dolfin team mailing list archive

[Johan Hake <hake@xxxxxxxxx>]

On Monday 30 June 2008 22:20:08 Anders Logg wrote:
> On Mon, Jun 30, 2008 at 01:48:33PM +0200, Martin Sandve Alnæs wrote:
> > 2008/6/30 Kent-Andre Mardal <kent-and@xxxxxxxxx>:
> > > On Sat, 2008-06-28 at 20:55 +0200, Johan Hake wrote:
> > >> Hello!
> > >>
> > >> I tried to compile an advection diffusion form using Streamline Upwind
> > >> Petrow Galerkin stabilazing method, similar to the one introduced in
> > >> DOLFIN 0.6.4. Using 3 dimensional and first degree Lagrange elements,
> > >> FFC/g++ chokes when the produced code is compiling. It wont finish
> > >> compile... Is this a known problem and a limitation of FFC/g++?
> > >>
> > >> In PyCC I use DOLFIN to assemble the same system matrix, but instead
> > >> of using one form for the whole system I split the form into at least
> > >> 4 different forms. These compiles smoothly and after I have assembled
> > >> the matrices I just add them together.
> > >>
> > >> This approach is not possible using the present DOLFIN linear algebra
> > >> interface, as sum of matrices is not supported. Is this something we
> > >> want for the future?
> > >>
> > >> Johan
> > >
> > > If the matrices come from the same dof_map one "should" be able to add
> > > to matrices together (and if not I guess you
> > > would not add them toghether). The problem is that the matrices
> > > themselves do not know what their created from.
> > >
> > > However, one could write a function
> > >
> > > matrix_add(mesh, dof_map, A, B, C);
> > >
> > > That does the trick in terms of the tabulate_dofs funcition in dof_map
> > > and the get, add functions in GenericMatrix.
> > >
> > > Kent
> >
> > Note that if the matrices have optimized sparsity patterns,
> > i.e. all zeros have been removed after the assembly,
> > having the same dof_map is not enough to say they can be added.
> >
> > If we later add the previously discussed feature with equation specific
> > local sparsity patterns to UFC and the Assembler, we may even
> > assemble directly into such equation-specific matrices.
>
> I think we should be able to add matrices together without needing to
> refer to a dof_map. The matrix does not know the dof_map that
> generated it but, it should be able to generate its sparsity pattern.
>
> Then we can extract the sparsity patterns for A and B, generate a new
> sparsity pattern, initialize the new matrix and then insert the
> values.

Sounds like a nice feature. I do not know how this complies with distributed 
matrices.

We should of course also look into the underlaying libraries. I had a 
halfhearted look into the Epetra interface but couldn't there and then find 
the right functions.

Johan