1: !
  2: !     "$Id: ex9f.F,v 1.11 2001/08/07 03:02:34 balay Exp $";
  3: !
  4: ! Description: Illustrates the use of VecCreateGhost()
  5: !
  6: !/*T
  7: !   Concepts: vectors^assembling vectors;
  8: !   Concepts: vectors^ghost padding;
  9: !   Processors: n
 10: !
 11: !   Description: Ghost padding is one way to handle local calculations that
 12: !      involve values from other processors. VecCreateGhost() provides
 13: !      a way to create vectors with extra room at the end of the vector 
 14: !      array to contain the needed ghost values from other processors, 
 15: !      vector computations are otherwise unaffected.
 16: !T*/

 18:       program main
 19:       implicit none

 21: !
 22: !  The following include statements are required for Fortran programs
 23: !  that use PETSc vectors:
 24: !     petsc.h       - base PETSc routines
 25: !     petscvec.h    - vectors
 26: !

 28:  #include include/finclude/petsc.h
 29:  #include include/finclude/petscvec.h

 31:       integer      rank,nlocal,nghost,ifrom(2),size,ierr,i,rstart,rend
 32:       integer      flag
 33:       PetscScalar  value,tarray(20)
 34:       Vec          lx,gx,gxs

 36:       nlocal = 6
 37:       nghost = 2

 39:       call PetscInitialize(PETSC_NULL_CHARACTER,ierr)
 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:        SETERRQ(1,'Must run with two processors',ierr)
 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
 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_CHARACTER,'-allocate',         &
 81:      &     flag,ierr)
 82:       if (flag .ne. 0) then
 83:         call VecCreateGhostWithArray(PETSC_COMM_WORLD,nlocal,            &
 84:      &        PETSC_DECIDE,nghost,ifrom,tarray,gxs,ierr)
 85:       else
 86:         call VecCreateGhost(PETSC_COMM_WORLD,nlocal,PETSC_DECIDE,        &
 87:      &       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:        do 10, i=rstart,rend-1
105:          value = i
106:          call VecSetValues(gx,1,i,value,INSERT_VALUES,ierr)
107:  10    continue

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

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

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

117:        call VecView(lx,PETSC_VIEWER_STDOUT_SELF,ierr)

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