1: !
  2: !
  3: ! Description: Illustrates the use of VecCreateGhost()
  4: !
  5: !/*T
  6: !   Concepts: vectors^assembling vectors;
  7: !   Concepts: vectors^ghost padding;
  8: !   Processors: n
  9: !
 10: !   Description: Ghost padding is one way to handle local calculations that
 11: !      involve values from other processors. VecCreateGhostBlock() provides
 12: !      a way to create vectors with extra room at the end of the vector
 13: !      array to contain the needed ghost values from other processors,
 14: !      vector computations are otherwise unaffected.
 15: !T*/

 17:       program main
 18:  #include <petsc/finclude/petscvec.h>
 19:       use petscvec
 20:       implicit none

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

 30:       nlocal = 6
 31:       nghost = 2
 32:       bs     = 2
 33:       nlocal = bs*nlocal

 35:       call PetscInitialize(PETSC_NULL_CHARACTER,ierr)
 36:       if (ierr .ne. 0) then
 37:         print*,'Unable to initialize PETSc'
 38:         stop
 39:       endif
 40:       call MPI_Comm_rank(PETSC_COMM_WORLD,rank,ierr)
 41:       call MPI_Comm_size(PETSC_COMM_WORLD,size,ierr)

 43:       if (size .ne. 2) then
 44:        SETERRA(PETSC_COMM_WORLD,1,'Requires 2 processors')
 45:       endif

 47: !
 48: !     Construct a two dimensional graph connecting nlocal degrees of
 49: !     freedom per processor. From this we will generate the global
 50: !     indices of needed ghost values
 51: !
 52: !     For simplicity we generate the entire graph on each processor:
 53: !     in real application the graph would stored in parallel, but this
 54: !     example is only to demonstrate the management of ghost padding
 55: !     with VecCreateGhost().
 56: !
 57: !     In this example we consider the vector as representing
 58: !     degrees of freedom in a one dimensional grid with periodic
 59: !     boundary conditions.
 60: !
 61: !        ----Processor  1-----------  ----Processor 2 --------
 62: !         0 1 2 3 4 5 6 7 8 9 10 11    12 13 14 15 16 17 18 19 20 21 22 23
 63: !                              |--|----|---|
 64: !         |-|--------------------------------------------------------|--|
 65: !


 68:       if (rank .eq. 0) then
 69:         ifrom(1) = 11
 70:         ifrom(2) = 6
 71:       else
 72:         ifrom(1) = 0
 73:         ifrom(2) = 5
 74:       endif

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

 80:       call PetscOptionsHasName(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,   &
 81:      &                         '-allocate',flag,ierr)
 82:       if (flag) then
 83:         call VecCreateGhostBlockWithArray(PETSC_COMM_WORLD,bs,nlocal,    &
 84:      &        PETSC_DECIDE,nghost,ifrom,tarray,gxs,ierr)
 85:       else
 86:         call VecCreateGhostBlock(PETSC_COMM_WORLD,bs,nlocal,             &
 87:      &       PETSC_DECIDE,nghost,ifrom,gxs,ierr)
 88:       endif


 91: !      Test VecDuplicate

 93:       call VecDuplicate(gxs,gx,ierr)
 94:       call VecDestroy(gxs,ierr)

 96: !      Access the local Form

 98:       call VecGhostGetLocalForm(gx,lx,ierr)

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

102:       call VecGetOwnershipRange(gx,rstart,rend,ierr)

104:       ione = 1
105:       do 10, i=rstart,rend-1
106:         value = i
107:         call VecSetValues(gx,ione,i,value,INSERT_VALUES,ierr)
108:  10   continue

110:       call VecAssemblyBegin(gx,ierr)
111:       call VecAssemblyEnd(gx,ierr)

113:       call VecGhostUpdateBegin(gx,INSERT_VALUES,SCATTER_FORWARD,ierr)
114:       call VecGhostUpdateEnd(gx,INSERT_VALUES,SCATTER_FORWARD,ierr)

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

118:       call VecView(lx,PETSC_VIEWER_STDOUT_SELF,ierr)

120:       call VecGhostRestoreLocalForm(gx,lx,ierr)
121:       call VecDestroy(gx,ierr)
122:       call PetscFinalize(ierr)
123:       end