[petsc-users] preconditioner for -snes_fd or -snes_fd_color

[Xiangdong]

On Fri, Apr 4, 2014 at 5:15 PM, Xiangdong <epscodes at gmail.com> wrote:

>
>
>
> On Fri, Apr 4, 2014 at 5:02 PM, Peter Brune <brune at mcs.anl.gov> wrote:
>
>>
>>
>>
>> On Fri, Apr 4, 2014 at 3:57 PM, Xiangdong <epscodes at gmail.com> wrote:
>>
>>> Hello everyone,
>>>
>>> If we use -snes_fd or -snes_fd_color, can we still pass a preconditioner
>>> to snes/ksp?
>>>
>>> In snes/ex1.c, when I use -snes_fd, the program never calls the
>>> FormJacobian1 to get the preconditioner.
>>>
>>
>> FormJacobian1 forms... wait for it... the Jacobian!  Not the
>> preconditioner.  -snes_fd and -snes_fd_color replace the Jacobian-forming
>> routine.
>>
>
> It looks like the FormJacobian1 forms both Jacobian J and preconditioner
> B. My observations are
>
> 1) Without any options, FormJacobian1 can provide both Jacobian J and
> preconditioner B.
> 2) With -snes_fd, the program does not call the FormJacobian1 function at
> all, even if we have SNESSetJacobian in the codes.
>
> If it does not call the FormJacobian1 and SNESSetJacobian is ignored with
> -snes_fd, how can I provide the preconditioner to snes?
>
> Thank you.
>
> Xiangdong
>
>
>
>>
>>
>>>
>>> Any suggestions about providing preconditioner to snes_fd or
>>> snes_fd_color?
>>>
>>>
>> Both of these explicitly create the approximate Jacobian matrix by using
>> finite differences, so they may be used with any preconditioner.  -snes_mf
>> is a different beast entirely and merely approximates the action of the
>> matrix without forming it, which is where the preconditioner issues come in.
>>
>
Given that -snes_fd is explicitly formed, so -pc_type gamg or ilu will
work. What happens if I want to provide my own preconditioner?

Thank you.

Xiangdong



>
>> - Peter
>>
>>
>>> Thanks.
>>>
>>> Xiangdong
>>>
>>
>>
>
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