[petsc-users] question on PETSc option '-snes_mf_operator'

[Zou (Non-US), Ling]

Thank you Matt.

I've noticed that there is a different option '-snes_fd', and I guess PETSc
will use finite difference method for both Jacobian (action) and
Preconditioner?

By the way, do you have any comments on my another question?
====================
One more question, if I pass '-snes' to PETSc, since it is direct Newton's
method, I assume it will explicitly construct a Jacobian. Does this
Jacobian come from user provided Jacobian or from the finite difference
Jacobian?
====================

Best,

Ling

On Thu, Sep 27, 2012 at 12:09 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Thu, Sep 27, 2012 at 2:01 PM, Zou (Non-US), Ling <ling.zou at inl.gov>wrote:
>
>> Dear all,
>>
>> I have a question on the PETSc option '-snes_mf_operator'.
>> I am reading the    <PETSc Users Manual Revision 3.3>. On page 100, 2nd
>> paragraph, it says:
>>
>>
>> ==============================================================================
>> "However, it allows us to check the analytic Jacobian we construct in
>> FormJacobian() by passing the -snes_mf_operator flag. This causes PETSc to
>> approximate the Jacobian using finite differencing of the function
>> evaluation (discussed in section 5.6), and the analytic Jacobian becomes
>> merely the preconditioner."
>>
>> ==============================================================================
>>
>> I wonder, if the '-snes_mf_operator' option is used, the Jacobian will
>> always be calculated from the finite difference method while
>> ignore whatever has been provided from user.
>>
> Yes, yhe action will be FD, but it will take the user provided operator to
> form a preconditioner from.
>
>    Matt
>
>
>> One more question, if I pass '-snes' to PETSc, since it is direct
>> Newton's method, I assume it will explicitly construct a Jacobian. Does
>> this Jacobian come from user provided Jacobian or from the finite
>> difference Jacobian?
>>
>>
>> Thanks,
>>
>>
>> Ling
>>
>>
>
>
> --
> 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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