[petsc-dev] MatSetFromOptions() after DMGetMatrix()

[Jed Brown]

On Sun, May 1, 2011 at 05:07, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
>   Jed,
>
>      You pose several issues here.
>
> 1) By default SBAIJ matrices generate an error if you set a value in the
> lower triangular portion.
>
>      I think this is the wrong default. I've changed it to ignore lower
> triangular values unless you explicitly set the option.
>

Excellent


>
> 2) MatSetType(A,MATSBAIJ); and friends deletes the matrix and creates a new
> one if the matrix is MATSEQSBAIJ or MATMPISBAIJ.
>
>      This is silly. Given our model of setting types however there is no
> trivial way to fix this. We could try to handle setting a heirarchy of types
> properly (how to know if something is a subclass? etc?) or do something just
> for Mat to handle the
>      rank = 1 and rank > 1 cases. I'm inclined to add a special Mat
> register for example MatRegisterBaseName("SBAIJ","SeqSBAIJ","MPISBAIJ"); and
> have MatSetType() first check if the type is in the BaseName list and if so
> grab the correct
>      classname based on rank, then it will realize the matrix is not having
> a type change and will not destroy it. Note that this means routines like
> MatCreateSBAIJ() will disappear.
>

I think you mean MatCreate_SBAIJ(). I also think this is better because
there is a bit of duplicate code and you can never actually have an instance
of MATSBAIJ.

The lookup in the other direction would take more effort, but there is a lot
of PetscTypeCompare that ends up needing to always check for both the SEQ
and MPI variants. But I think managing a full type hierarchy is a very deep
rabbit hole so I'm not convinced it's worth it.


> 3) The general wordyness of setting up the matrix fill from the DM.
>
>     My planned model for the future was to change DMGetMatrix() to
> DMFillMatrix() and have code something like
>
>     MatCreate(comm,&Ap);
>     MatSetOptionsPrefix(Ap,"Ap_");
>     MatSetFromOptions(Ap);
>      MatSetOption(Ap,MAT_SYMMETRIC,PETSC_TRUE);
>      DMFillMatrix(fsu,Ap);
>
>     If is actually not any shorter but I think it is a better model.
>

This is much less wordy because all the code is in one place (no separate
part for PetscOptionsList) and there are no if statements. It would also be
possible to write a helper like

DMCreateMatWithOptions(fsu,"Ap_",MATSBAIJ,MAT_SPD,MAT_NEW_NONZERO_LOCATION_ERR,...,0,&Ap);

(or without variadic handling of options).
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