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,mySize
 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:       call PetscInitialize(PETSC_NULL_CHARACTER,ierr)
 31:       if (ierr /= 0) then
 32:         print*,'PetscInitialize failed'
 33:         stop
 34:       endif
 35:       call MPI_Comm_rank(PETSC_COMM_WORLD,rank,ierr)
 36:       call MPI_Comm_size(PETSC_COMM_WORLD,mySize,ierr)

 38:       if (mySize /= 2) then; SETERRA(PETSC_COMM_WORLD,PETSC_ERR_WRONG_MPI_SIZE,'Requires 2 processors'); endif

 40: !
 41: !     Construct a two dimensional graph connecting nlocal degrees of
 42: !     freedom per processor. From this we will generate the global
 43: !     indices of needed ghost values
 44: !
 45: !     For simplicity we generate the entire graph on each processor:
 46: !     in real application the graph would stored in parallel, but this
 47: !     example is only to demonstrate the management of ghost padding
 48: !     with VecCreateGhost().
 49: !
 50: !     In this example we consider the vector as representing
 51: !     degrees of freedom in a one dimensional grid with periodic
 52: !     boundary conditions.
 53: !
 54: !        ----Processor  1---------  ----Processor 2 --------
 55: !         0    1   2   3   4    5    6    7   8   9   10   11
 56: !                               |----|
 57: !         |-------------------------------------------------|
 58: !

 60:       if (rank .eq. 0) then
 61:         ifrom(1) = 11
 62:         ifrom(2) = 6
 63:       else
 64:         ifrom(1) = 0
 65:         ifrom(2) = 5
 66:       endif

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

 72:       call PetscOptionsHasName(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,   &
 73:      &                         '-allocate',flag,ierr)
 74:       if (flag) then
 75:         call VecCreateGhostWithArray(PETSC_COMM_WORLD,nlocal,            &
 76:      &        PETSC_DECIDE,nghost,ifrom,tarray,gxs,ierr)
 77:       else
 78:         call VecCreateGhost(PETSC_COMM_WORLD,nlocal,PETSC_DECIDE,        &
 79:      &       nghost,ifrom,gxs,ierr)
 80:       endif

 82: !      Test VecDuplicate

 84:        call VecDuplicate(gxs,gx,ierr)
 85:        call VecDestroy(gxs,ierr)

 87: !      Access the local Form

 89:        call VecGhostGetLocalForm(gx,lx,ierr)

 91: !     Set the values from 0 to 12 into the 'global' vector

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

 95:        ione = 1
 96:        do 10, i=rstart,rend-1
 97:          value = real(i)
 98:          call VecSetValues(gx,ione,i,value,INSERT_VALUES,ierr)
 99:  10    continue

101:        call VecAssemblyBegin(gx,ierr)
102:        call VecAssemblyEnd(gx,ierr)

104:        call VecGhostUpdateBegin(gx,INSERT_VALUES,SCATTER_FORWARD,ierr)
105:        call VecGhostUpdateEnd(gx,INSERT_VALUES,SCATTER_FORWARD,ierr)

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

109:        call PetscViewerGetSubViewer(PETSC_VIEWER_STDOUT_WORLD,PETSC_COMM_SELF,subviewer,ierr)
110:        call VecView(lx,subviewer,ierr)
111:        call PetscViewerRestoreSubViewer(PETSC_VIEWER_STDOUT_WORLD,PETSC_COMM_SELF,subviewer,ierr)

113:        call VecGhostRestoreLocalForm(gx,lx,ierr)
114:        call VecDestroy(gx,ierr)
115:        call PetscFinalize(ierr)
116:        end

118: !/*TEST
119: !
120: !     test:
121: !       nsize: 2
122: !
123: !     test:
124: !       suffix: 2
125: !       nsize: 2
126: !       args: -allocate
127: !
128: !TEST*/