[petsc-dev] [petsc-users] VECMPICUSP with ghosted vector

[Matthew Knepley]

On Mon, Feb 6, 2012 at 1:11 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Feb 6, 2012, at 12:47 PM, Jed Brown wrote:
>
> > On Mon, Feb 6, 2012 at 21:42, Matthew Knepley <knepley at gmail.com> wrote:
> > I don't like this because it would mean calling VecSetUp() all over the
> place. Couldn't the ghosting flag be on the same
> > level as the sizes?
> >
> > Maybe VecSetUp() is wrong because that would imply collective. This
> memory allocation is simple and need not be collective.
> >
> > Ghosting information is an array, so placing it in VecSetSizes() would
> seem unnatural to me. I wouldn't really want
> VecSetGhosts(Vec,PetscInt,const PetscInt*) to be order-dependent with
> respect to VecSetType(), but maybe the VecSetUp() would be too messy.
>
>    Only some vectors support ghosting, so the usual PETSc way (like with
> KSPGMRESRestart()) is to calling the specific setting routines ONLY AFTER
> the type has been set.  Otherwise all kinds of oddball type specific stuff
> needs to be cached in the object and then pulled out later; possible but is
> that desirable? Who decides what can be set before the type and what can be
> set after? Having a single rule, anything appropriate for a subset of the
> types must be set after the type is set is a nice simple model.
>
>   On the other hand you could argue that ALL vector types should support
> ghosting as a natural thing (with sequential vectors just have 0 length
> ghosts conceptually) then it would be desirable to allow setting the ghost
> information in any ordering.
>

I will argue this.


>   Sadly we now pretty much require MatSetUp() or a
> MatXXXSetPreallocation() to be called so why not always have VecSetUp()
> always called?
>

Because I don't think we need it and it is snother layer of complication
for the user and us. I think
we could make it work where it was called automatically when necessary, but
that adds another
headache for maintenance and extension.

    Matt


>   We have not converged yet,
>
>    Barry
>
>
>


-- 
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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