Actual source code: ex9f.F90

  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. VecCreateGhost() 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: !

 13: program main
 14: #include <petsc/finclude/petscvec.h>
 15:   use petscvec
 16:   implicit none

 18:   PetscMPIInt rank, size
 19:   PetscInt nlocal, nghost, ifrom(2)
 20:   PetscErrorCode ierr
 21:   PetscInt i, rstart, rend, ione
 22:   PetscBool flag
 23:   PetscScalar value, tarray(20)
 24:   Vec lx, gx, gxs
 25:   PetscViewer subviewer

 27:   nlocal = 6
 28:   nghost = 2

 30:   PetscCallA(PetscInitialize(ierr))
 31:   PetscCallMPIA(MPI_Comm_rank(PETSC_COMM_WORLD, rank, ierr))
 32:   PetscCallMPIA(MPI_Comm_size(PETSC_COMM_WORLD, size, ierr))

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

 36: !
 37: !     Construct a two dimensional graph connecting nlocal degrees of
 38: !     freedom per processor. From this we will generate the global
 39: !     indices of needed ghost values
 40: !
 41: !     For simplicity we generate the entire graph on each processor:
 42: !     in real application the graph would stored in parallel, but this
 43: !     example is only to demonstrate the management of ghost padding
 44: !     with VecCreateGhost().
 45: !
 46: !     In this example we consider the vector as representing
 47: !     degrees of freedom in a one dimensional grid with periodic
 48: !     boundary conditions.
 49: !
 50: !        ----Processor  1---------  ----Processor 2 --------
 51: !         0    1   2   3   4    5    6    7   8   9   10   11
 52: !                               |----|
 53: !         |-------------------------------------------------|
 54: !

 56:   if (rank == 0) then
 57:     ifrom(1) = 11
 58:     ifrom(2) = 6
 59:   else
 60:     ifrom(1) = 0
 61:     ifrom(2) = 5
 62:   end if

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

 68:   PetscCallA(PetscOptionsHasName(PETSC_NULL_OPTIONS, PETSC_NULL_CHARACTER, '-allocate', flag, ierr))
 69:   if (flag) then
 70:     PetscCallA(VecCreateGhostWithArray(PETSC_COMM_WORLD, nlocal, PETSC_DECIDE, nghost, ifrom, tarray, gxs, ierr))
 71:   else
 72:     PetscCallA(VecCreateGhost(PETSC_COMM_WORLD, nlocal, PETSC_DECIDE, nghost, ifrom, gxs, ierr))
 73:   end if

 75: !      Test VecDuplicate

 77:   PetscCallA(VecDuplicate(gxs, gx, ierr))
 78:   PetscCallA(VecDestroy(gxs, ierr))

 80: !      Access the local Form

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

 84: !     Set the values from 0 to 12 into the 'global' vector

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

 88:   ione = 1
 89:   do 10, i = rstart, rend - 1
 90:     value = real(i)
 91:     PetscCallA(VecSetValues(gx, ione, [i], [value], INSERT_VALUES, ierr))
 92: 10  continue

 94:     PetscCallA(VecAssemblyBegin(gx, ierr))
 95:     PetscCallA(VecAssemblyEnd(gx, ierr))

 97:     PetscCallA(VecGhostUpdateBegin(gx, INSERT_VALUES, SCATTER_FORWARD, ierr))
 98:     PetscCallA(VecGhostUpdateEnd(gx, INSERT_VALUES, SCATTER_FORWARD, ierr))

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

102:     PetscCallA(PetscViewerGetSubViewer(PETSC_VIEWER_STDOUT_WORLD, PETSC_COMM_SELF, subviewer, ierr))
103:     PetscCallA(VecView(lx, subviewer, ierr))
104:     PetscCallA(PetscViewerRestoreSubViewer(PETSC_VIEWER_STDOUT_WORLD, PETSC_COMM_SELF, subviewer, ierr))

106:     PetscCallA(VecGhostRestoreLocalForm(gx, lx, ierr))
107:     PetscCallA(VecDestroy(gx, ierr))
108:     PetscCallA(PetscFinalize(ierr))
109:   end

111: !/*TEST
112: !
113: !     test:
114: !       nsize: 2
115: !
116: !     test:
117: !       suffix: 2
118: !       nsize: 2
119: !       args: -allocate
120: !
121: !TEST*/