[petsc-dev] DMComplex and PCFieldSplit

[Matthew Knepley]

On Fri, Feb 24, 2012 at 3:05 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Feb 24, 2012, at 2:41 PM, Matthew Knepley wrote:
>
> > On Fri, Feb 24, 2012 at 2:39 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> > On Feb 24, 2012, at 2:23 PM, Jed Brown wrote:
> >
> > > We need a richer field interface. Some way for a client of DM to
> discover vector and tensor structure.
> >
> >   No idea what a tensor is, but DMGetSubDMS(dm,PetscInt *cnt,IS *ises,DM
> *dms);   and DMPushSubDMType(DM,"name of a type of splitting like u,v,p or
> velocity,p"); DMPopSubDMType(); and DMGetAvailableSubDMTypes(DM,char **);
> >
> >  Do you need more than that?
> >
> > Yuck Yuck Yuck. Why are we persisting in shoving all of the index stuff
> into the DM interface. I think this is so
> > unwieldy and complicated. PetscSection is small, powerful, and does
> everything suggested and more. I see
> > no argument in support of stuff like the above.
>
>   I have no idea what a PetscSection is and
>
> PetscSection - This is a mapping from DMMESH points to sets of values,
> which is
>  our presentation of a fibre bundle.
>
> doesn't help explain it.   PetscSection is small? It has more methods than
> DM, that is not small.
>

Don't make me return the count. It has fewer, and definitely fewer
concepts. Most of those are
simple get/set.


> PetscSectionGetFieldComponents() seems to be intuitive, cool. By why does
> it return an array of integers? The universal way in PETSc would have it
> returning an IS. Wait a minute, it returns one int, the number of
> components? Then why is called GetFieldComponents and not
> GetNumFieldComponents()
>

It could be changed to GetNumFieldComponents.


> And what is all this constraint stuff? PetscSectionGetConstraintDof() Huh?
> It looks inside the "section" and gets an array of ints from a section
> inside the section?
>

Yes. It keeps track of constrained dofs. People, who through laziness or
willful blindness, refuse to acknowledge that some
problems make more sense when constrained dofs ar eliminated.

Here's my guess: a PetscSection is way of "chopping up" a canonical vector
> into a bunch of fields. Fine, but then why does it have a bunch of other
> stuff in it (like constraints, what the hell are constraints)? And why
> isn't a PetscSection just an array of IS that define the fields?
>

Constraints are just that. IS is designed to be immutable. Building on it
internally is not a good idea for this reason.


> The reason to shove all the index stuff into the DM interface is because
> generally one doesn't just want to know the entries of a field but one also
> wants to create vectors for those field variables only, create matrices,
> know something about the mesh for that field so one can generate
> interpolations and coarsen for multigrid. One cannot do any of those things
> with PetscSection.
>

Yes you can.


>  Please explain in freshman linear algebra what a PetscSection is.


Exactly what it says in the help. It is a mapping from mesh points
(vertices, edges, faces, cells) to dofs. How can this be easier?

   Matt


>
>    Barry
>
>
>
>
>
>
> >
> >    Matt
> >
> >    Barry
> >
> > Note that current DMDA's default subdms served up would be a DM for each
> field consisting of one of the degrees of freedom per node and it could
> also serve up  DMs that represent any collections of those for example dof
> 0 and 1 per node, then dof 2 per node. We'll need to come up with some
> naming convention for these.
> >
> >
> > > Possibly also (eventually) a way to solve equations of state so that
> we can formulate reduced problems in non-conservative variables.
> > >
> > > On Feb 24, 2012 2:19 PM, "Matthew Knepley" <knepley at gmail.com> wrote:
> > > On Fri, Feb 24, 2012 at 1:58 PM, Jed Brown <jedbrown at mcs.anl.gov>
> wrote:
> > > http://petsc.cs.iit.edu/petsc/petsc-dev/rev/518ff70e8a0a
> > >
> > > Matt, I want to get rid of these conditionals, not add more. We should
> have a DM base interface for getting fields on which to split, that common
> interface should _replace_ the DMComposite specialization.
> > >
> > > I agree. However, I won't put anything in until I do it by hand once.
> > >
> > > Also, I was tempted to just promote DMGetGlobalISes(), but its not
> quite right. I am planning
> > > to put the IS method in PetscSection. I am hoping eventually DMDA uses
> PetscSection for
> > > layout.
> > >
> > >    Matt
> > >
> > > --
> > > What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> > > -- Norbert Wiener
> >
> >
> >
> >
> > --
> > What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> > -- Norbert Wiener
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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