[petsc-users] Question about using MatSNESMFWPSetComputeNormU

[Matthew Knepley]

On Thu, Aug 18, 2016 at 12:04 PM, 최경준 <kyungjun.choi92 at gmail.com> wrote:

> 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?
>

Yes, but again the solve will likely perform very poorly.

  Thanks,

     Matt


> 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
>>
>
>


-- 
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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