Actual source code: ex14f.F90
petsc-3.13.6 2020-09-29
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      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. VecCreateGhostBlock() 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:  #include <petsc/finclude/petscvec.h>
     19:       use petscvec
     20:       implicit none

     22:       PetscMPIInt size,rank
     23:       PetscInt nlocal,nghost,ifrom(2)
     24:       PetscInt i,rstart,rend,bs,ione
     25:       PetscBool       flag
     26:       PetscErrorCode ierr
     27:       PetscScalar  value,tarray(20)
     28:       Vec          lx,gx,gxs
     29:       PetscViewer singleton

     31:       nlocal = 6
     32:       nghost = 2
     33:       bs     = 2
     34:       nlocal = bs*nlocal

     36:       call PetscInitialize(PETSC_NULL_CHARACTER,ierr)
     37:       if (ierr .ne. 0) then
     38:         print*,'Unable to initialize PETSc'
     39:         stop
     40:       endif
     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; SETERRA(PETSC_COMM_WORLD,PETSC_ERR_WRONG_MPI_SIZE,'Requires 2 processors'); endif

     46: !
     47: !     Construct a two dimensional graph connecting nlocal degrees of
     48: !     freedom per processor. From this we will generate the global
     49: !     indices of needed ghost values
     50: !
     51: !     For simplicity we generate the entire graph on each processor:
     52: !     in real Section 1.5 Writing Application Codes with PETSc the graph would stored in parallel, but this
     53: !     example is only to demonstrate the management of ghost padding
     54: !     with VecCreateGhost().
     55: !
     56: !     In this example we consider the vector as representing
     57: !     degrees of freedom in a one dimensional grid with periodic
     58: !     boundary conditions.
     59: !
     60: !        ----Processor  1-----------  ----Processor 2 --------
     61: !         0 1 2 3 4 5 6 7 8 9 10 11    12 13 14 15 16 17 18 19 20 21 22 23
     62: !                              |--|----|---|
     63: !         |-|--------------------------------------------------------|--|
     64: !

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

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

     79:       call PetscOptionsHasName(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,   &
     80:      &                         '-allocate',flag,ierr)
     81:       if (flag) then
     82:         call VecCreateGhostBlockWithArray(PETSC_COMM_WORLD,bs,nlocal,    &
     83:      &        PETSC_DECIDE,nghost,ifrom,tarray,gxs,ierr)
     84:       else
     85:         call VecCreateGhostBlock(PETSC_COMM_WORLD,bs,nlocal,             &
     86:      &       PETSC_DECIDE,nghost,ifrom,gxs,ierr)
     87:       endif

     90: !      Test VecDuplicate

     92:       call VecDuplicate(gxs,gx,ierr)
     93:       call VecDestroy(gxs,ierr)

     95: !      Access the local Form

     97:       call VecGhostGetLocalForm(gx,lx,ierr)

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

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

    103:       ione = 1
    104:       do 10, i=rstart,rend-1
    105:         value = i
    106:         call VecSetValues(gx,ione,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 PetscViewerGetSubViewer(PETSC_VIEWER_STDOUT_WORLD,PETSC_COMM_SELF,singleton,ierr)
    118:       call VecView(lx,singleton,ierr)
    119:       call PetscViewerRestoreSubViewer(PETSC_VIEWER_STDOUT_WORLD,PETSC_COMM_SELF,singleton,ierr)

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

    127: !/*TEST
    128: !
    129: !     test:
    130: !       nsize: 2
    131: !
    132: !TEST*/