[petsc-dev] DMGetMatrix --> DMGetMatrices?

[Jed Brown]

The current API guarantees the FormFunction is called before FormJacobian.
Do you have a use case where that is not satisfied?
On Feb 17, 2012 11:13 PM, "Dmitry Karpeev" <karpeev at mcs.anl.gov> wrote:

> 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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