dolfin team mailing list archive

["Martin Sandve Alnæs" <martinal@xxxxxxxxx>]

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.

--
Martin