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*/