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*/