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

[김성익]

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.


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/>
>
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