[petsc-dev] Getting "from" and "to" IS from a VecScatter

[Matthew Knepley]

On Wed, Jun 20, 2012 at 8:40 PM, Blaise Bourdin <bourdin at lsu.edu> wrote:

> Hi,
>
> I want to change the way I handle boundary conditions in my finite
> elements code:
> Right now, I skip the rows and columns during the matrices assembly, using
> a flag (a SectionInt). Using an IS denoting all dof with a BC and
> MatZeroRowsColumns, MatZeroRowsColumnsIS or MatZeroRowsColumnsLocal would
> make much more sense.
>
> The difficulty comes up when applying a Dirichlet boundary condition on
> only one of the dof associated with a vertex (roller boundary conditions in
> elasticity, for instance). What I can easily get from Sieve is the point
> number associated with a vertex, for instance, from which I can obtain the
> _local_ index of the dof associated with a given component of a field at
> this point. For vectors, I can then set values on the local Vec or the
> Section. For matrices, though I can't just use MatGetLocalSubMatrix since I
> need also to modify the out of diagonal block terms.
>
> I guess I need the LocalToGlobalMapping or to renumber the IS describing
> my BC and convert it to global indices. I am sure that this information is
> somewhere in Sieve (in the overlap, I guess), but I was thinking that since
> getting a Local to Global scatter is trivial, there may be a way to obtain
> the mapping or renumbering the IS from the scatter.
>
> Now that I think about it more, the right way is most likely to get the
> information I need from Sieve.


Yes, in the new version there is jsut a LocalToGlobalMapping. In C++, you
can use the GlobalNumbering,

  numbering->getIndex(point) + dof offset

     Matt


>
> Blaise
>
>
> On Jun 20, 2012, at 7:23 PM, Barry Smith wrote:
>
> >
> >  Why do you need them? Maybe there is a better way?
> >
> >
> > On Jun 20, 2012, at 6:19 PM, Blaise Bourdin wrote:
> >
> >> Hi,
> >>
> >> I need to reconstruct the IS that were used to build a VecScatter
> between a local and a global Vec.
> >>
> >> From what I see in VecScatterView_MPI (vpscat.c:13), it looks like I
> can get the indices from from->local.vslots[i],to->local.vslots[i], i=0 ..
> i<to->local.n where from and to are obtained with
> >> VecScatter_MPI_General *to=(VecScatter_MPI_General*)ctx->todata;
> >> VecScatter_MPI_General *from=(VecScatter_MPI_General*)ctx->fromdata;
> >>
> >> From there, constructing the IS is trivial. Is there a way that does
> not involve poking inside the VecScatter data structure?
> >>
> >> Blaise
> >>
> >>
> >> --
> >> Department of Mathematics and Center for Computation & Technology
> >> Louisiana State University, Baton Rouge, LA 70803, USA
> >> Tel. +1 (225) 578 1612, Fax  +1 (225) 578 4276
> http://www.math.lsu.edu/~bourdin
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >
>
> --
> Department of Mathematics and Center for Computation & Technology
> Louisiana State University, Baton Rouge, LA 70803, USA
> Tel. +1 (225) 578 1612, Fax  +1 (225) 578 4276
> http://www.math.lsu.edu/~bourdin
>
>
>
>
>
>
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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