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

Blaise Bourdin bourdin at lsu.edu
Thu Jun 21 09:06:34 CDT 2012

On Jun 21, 2012, at 7:42 AM, Matthew Knepley wrote:

> 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

And for the record, in the old version, where a DMMesh can be associated with several data layout, the following seems to work (adapted from DMCreateLocalToGlobalMapping_Mesh):

  Obj<PETSC_MESH_TYPE::real_section_type> s;
  ierr = DMMeshGetMesh(dm, m);CHKERRQ(ierr);
  ierr = SectionRealGetSection(vsec,s);CHKERRQ(ierr);
  const ALE::Obj<PETSC_MESH_TYPE::order_type>& globalOrder = m->getFactory()->getGlobalOrder(m, s->getName(), s);

  offset = globalOrder->getIndex(point) + dof ;

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

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