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

 34:       PetscCheckA(size .eq. 2,PETSC_COMM_WORLD,PETSC_ERR_WRONG_MPI_SIZE,'Requires 2 processors')

 36: !
 37: !     Construct a two dimensional graph connecting nlocal degrees of
 38: !     freedom per processor. From this we will generate the global
 39: !     indices of needed ghost values
 40: !
 41: !     For simplicity we generate the entire graph on each processor:
 42: !     in real application the graph would stored in parallel, but this
 43: !     example is only to demonstrate the management of ghost padding
 44: !     with VecCreateGhost().
 45: !
 46: !     In this example we consider the vector as representing
 47: !     degrees of freedom in a one dimensional grid with periodic
 48: !     boundary conditions.
 49: !
 50: !        ----Processor  1---------  ----Processor 2 --------
 51: !         0    1   2   3   4    5    6    7   8   9   10   11
 52: !                               |----|
 53: !         |-------------------------------------------------|
 54: !

 56:       if (rank .eq. 0) then
 57:         ifrom(1) = 11
 58:         ifrom(2) = 6
 59:       else
 60:         ifrom(1) = 0
 61:         ifrom(2) = 5
 62:       endif

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

 68:       PetscCallA(PetscOptionsHasName(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,'-allocate',flag,ierr))
 69:       if (flag) then
 70:         PetscCallA(VecCreateGhostWithArray(PETSC_COMM_WORLD,nlocal,PETSC_DECIDE,nghost,ifrom,tarray,gxs,ierr))
 71:       else
 72:         PetscCallA(VecCreateGhost(PETSC_COMM_WORLD,nlocal,PETSC_DECIDE,nghost,ifrom,gxs,ierr))
 73:       endif

 75: !      Test VecDuplicate

 77:        PetscCallA(VecDuplicate(gxs,gx,ierr))
 78:        PetscCallA(VecDestroy(gxs,ierr))

 80: !      Access the local Form

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

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

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

 88:        ione = 1
 89:        do 10, i=rstart,rend-1
 90:          value = real(i)
 91:          PetscCallA(VecSetValues(gx,ione,i,value,INSERT_VALUES,ierr))
 92:  10    continue

 94:        PetscCallA(VecAssemblyBegin(gx,ierr))
 95:        PetscCallA(VecAssemblyEnd(gx,ierr))

 97:        PetscCallA(VecGhostUpdateBegin(gx,INSERT_VALUES,SCATTER_FORWARD,ierr))
 98:        PetscCallA(VecGhostUpdateEnd(gx,INSERT_VALUES,SCATTER_FORWARD,ierr))

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

102:        PetscCallA(PetscViewerGetSubViewer(PETSC_VIEWER_STDOUT_WORLD,PETSC_COMM_SELF,subviewer,ierr))
103:        PetscCallA(VecView(lx,subviewer,ierr))
104:        PetscCallA(PetscViewerRestoreSubViewer(PETSC_VIEWER_STDOUT_WORLD,PETSC_COMM_SELF,subviewer,ierr))

106:        PetscCallA(VecGhostRestoreLocalForm(gx,lx,ierr))
107:        PetscCallA(VecDestroy(gx,ierr))
108:        PetscCallA(PetscFinalize(ierr))
109:        end

111: !/*TEST
112: !
113: !     test:
114: !       nsize: 2
115: !
116: !     test:
117: !       suffix: 2
118: !       nsize: 2
119: !       args: -allocate
120: !
121: !TEST*/