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

[Matthew Knepley]

On Thu, Sep 27, 2012 at 2:13 PM, Zou (Non-US), Ling <ling.zou at inl.gov>wrote:

> 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?
>

-snes_fd uses FD to create the entire dense Jacobian. This is just for
testing. -snes_mf uses FD to evaluate the action
of the Jacobian on a vector.


> 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?
> ====================
>

The option -snes does not do anything. I am not sure what you are asking
here. If you mean,
what is used when you pass -snes_mf or -snes_mf_operator, it is FD, not the
user provided
Jacobian routine if it exists.

   Matt


> 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
>>
>
>


-- 
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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