Re: adding SNESSetLinearSolve()

["Lisandro Dalcin" <dalcinl@xxxxxxxxx>]

On 10/31/07, Barry Smith <bsmith@xxxxxxxxxxx> wrote:
>   Lisandro,
>    A followup to our previous discussion. It sounds to me like you
> are "actually" solving an n+1 unknown nonlinear problem where the
> "special" unknown is kept secret from SNES and managed somehow by the
> application code?

That's exactly the case. Furthermore, this 'special' unknown in just a
regularization parameter who tends to zero as the nonlinear solution
is reached. Unfortunatelly, this unknow is coupled with all other
degree of freedom, thus generating a full dense row and a dense column
in the Jacobian matrix. But fortunatelly, the special unknown is just
a single scalar, thus computing the schur complement is feasible, but
requires two linear solves with the other 'sparse' block of the
Jacobian.

>   You can guess how I feel about this :-).

Yes, of course. I agree that PETSc API must be consistent and clean.
But I also feel that some time I need more features. Please remember I
use PETSc exclusively from Python. And then is so easy to manage
complicated application setups. But at some point I need more
low-level support from PETSc to make it working.

For example, I would love to have SNESSetPresolve/SNESSetPostSolve and
SNESSetPreStep/SNESSetPostStep, and perhaps a
SNESSetPreLinearSolve/SNESSetPostLinearSolve. Of course, this make the
API grow with features that are not frecuently needed.

> PETSc/SNES is suppose
> to be good enough to allow you to feed it the ENTIRE nonlinear problem
> in a way that would allow as efficient solution as if you "handled the
> special unknown" specially.

Even for my particular case? Can I pass-over the issue with the full
denses rows and columns?

> In particular for this problem the intended
> solution is to use the PETSc DMComposite object via DMCompositeCreate()
>   You may want to look at this construct to see if it is suitable
> for your friends needs. And what we need to add (note that though DMComposite
> can be used with DMMG it does not have to be, it can be used directly
> with a SNES also.

I'll definitely take a loop ASAP.

Regards,

-- 
Lisandro Dalcín
---------------
Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
PTLC - Güemes 3450, (3000) Santa Fe, Argentina
Tel/Fax: +54-(0)342-451.1594