 62:   PetscCallA(PetscOptionsHasName(PETSC_NULL_OPTIONS, PETSC_NULL_CHARACTER, '-allocate', flag, ierr))
 63:   if (flag) then
 64:     PetscCallA(VecCreateGhostBlockWithArray(PETSC_COMM_WORLD, bs, nlocal, PETSC_DECIDE, nghost, ifrom, tarray, gxs, ierr))
 65:   else
 66:     PetscCallA(VecCreateGhostBlock(PETSC_COMM_WORLD, bs, nlocal, PETSC_DECIDE, nghost, ifrom, gxs, ierr))
 67:   end if

 69: !      Test VecDuplicate

 71:   PetscCallA(VecDuplicate(gxs, gx, ierr))
 72:   PetscCallA(VecDestroy(gxs, ierr))

 74: !      Access the local Form

 76:   PetscCallA(VecGhostGetLocalForm(gx, lx, ierr))

 78: !     Set the values from 0 to 12 into the "global" vector

 80:   PetscCallA(VecGetOwnershipRange(gx, rstart, rend, ierr))

 82:   do i = rstart, rend - 1
 83:     value = i
 84:     PetscCallA(VecSetValues(gx, 1_PETSC_INT_KIND, [i], [value], INSERT_VALUES, ierr))
 85:   end do

 87:   PetscCallA(VecAssemblyBegin(gx, ierr))
 88:   PetscCallA(VecAssemblyEnd(gx, ierr))

 90:   PetscCallA(VecGhostUpdateBegin(gx, INSERT_VALUES, SCATTER_FORWARD, ierr))
 91:   PetscCallA(VecGhostUpdateEnd(gx, INSERT_VALUES, SCATTER_FORWARD, ierr))

 93: !     Print out each vector, including the ghost padding region.

 95:   PetscCallA(PetscViewerGetSubViewer(PETSC_VIEWER_STDOUT_WORLD, PETSC_COMM_SELF, singleton, ierr))
 96:   PetscCallA(VecView(lx, singleton, ierr))
 97:   PetscCallA(PetscViewerRestoreSubViewer(PETSC_VIEWER_STDOUT_WORLD, PETSC_COMM_SELF, singleton, ierr))

 99:   PetscCallA(VecGhostRestoreLocalForm(gx, lx, ierr))
100:   PetscCallA(VecDestroy(gx, ierr))
101:   PetscCallA(PetscFinalize(ierr))
102: end

104: !/*TEST
105: !
106: !     test:
107: !       nsize: 2
108: !
109: !TEST*/