dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

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.

-- 
Anders

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