[petsc-users] SNES and ghosted vector of unknowns

[Dominik Szczerba]

Thanks Barry for the hint.
I am indeed solving my nonlinear system using the Picard scheme.
I was once advised to set the Jacobian to my coefficient matrix "A" in
the FormJacobian function passed to SNES to achieve this goal (I am
still trying to get it to work with limited success though).
Are you suggesting I should abandon this approach in favor of the new
functionality in petsc-dev?

Regards,
Dominik

On Sat, Apr 21, 2012 at 7:10 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>   Domink,
>
>     Sounds like you want to do a "Picard" iteration. You should use http://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/SNES/SNESSetPicard.html  in petsc-dev
>     Note with this you never compute f(x) = A(x)x - b(x)   you provide a function to compute b(x) and A(x).
>
>     Please let us know if this new functionality doesn't serve your purpose.
>
>      You would do the ghost point update in the function that computes b().
>
>
>    Barry
>
>
> On Apr 21, 2012, at 12:01 PM, Dominik Szczerba wrote:
>
>> Thank a lot for the hints, but I still one little clarification. The
>> FormFunction function has a signature
>>
>> PetscErrorCode ScalarNonLinearSolver::FormFunction(SNES snes, Vec x,
>> Vec f, void *ctx)
>>
>> where I am supposed to compute A(x), b(x) and then fill f with f(x) =
>> A(x)x - b(x)
>>
>> The question is: should I call the ghost update on the passed argument
>> x, or on my own x that I maintain in my application as a part of a KSP
>> system? Or are they supposed to be actually the same?
>>
>> Thanks a lot
>> Dominik
>>
>> On Fri, Apr 20, 2012 at 4:13 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>>> On Fri, Apr 20, 2012 at 07:08, Matthew Knepley <knepley at gmail.com> wrote:
>>>>
>>>> You can use
>>>>
>>>>
>>>>   http://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/SNES/SNESSetUpdate.html
>>>
>>>
>>> Please don't do it this way, line searches and other methods will not work.
>>> A ghosted vector is defined by the global entries, not the local form.
>>> Update the local form when you want it to be current.
>