[petsc-dev] Questions about Petsc

[Matthew Knepley]

On Fri, Sep 6, 2013 at 1:33 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> Please send these questions to petsc-dev (Cc'd now) or petsc-users so
> that others can comment.
>
> Lulu Liu <lulu.liu at kaust.edu.sa> writes:
>
> > Hi,
> >
> > I want to implement ASPIN-like ideas, but based on field splitting. You
> > could find the details in PDF file.
>
> You are proposing left-preconditioned nonlinear additive fieldsplit.
> This is a sensible algorithm that we have discussed before.  The only
> part that needs to be implemented is the fieldsplit solve: separation of
> G and H, then solving each separately.
>
> Unfortunately, I think it's kinda tricky to come up with a generally
> useful interface.  In particular, many applications cannot partition the
> variables like you have done, meaning that the preconditioner is
> becomes: change variables, solve in other bases, then change back.  At
> this point, I would recommend implementing the solve with \hat F using
> SNESShell.
>

Did we throw away the SNES 'multiblock' code? It was supposed to do this.

   Matt


> > My questions:
> >
> > I have a nonlinear system including two equations. In my implementation,
> I
> > need to solve each equation separately, also I need to solve the coupled
> > system. I feel confused about ordering:
> >
> > I know the unknowns are ordered as [p1,s1,p2,s2,p3,s3,......] in PETSc,
> > however, it seems that I need the variables like [p1,p2,p3,...
> s1,s2,s3,...]
>
> No, just use a strided IS to describe each set of variables.  You can
> VecScatter from the packed/monolithic global vector to the split space.
>
> > Is there any example to do the similar things in PETSc?
> > Do you have any ideas to implement the algorithm easier?
> > How to quickly extract [p1,p2,...p_{N}] or [s1,s2,....] from
> > [p1,p2,....s1,s2,s3,...] ?
> > How to quickly get the composed [p1,p2,...s1,s2,....] if I have
> [p1,p2,...]
> > and [s1,s2,..]?
> >
>
>


-- 
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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