[petsc-dev] DMGetMatrix --> DMGetMatrices?

[Dmitry Karpeev]

The need for computation reuse (between J and J_pre, for example) might
require a change in the SNES callback model.
Right now each call -- FormFunction, FormJacobian -- is stateless.  It
takes a Vec X and computes the corresponding quantities -- residual, J,
J_pre -- solely from X.  Any callback that  wanted to reuse any other
callback's computation would have to be able to make sure the reuse is
safe: am I being called with the same X as this previous callback that
already precomputed a bunch of stuff for me?  This is only possible with a
costly, collective (and imprecise) vector comparison, or a fragile state
tracker within X.  If this is unacceptable, the callbacks must carry (and
share) a state, and, thus, become objects.

That's probably undesirable within the basic SNES callback model, but may
be natural within the DM-based callback model: a SNES sets its DM's Vec X
and calls DMFunction, DMJacobian without any argument -- those calls pull
out X and compute whatever they need, including some common data for others
(e.g., PC DM) to reuse.  A new call to DMSetVec() will reset these common
data structures.  This way the DM can also be viewed as a "factory".

This would probably make my interaction with libMesh a bit easier, but
isn't strictly necessary.  Still, it would be good to know any thoughts on
this.

Dmitry.

On Sun, Feb 12, 2012 at 10:32 AM, Dmitry Karpeev <karpeev at mcs.anl.gov>wrote:

>
>
> On Sat, Feb 11, 2012 at 8:36 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>
>> On Sat, Feb 11, 2012 at 08:32, Dmitry Karpeev <karpeev at mcs.anl.gov>wrote:
>>
>>> This sounds fine to me, but what's the API for doing this? You want to
>>> do KSPSetDM() and PCSetDM(),
>>> so that the user has to explicitly pull out the PC to set its DM?
>>>
>>
>> The PC *always* owns both matrices anyway.
>>
>
> I think there is a rather natural API for handling DM grouping, code reuse
> and dealing with the issue of single matrix per DM:
>  DM should support DMGetKSPDM(), DMGetPCDM() -- similar to
> DMDAGetCoordinateDA() --  each of which will compute the correct matrix.
> These calls can return the original DM (or PETSC_NULL) by default.  SNES
> will pull out the KSP DM out to set it on the KSP, KSP will do the same for
> the PC, etc. DM already has Vec x, so all of the DMs can share x and
> compute their respective matrices at the same x and reuse whatever
> computation is reusable.  That's up to the implementation, though.
>
> Dmitry.
>
>
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