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

[Zou (Non-US), Ling]

Thank you Dmitry.

Yes, I am now fully understanding what '-snes_mf' and '-snes_mf_operator'
are doing. Got a bit confused by the '-snes' keyword used in Moose. As you
mentioned, this is for input completeness sake, so I guess there are
default options as this keyword is used. I will dig out what are those
default options.

Appreciate your answer.

Ling


On Thu, Sep 27, 2012 at 12:30 PM, Dmitry Karpeev <karpeev at mcs.anl.gov>wrote:

>
>
> On Thu, Sep 27, 2012 at 1:22 PM, Matthew Knepley <knepley at gmail.com>wrote:
>
>> 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?
>>> ====================
>>>
>>  Ling,
> -snes isn't really a PETSc option.  I think Moose recommends using it for
> "completeness" and in contrast to -snes_mf or -snes_mf_operator,
> so that users aren't confused about what's being used to compute the
> Jacobian when both -snes_mf and -snes_mf_operator are omitted.
> I'll copy this to to moose-users, in case it is useful there.
>
> -snes_mf will implement the action of the Jacobian approximately by
> differencing the residual. No preconditioner matrix will be used.
> -snes_mf_operator is like -snes_mf, except the user-provided
> preconditioner matrix will be used.
> -snes_fd will *assemble* both the Jacobian and the preconditioner matrix
> using the same residual-differencing algorithm as in -snes_mf.
>
> Dmitry.
>
>
>
>> 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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