[petsc-users] Question About Assembly matrix and declaration of KSP & pc

[Matthew Knepley]

On Wed, Nov 30, 2022 at 8:25 AM 김성익 <ksi2443 at gmail.com> wrote:

>
> In your comments,
> KSPSetOperators is called if you want to change the system matrix.
>
> "change the system matrix" means the components of matrix are changed?
> I mean the values of some components of matrix are changed.
>

If you just change values in the matrix, you do not have to call it again.

  Thanks,

     Matt


> Thanks,
> Hyung Kim
>
>
> 2022년 11월 30일 (수) 오후 10:00, Matthew Knepley <knepley at gmail.com>님이 작성:
>
>> On Wed, Nov 30, 2022 at 7:51 AM 김성익 <ksi2443 at gmail.com> wrote:
>>
>>> Thank you for your comments.
>>> However, I have more questions.
>>>
>>> 1. Generally,  (KSPCreate, KSPSetOperators, KSPGetPC, PCSetType,
>>> PCFactorSetMatSolverType, KSPSetFromOptions )
>>> above functions are should be called after  each "MatassemblyEnd??"
>>>
>>
>> KSPCreate is called once.
>>
>> You do not need PCSetType, PCFactorSetMatSolverType, KSPSetFromOptions
>> more than once, unless you want to change the solver type.
>>
>> KSPSetOperators is called if you want to change the system matrix.
>>
>> KSPSolve is called when you want to change the rhs.
>>
>>   Thanks,
>>
>>      Matt
>>
>>
>>> 2.   Though reading the user guide, I don't fully understand under what
>>> circumstances the functions mentioned above should be called again. Can you
>>> explain when each function should be called?
>>>
>>> Thanks,
>>>
>>> Hyung Kim
>>>
>>> 2022년 11월 30일 (수) 오후 8:37, Mark Adams <mfadams at lbl.gov>님이 작성:
>>>
>>>>
>>>>
>>>> On Wed, Nov 30, 2022 at 5:08 AM 김성익 <ksi2443 at gmail.com> wrote:
>>>>
>>>>> Hello,
>>>>>
>>>>>
>>>>>
>>>>> I’m working on FEM using PETSc.
>>>>>
>>>>> As everyone knows, it is necessary to repeatedly solve Ax=B.
>>>>>
>>>>> Regarding this, I have 4 questions.
>>>>>
>>>>>
>>>>>
>>>>> 1.     There are many steps for preparing KSPSolve. For example
>>>>> KSPcreate, KSPSetOperators, KSPGetPC, PCSetType, PCFactorSetMatSolverType,
>>>>> KSPSetFromOptions…
>>>>> In Nonlinear FEM, there are repeatedly kspsolve for getting answer
>>>>> vector. Is it correct to do all of the aforementioned processes (KSPcreate,
>>>>> KSPSetOperators ~~~) for each KSPSolve? Or should I declare it only once at
>>>>> the beginning and not call it again?
>>>>>
>>>>
>>>> You just do these once at setup but for nonlinear problems
>>>> KSPSetOperators tells the solver that you have a new matrix and so "matrix
>>>> setup" work needs to be done.
>>>>
>>>>
>>>>>
>>>>> 2.     If the answer to question 1 is that it must be repeated every
>>>>> time, should this work be done right before kspsolve, that is, when the
>>>>> global matrix assembly is finished, or is it irrelevant to performance at
>>>>> any time?
>>>>>
>>>>
>>>> KSPSetOperators should be set after the new matrix values are set but
>>>> it might work before. It just sets a pointer to the matrix and flags it as
>>>> not setup.
>>>>
>>>>
>>>>>
>>>>>
>>>>> 3.     When performing FEM, local matrices are often scattered in
>>>>> global matrices depending on connectivity. In this case, which is better in
>>>>> terms of performance: adding the values one by one with MatSetValue or
>>>>> adding them all at once with MatSetValues even if they are scattered?
>>>>>
>>>>
>>>> You want to add one element matrix at a time, generally.
>>>>
>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> 4.     I would like to measure the time of each section of the
>>>>> process. Which method is recommended?
>>>>>
>>>>
>>>> PETSc methods are timed separateluy, but setup gets folded into
>>>> KSPSolve unless you call SNESSetUp before the SNES[KSP]Solve.
>>>> You can add you own timers also
>>>> https://petsc.org/release/docs/manualpages/Profiling/PetscLogEventRegister/
>>>>
>>>> Mark
>>>>
>>>>
>>>>
>>>>>
>>>>>
>>>>> Thank you for your help.
>>>>>
>>>>>
>>>>>
>>>>> Hyung Kim
>>>>>
>>>>
>>
>> --
>> 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
>>
>> https://www.cse.buffalo.edu/~knepley/
>> <http://www.cse.buffalo.edu/~knepley/>
>>
>

-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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