[petsc-dev] SNES, Coloring, DM, and TS

[Dmitry Karpeev]

On Mon, Feb 27, 2012 at 10:48 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> I'm getting lost with too many ways to do the same thing. The DM has only
> one slot for a residual function, I'm not wild about having TS overwrite
> that. So TS uses SNESSetFunction(), but now I need it on levels and I want
> coloring to work. But the coloring setup code is inside DMComputeJacobian()
> and I'd rather not duplicate it in SNES
>
> if (snes->ops->computefunction && !snes->ops->computejacobian && snes->dm
> && !snes->dm->ops->function) {
>   /* set up coloring using DM, but apply using snes->ops->computefunction
> */
> }
>
> or some such nastiness.
>
> I would like to either _always_ use DM (possibly created implicitly by
> TS/SNES/KSP and just used to hold function pointers) or _never_ use DM.
>
> The problem with never using DM is that we need a way for the user to
> decide to implement different physics on coarse levels. They currently do
> this by implementing DMCoarsen() or using DMCoarsenHookAdd(). If the
> function pointers are stored in SNES and TS, the hook would have to be able
> to access them somehow.
>
> The problem with always using DM is that we effectively create a
> dependency loop because the user is telling DM about a nonlinear or
> transient problem even though that concept is not part of the DM interface.
>

Why is a nonlinear problem not part of the DM interface?  In my mind a DM
encapsulates a problem, that also knows
how to linearize/split/coarsen/refine itself by returning the corresponding
subDMs.

Dmitry.


>
>
> As a related matter, I'm stuck with a reference counting loop where I want
> to attach special Vecs to a DM. I can wrap them in a PetscContainer to
> circumvent the code that checks for reference cycles, but that makes
> freeing them delicate. We could make a named cache similar to DM work
> vectors, but where restored Vecs are not available for reuse.
>
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