Date: 2013-07-02T14:40:21+00:00

Actual source code: ex9f.F
petsc-3.4.2 2013-07-02

      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. VecCreateGhost() 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:       implicit none

     20: !
     21: !  The following include statements are required for Fortran programs
     22: !  that use PETSc vectors:
     23: !     petscsys.h       - base PETSc routines
     24: !     petscvec.h    - vectors
     25: !

     27: #include <finclude/petscsys.h>
     28: #include <finclude/petscvec.h>

     30:       PetscMPIInt rank,size
     31:       PetscInt nlocal,nghost,ifrom(2)
     32:       PetscInt ierr,i,rstart,rend,ione
     33:       PetscBool   flag
     34:       PetscScalar  value,tarray(20)
     35:       Vec          lx,gx,gxs

     37:       nlocal = 6
     38:       nghost = 2

     40:       call PetscInitialize(PETSC_NULL_CHARACTER,ierr)
     41:       call MPI_Comm_rank(PETSC_COMM_WORLD,rank,ierr)
     42:       call MPI_Comm_size(PETSC_COMM_WORLD,size,ierr)

     44:       if (size .ne. 2) then
     45:        SETERRQ(PETSC_COMM_SELF,1,'Must run with two processors',ierr)
     46:       endif

     48: !
     49: !     Construct a two dimensional graph connecting nlocal degrees of
     50: !     freedom per processor. From this we will generate the global
     51: !     indices of needed ghost values
     52: !
     53: !     For simplicity we generate the entire graph on each processor:
     54: !     in real application the graph would stored in parallel, but this
     55: !     example is only to demonstrate the management of ghost padding
     56: !     with VecCreateGhost().
     57: !
     58: !     In this example we consider the vector as representing
     59: !     degrees of freedom in a one dimensional grid with periodic
     60: !     boundary conditions.
     61: !
     62: !        ----Processor  1---------  ----Processor 2 --------
     63: !         0    1   2   3   4    5    6    7   8   9   10   11
     64: !                               |----|
     65: !         |-------------------------------------------------|
     66: !

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

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

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

     92: !      Test VecDuplicate

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

     97: !      Access the local Form

     99:        call VecGhostGetLocalForm(gx,lx,ierr)

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

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

    105:        ione = 1
    106:        do 10, i=rstart,rend-1
    107:          value = i
    108:          call VecSetValues(gx,ione,i,value,INSERT_VALUES,ierr)
    109:  10    continue

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

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

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

    119:        if (rank .eq. 0) then
    120:           call VecView(lx,PETSC_VIEWER_STDOUT_SELF,ierr)
    121:        endif
    122:        call MPI_Barrier(PETSC_COMM_WORLD,ierr)
    123:        if (rank .eq. 1) then
    124:           call VecView(lx,PETSC_VIEWER_STDOUT_SELF,ierr)
    125:        endif

    127:        call VecGhostRestoreLocalForm(gx,lx,ierr)
    128:        call VecDestroy(gx,ierr)
    129:        call PetscFinalize(ierr)
    130:        end