Actual source code: ex14f.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. 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*/