[petsc-dev] SNESSolve and changing dimensions

[Matthew Knepley]

On Wed, Apr 3, 2019 at 9:31 AM Pierre Jolivet <pierre.jolivet at enseeiht.fr>
wrote:

> On 3 Apr 2019, at 3:15 PM, Stefano Zampini <stefano.zampini at gmail.com>
> wrote:
>
> Pierre,
>
> using MatHeaderReplace should be discouraged in a SNES/TS callback ;-)
>
>
> Gotcha!
>
> I coded this way to have the possibility of changing the MatType while
> running SNES or TS, not for variable sizes.
>
> A tricky way of achieving what you want is to code a proper hook function
> (see
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/SNES/SNESSetUpdate.html),
> which gets called right before each linear solve.
> This is very tricky (and implementation dependent) because you also need
> to update/interpolate all the relevant vectors SNES may use later for line
> search
>
> The proper solution would be to extend DMAdaptor to the case of
> user-defined refinements and stopping criterion callbacks. Matt?
>
>
> Following Matt’s message, I’ve permuted the two “loops" (SNESSolve and
> mesh adaptation), but if there is some way to do the adaptation in the
> inner loop coming along, I’ll give it a go.
>

For now, this is the best.

Can you tell me a little about what you are adapting, and why adapting
after only a linearized solve would be preferable
to adapting after the nonlinear solve?

  Thanks,

    Matt


> Thanks,
> Pierre
>
> Il giorno mer 3 apr 2019 alle ore 14:52 Matthew Knepley via petsc-dev <
> petsc-dev at mcs.anl.gov> ha scritto:
>
>> On Wed, Apr 3, 2019 at 5:43 AM Pierre Jolivet via petsc-dev <
>> petsc-dev at mcs.anl.gov> wrote:
>>
>>> I’d like to do mesh adaptation in a Newton loop.
>>> I’m using a SNES with SNESSetFunction and SNESSetJacobian.
>>> My first question is: may the dimension of the linear systems change
>>> throughout the nonlinear iterations?
>>>
>>
>> No. Everything we do is algebraic right now. You would have to
>> reformulate Newton in
>> some Banach space, and make sure all the measures you were using were
>> discretization
>> independent. Then you could allow the underlying linear algebra to
>> change, but you would
>> need operations for projecting the the new space, making BC in the new
>> space, etc. There
>> is a lot of machinery in the continuous space that PETSc does not have.
>>
>> It sounds like it should be setup as an FAS.
>>
>>   Thanks,
>>
>>      Matt
>>
>>
>>> If so, what are the proper things to do? I tried to do as Stefano in
>>> MFEM and use MatHeaderReplace
>>> https://github.com/mfem/mfem/blob/master/linalg/petsc.cpp#L3833 inside
>>> the function supplied to SNESSetJacobian, but I end up with a [0]PETSC
>>> ERROR: PCApply() line 455 in petsc/src/ksp/pc/interface/precon.c
>>> Preconditioner number of local rows 6561 does not equal resulting vector
>>> number of rows 1681 further down the road. Should I also call something
>>> like VecHeaderReplace (which apparently does not exist) on the residual Vec?
>>>
>>> Thanks,
>>> Pierre
>>>
>>
>>
>> --
>> 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
>>
>> https://www.cse.buffalo.edu/~knepley/
>> <http://www.cse.buffalo.edu/~knepley/>
>>
>
>
> --
> Stefano
>
>
>

-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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