[petsc-dev] Questions about Petsc

[Matthew Knepley]

On Fri, Sep 6, 2013 at 2:26 PM, Peter Brune <prbrune at gmail.com> wrote:

>
>
>
> On Fri, Sep 6, 2013 at 1:53 PM, Matthew Knepley <knepley at gmail.com> wrote:
>
>> 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.
>>
>>
> Multiblock never actually ran and therefore it has been removed from
> compilation for some time.  The code is still sitting there, though.  We've
> (Jed and I) been discussing how to put this together for some time and we
> haven't settled on a model of prying apart the fields and field
> residual/Jacobian assembly in such a way that makes this easy enough for
> users to futz around with.  We could easily do something akin to what we do
> with (N)ASM and have hooks that spirit the whole solution to where the
> evaluation needs it.
>

At one point it ran :) So I think the way to do this is to only activate it
if the user has a DM+PetscSection. Then we have enough information
to take apart the nonlinear residual evaluation. In fact,
DMPlexComputeResidual() does it field-by-field right now.

   Matt


> So, what do you do?  Do you do whole-system residual evaluation followed
> by restriction to the subsystem?  Do you require the user to give you a
> number of subproblems living on potentially different discretizations and
> ways of gluing them together?  These things need to be decided.
>
> ASPIN-like things with this would require having the outer solver know how
> to pull apart the inner solver and get at its tasty, tasty fieldsplit
> Jacobians.  It would be a pretty easy extension of the preexisting ASPIN
> stuff.
>
> - Peter
>
>    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
>>
>
>


-- 
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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