[petsc-dev] [petsc-users] Multigrid with defect correction

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:
>> The DMSNES and friends are the parts that are solver specific, since a given DM can be associated with several solvers.
>
>    No, that is not the reason they exist. Every DM actually contains its DMKSP, DMSNES, and DMTS so at any point in time a DM can actually have only one DMXXX. We had arguments over sharing DM with multiple solvers but I think that is a goofy optimization.

libpetscdm.so does not depend on solvers.  The DMKSP, DMSNES, are
basically private DM plugins that are packaged with their respective
solvers.  The old DMMG used void* to avoid a source-level dependency,
but was more limited in capability and much less safe.  (Remember that
the DMSNES design was a replacement for DMMG, simplifying a complicated
and brittle interface with messy dependencies, not some complexity that
was just added to SNES.)

>> That is why the resolution-dependent information can go there. You could have stuck all that info in the solver I guess, but perhaps Jed has a good reason not to do that (maybe to make it easier to pass around, or maybe extensible to different DMs).

We had lots of functions that needed to be forwarded for nonlinear
preconditioning and the like.

>    Hmm, since PC/KSP/SNES/TS know about DM why don't we just stick that information in the solver. Seems simpler, there must be some reason.

That information would need to be forwarded to sub-solvers (nonlinear
preconditioners, nonlinear DD subdomain solvers, FAS).  It is very
useful to have reference counting and to minimize order-dependence.

>> I am not against deprecation there, but we have to preserve an easy way for people to just feed there crap in, meaning no creating a DMSHELL or other setup.
>> 
>>    Matt
>>  
>> Perhaps we need to revisit what DM is? Looking at the struct _p_DM is rather scary.

I'm more suspicious about the sections and constraints than about the
coarsen and subdomain hooks.

I know you're skeptical of the DMSNES design, but I think it is highly
susceptible to second system syndrome so let's not leap to conclusions
about its replacement.
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