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: !

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

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

     27:   nlocal = 6
     28:   nghost = 2
     29:   bs = 2
     30:   nlocal = bs*nlocal

     32:   PetscCallA(PetscInitialize(ierr))
     33:   PetscCallMPIA(MPI_Comm_rank(PETSC_COMM_WORLD, rank, ierr))
     34:   PetscCallMPIA(MPI_Comm_size(PETSC_COMM_WORLD, size, ierr))

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

     38: !
     39: !     Construct a two dimensional graph connecting nlocal degrees of
     40: !     freedom per processor. From this we will generate the global
     41: !     indices of needed ghost values
     42: !
     43: !     For simplicity we generate the entire graph on each processor:
     44: !     in real application the graph would stored in parallel, but this
     45: !     example is only to demonstrate the management of ghost padding
     46: !     with VecCreateGhost().
     47: !
     48: !     In this example we consider the vector as representing
     49: !     degrees of freedom in a one dimensional grid with periodic
     50: !     boundary conditions.
     51: !
     52: !        ----Processor  1-----------  ----Processor 2 --------
     53: !         0 1 2 3 4 5 6 7 8 9 10 11    12 13 14 15 16 17 18 19 20 21 22 23
     54: !                              |--|----|---|
     55: !         |-|--------------------------------------------------------|--|
     56: !

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

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

     70:   PetscCallA(PetscOptionsHasName(PETSC_NULL_OPTIONS, PETSC_NULL_CHARACTER, '-allocate', flag, ierr))
     71:   if (flag) then
     72:     PetscCallA(VecCreateGhostBlockWithArray(PETSC_COMM_WORLD, bs, nlocal, PETSC_DECIDE, nghost, ifrom, tarray, gxs, ierr))
     73:   else
     74:     PetscCallA(VecCreateGhostBlock(PETSC_COMM_WORLD, bs, nlocal, PETSC_DECIDE, nghost, ifrom, gxs, ierr))
     75:   end if

     77: !      Test VecDuplicate

     79:   PetscCallA(VecDuplicate(gxs, gx, ierr))
     80:   PetscCallA(VecDestroy(gxs, ierr))

     82: !      Access the local Form

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

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

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

     90:   ione = 1
     91:   do 10, i = rstart, rend - 1
     92:     value = i
     93:     PetscCallA(VecSetValues(gx, ione, [i], [value], INSERT_VALUES, ierr))
     94: 10  continue

     96:     PetscCallA(VecAssemblyBegin(gx, ierr))
     97:     PetscCallA(VecAssemblyEnd(gx, ierr))

     99:     PetscCallA(VecGhostUpdateBegin(gx, INSERT_VALUES, SCATTER_FORWARD, ierr))
    100:     PetscCallA(VecGhostUpdateEnd(gx, INSERT_VALUES, SCATTER_FORWARD, ierr))

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

    104:     PetscCallA(PetscViewerGetSubViewer(PETSC_VIEWER_STDOUT_WORLD, PETSC_COMM_SELF, singleton, ierr))
    105:     PetscCallA(VecView(lx, singleton, ierr))
    106:     PetscCallA(PetscViewerRestoreSubViewer(PETSC_VIEWER_STDOUT_WORLD, PETSC_COMM_SELF, singleton, ierr))

    108:     PetscCallA(VecGhostRestoreLocalForm(gx, lx, ierr))
    109:     PetscCallA(VecDestroy(gx, ierr))
    110:     PetscCallA(PetscFinalize(ierr))
    111:   end

    113: !/*TEST
    114: !
    115: !     test:
    116: !       nsize: 2
    117: !
    118: !TEST*/