[petsc-dev] questions regarding TSDM

[Peter Brune]

On Tue, Nov 20, 2012 at 10:21 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Nov 20, 2012, at 9:10 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>
> > On Tue, Nov 20, 2012 at 3:08 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> > The object is called TSDM but methods are DMTSXXX()?
> >
> > DMTS... is right because they are methods on a DM, though they are
> packaged with TS. The user _never_ sees the internal "TSDM" object. I think
> the original motivation for that naming structure was to prevent conflict
> with a TS/SNES implementation of the same name, but I would not object to
> switching to DMTS
> >
> >
> > TSDM is itself not a PETSc object but it is wrapped into a
> PetscContainer (which is a PETSc object), so why not just make TSDM a PETSc
> object? Is it because of circular references?
> >
> > I don't mind making it a PetscObject.
> >
> > What's with the complicated DMTSGetContextWrite() (copy on write
> crapola) stuff? This kind of paradigm is not used anywhere else in PETSc,
> is it really needed? Seems overly complex, is it just to save a few small
> objects?
> >
> > It's for interface simplicity. If the user sets the the residual routine
> on the finest level and then changes it, they probably expect their change
> to propagate through the levels. But if they get out a coarse level and
> explicitly change it there, they probably intended to use a different
> discretization/physics on the coarse level.
>
>    How do they "get out a coarse level and explicitly change it" for
>
> 1) KSP?
> 2) SNES?
> 3) TS?
>
> >
> >
> > The DMTSGetContext() business uses PetscObjectCompose to attach the TSDM
> to a dm. Why not just have DM's have opaque pointers to KSPDM, SNESDM, and
> TSDM built into the DM and things
> > like DMGetKSPDM() that do the usual dm->kspdm access instead of
> composing business.  Since the business of DM's is to talk to TS, SNES, and
> KSP there is no reason to go through the more complicated object compose
> business is there? (The object compose business is for out-of-the-ordinary
> stashing of stuff, not ordinary stashing of stuff, it is kind of like using
> object compose to attach the ksp to the snes).
> >
> > Okay, but when the optimizers need to attach something, do we add
> special support for that in DM as well?
>
>    Sure
>
> > What about UQ and so on? The idea here was to keep DM as ignorant as
> possible, but allow it to carry this extra data around for the various
> solver objects that need it.
>
>   Ok, objectcompose is certainly more extensible.
>
>   BTW: DMTSGetContext() is kind of a funky name, context is a very general
> term, this function returns a TSDM  (or DMTS) so could be DMGetDMTS() or
> DMTSGetDMTS()
>
>
Should it be DMGetTSDM?

I'v'e thought this (DMTS vs TSDM) an awkward distinction for a while too.
However, would removing weasel words like "Context" from the names make it
more clear?

- Peter


>    Barry
>
>
> >
> >   Perhaps some refactoring could bring this stuff down to the
> simple-minded design of the rest of PETSc?
> >
> >     Barry
> >
> >
>
>
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