[petsc-dev] DMGetMatrix --> DMGetMatrices?

[Dmitry Karpeev]

On Fri, Feb 10, 2012 at 4:28 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> On Fri, Feb 10, 2012 at 16:13, Dmitry Karpeev <karpeev at mcs.anl.gov> wrote:
>
>> Well, two is the number that's used in KSP and SNES.
>>
>
> DM can be used outside of KSP and SNES.
>
> It may well make sense to have J and Jpre corresponding to the implicit
> part of an equation plus a "mass matrix" to be used for "explicit stages"
> in an IMEX method. That is three (or even four if the mass matrix needs a
> preconditioner) extracted from the same DM (in different solves).
>

The mass matrix (plus a lumped mass matrix to build the preconditioner) for
an "explicit" integrator is the very example I had in mind. Time
integrators, however, have a different API from SNES and KSP.  Maybe
multiple DMs would be appropriate in that situation (if necessary, sharing
the backend mesh or whatever other data structures for efficiency) -- a DM
for each explicit and implicit stage.  Yet, the SNES/KSP for each stage
requires two matrices, so it would be natural for DM to reflect that.


>
>>
>> With the current API SNES actually uses the same matrix for the Jacobian
>> and preconditioner, if the matrix is obtained from a DM, which is limiting.
>>  It is natural to have a "geometric" backend (e.g., libMesh or MOOSE)
>> interact with the algebraic objects (PETSc's SNES, KSP) through the DM as
>> much as possible, since the DM (its impl, rather) carries all of the
>> necessary mesh and discretization information in one place. Yet, currently
>> this interaction is limited as compared to the "bare" SNES/KSP interface,
>> in part because of the one matrix limitation.
>>
>
> I agree that it's clumsy that -snes_mf_operator is the only "nice" way to
> get matrix-free in some cases, and it only gives you one kind of
> matrix-free.
>

I would also vote for a more transparent name for that option --
"mf_operator" seems rather obscure.

Dmitry.
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