[petsc-dev] PetscSection

[Jed Brown]

On Thu, Nov 8, 2012 at 10:58 PM, Matthew Knepley <knepley at gmail.com> wrote:

> I don't care about expensive VecGetArray(). We do it absolutely
> everywhere in PETSc.
> We recommend that users do it. This is not a real objection.
>

We normally do one VecGetArray(), then iterate over the whole local part.
That's different from doing it in an inner loop. Many other packages do it
in inner loops (overloading operator[] is popular among some), but that
restricts the Vec types that can perform well with that package.


> > What other sections do users need? A trace space?
>
> Blaise uses a couple for every problem. We have several in PyLith
> (ground surface,
> fault).
>

What is the difference between using a PetscSection for each thing and
using an IS for those points on the feature? I guess just the extra
indirection (IS) versus more duplication (a PetscSection is heavier than an
IS).


> >
> > How do you intend to support many threads calling
> DMComplexVecGetClosure(),
> > perhaps each with multiple buffers, without expensive instructions?
>
> GetClosure() is not a problem. All threads are reading from the same
> topology, and
> writing to separate output buffers. SetClosure() needs to be locked.
>

You need separate buffers per thread, but we can make DMGetWorkArray() have
a separate cache per thread. VecGetArray() is still not trivially cheap
because we need atomics. Maybe I'm being overly paranoid, but I don't like
the granularity that imposes.


> >>
> >>
> >> These are for multi-field splitting/
> >
> >
> > Clearly, but why do they need to be in PetscSection? We already need DMs
> > associated with fields.
>

Answering my own question: the distinction between DM and Section is that
the latter can address a discontinuous part of a Vec so addressing by field
does not imply a copy.


>
> Someone needs to know the splitting. If you had a DM, this info is
> still stuck down
> somewhere, so that DM is holding other DMs.
>

It's still not clear to me why they are needed. If PetscSection is
user-visible, can't they just be different Sections?
DMComplexGetFieldSection(dm,field,&fsection);
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