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

[Dmitry Karpeev]

On Mon, Feb 6, 2012 at 1:30 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Feb 6, 2012, at 1:27 PM, Matthew Knepley wrote:
>
> > On Mon, Feb 6, 2012 at 1:23 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >
> > On Feb 6, 2012, at 1:14 PM, Matthew Knepley wrote:
> >
> > > 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.
> >
> >   Ok, then just like VecSetSizes() we stash this information if given
> before the type is set and use it when the type is set.  However if it is
> set after the type is set (and after the sizes are set) then we need to
> destroy the old datastructure and build a new one which means messier code.
>   By instead actually allocating the data structure at VecSetUp() the code
> is cleaner because we never need to take down and rebuild a data structure
> and yet order doesn't matter.  Users WILL need to call VecSetUp() before
> VecSetValues() and possibly a few other things like they do with Mat now.
> >
> > We just disallow setting it after the type, just like sizes. I don't see
> the argument against this.
>
>    We allow setting the sizes after the type.
>

Since we are on a related subject: should then all type-specific processing
of sizes be moved out of MatSetSizes()
into MatSetUp?  By this I mean this code:
 if (A->ops->setsizes) {
    /* Since this will not be set until the type has been set, this will
NOT be called on the initial
       call of MatSetSizes() (which must be called BEFORE MatSetType() */
    ierr = (*A->ops->setsizes)(A,m,n,M,N);CHKERRQ(ierr);
  } else {

This eliminates the need to check for the presence of various type-specific
setup methods  -- they will all be called in MatSetUp after the type is
guaranteed to have been set.  This would also make MatSetSizes not
collective.  I imagine that Vec could be organized the same way.  I
actually would prefer VecSetUp to explicitly delineate the end of the
"factory" phase.

Dmitry.

>
>
> >
> >    Matt
> >
> >
> >   Barry
> >
> > >
> > >   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
> >
> >
> >
> >
> > --
> > 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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