[petsc-users] Question about using MatSNESMFWPSetComputeNormU

[최경준]

Then in order not to use preconditioner,

is it ok if I just put A matrix-free matrix (made from MatCreateSNESMF())
into the place where preA should be?


The flow goes like this
- call SNESCreate
- call SNESSetFunction(snes, r, FormResidual, userctx, ier)
- call MatCreateSNESMF(snes, A, ier)
- call SNESSetJacobian(snes, A, A, FormJacobian, userctx, ier)
- call SNESSetFromOptions()

- call SNESGetKSP(snes, ksp, ier)
- call KSPSetType(ksp, KSPGMRES, ier)
- call KSPGetPC(ksp, pc, ier)
- call PCSetType(pc, PCNONE, ier)
- call KSPGMRESSetRestart(ksp, 30, ier)

- call SNESSolve()
.
.


and inside the FormJacobian routine
- call SNESComputeJacobian(snes, v, J, pJ, userctx, ier)  --> J and pJ must
be pointed with A and A.



Thank you again,

Kyungjun.

2016-08-19 1:44 GMT+09:00 Matthew Knepley <knepley at gmail.com>:

> On Thu, Aug 18, 2016 at 11:42 AM, 최경준 <kyungjun.choi92 at gmail.com> wrote:
>
>> Thanks for your helpful answers.
>>
>> Here's another question...
>>
>> As I read some example PETSc codes, I noticed that there should be a
>> preconditioning matrix (e.g. approx. jacobian matrix) when using
>> MatCreateSNESMF().
>>
>> I mean,
>> after calling MatCreateSNESMF(snes, A, ier),
>> there should be another matrix preA(preconditioning matrix) to use
>> SNESSetJacobian(snes, A, preA, FormJacobian, ctx, ier).
>>
>>
>> 1) Is there any way that I can use matrix-free method without making
>> preconditioning matrix?
>>
>
> Don't use a preconditioner. As you might expect, this does not often work
> out well.
>
>
>> 2) I have a reference code, and the code adopts
>>
>> MatFDColoringCreate()
>> and finally uses
>> SNESComputeJacobianDefaultColor() at FormJacobian stage.
>>
>> But I can't see the inside of the fdcolor and I'm curious of this
>> mechanism. Can you explain this very briefly or tell me an example code
>> that I can refer to. ( I think none of PETSc example code is using
>> fdcolor..)
>>
>
> This is the default, so there is no need for all that code. We use naive
> graph 2-coloring. I think there might be a review article by Alex Pothen
> about that.
>
>   Thanks,
>
>     Matt
>
>
>>
>> Best,
>>
>> Kyungjun.
>>
>> 2016-08-19 0:54 GMT+09:00 Matthew Knepley <knepley at gmail.com>:
>>
>>> On Thu, Aug 18, 2016 at 10:39 AM, 최경준 <kyungjun.choi92 at gmail.com> wrote:
>>>
>>>> 1) I wanna know the difference between applying option with command
>>>> line and within source code.
>>>> From my experience, command line option helps set other default
>>>> settings that I didn't applied, I guess.
>>>>
>>>
>>> The command line arguments are applied to an object when
>>> *SetFromOptions() is called, so in this case
>>> you want SNESSetFromOptions() on the solver. There should be no
>>> difference from using the API.
>>>
>>>
>>>> 2) I made a matrix-free matrix with MatCreateSNESMF function, and every
>>>> time I check my snes context with SNESView,
>>>>
>>>>     Mat Object:     1 MPI processes
>>>>       type: mffd
>>>>       rows=11616, cols=11616
>>>>         Matrix-free approximation:
>>>>           err=1.49012e-08 (relative error in function evaluation)
>>>>           The compute h routine has not yet been set
>>>>
>>>> at the end of line shows there's no routine for computing h value.
>>>> I used MatMFFDWPSetComputeNormU function, but it didn't work I think.
>>>> Is it ok if I leave the h value that way? Or should I have to set h
>>>> computing routine?
>>>>
>>>
>>> I am guessing you are calling the function on a different object from
>>> the one that is viewed here.
>>> However, there will always be a default function for computing h.
>>>
>>>   Thanks,
>>>
>>>     Matt
>>>
>>>
>>>> Kyungjun.
>>>>
>>>> 2016-08-18 23:18 GMT+09:00 Matthew Knepley <knepley at gmail.com>:
>>>>
>>>>> On Thu, Aug 18, 2016 at 8:35 AM, 최경준 <kyungjun.choi92 at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> Hi, I'm trying to set my SNES matrix-free with Walker & Pernice way
>>>>>> of computing h value.
>>>>>>
>>>>>> I found above command (MatSNESMFWPSetComputeNormU) but my fortran
>>>>>> compiler couldn't fine any reference of that command.
>>>>>>
>>>>>> I checked Petsc changes log, but there weren't any mentions about
>>>>>> that command.
>>>>>>
>>>>>> Should I have to include another specific header file?
>>>>>>
>>>>>
>>>>> We have this function
>>>>>
>>>>>   http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpage
>>>>> s/Mat/MatMFFDWPSetComputeNormU.html
>>>>>
>>>>> but I would recommend using the command line option
>>>>>
>>>>>   *-mat_mffd_compute_normu*
>>>>>
>>>>>   Thanks,
>>>>>
>>>>>      Matt
>>>>>
>>>>> Thank you always.
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> 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
>>>
>>
>>
>
>
> --
> 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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