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

[Xiangdong]

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