[petsc-dev] DMGetMatrix --> DMGetMatrices?

[Dmitry Karpeev]

On Fri, Feb 10, 2012 at 5:14 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> On Fri, Feb 10, 2012 at 17:08, Dmitry Karpeev <karpeev at mcs.anl.gov> wrote:
>
>> I think it's completely natural for a DM to assemble two operators -- the
>> discretizations for the two are likely to be related anyway -- as soon as
>> we decide that it's natural for KSP to take in two matrices and, more
>> importantly, for the callback set with DMSetJacobian() to compute two
>> matrices: if a DM knows how to compute  two "Jacobians", why wouldn't it
>> know how to create/preallocate the two corresponding matrices?
>
>
> Lots of functions get messy if the DM has multiple ways to do something.
> Should DMCreateLocalVector() use the full stencil or the preconditioning
> stencil, what should DMGlobalToLocalBegin() be updating, etc.
>

As you yourself point out, the Jacobian and the preconditioning matrix
should operate between the same spaces, so there is no ambiguity regarding
the vectors and the LocalToGlobal/GlobalToLocal mappings for them.

>
> Barry's solution of having separate DMs sounds cleaner to me
>

Okay.  I think what saves us here is the assumption that the operators that
go into SNES (and KSP) act on a single space (i.e., the input and output
spaces are the same).  Yes, to be able to solve equations we have to have
spaces
of the same dimension, and, sure, it's natural to assume they have the same
PetscLayout, but it's not at all clear they should have the same local form
(i.e., ISLocalToGlobal for the row and column spaces).  If we did allow
different local forms, does that mean that we should have two different DMs
describing the intput and output spaces of *each* J and J_pre?  That would
mean 4 DMs per SNES/KSP.  This is rhetorical, I guess, since we do assume
identical input/output spaces, and 2 DMs seems okay to me.

Dmitry.

> , at least modulo needing conventions about which DM on which to
> PetscObjectCompose() things needed by certain callbacks (e.g. in the FAS
> with TS stuff I'm doing).
>
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