[petsc-users] Help with fieldsplit performance

[Edoardo alinovi]

Hi Matt,

Thanks for your feedback! Would you suggest me the command line to get the
number of sub iterations?

Il Dom 5 Feb 2023, 20:14 Matthew Knepley <knepley at gmail.com> ha scritto:

> On Sun, Feb 5, 2023 at 1:26 PM Edoardo alinovi <edoardo.alinovi at gmail.com>
> wrote:
>
>> Hello Petsc's crew,
>>
>> I would like to ask for some support in setting up the fieldsplit
>> preconditioner in order to obtain better performance. I have already found
>> some posts on the topic and keep experimenting, but I would like to hear
>> your opinion as experts :)
>>
>> I have my fancy  CFD pressure based coupled solver already validated on
>> some basic problems, so I am confident the matrix is OK. However, I am
>> struggling a bit in finding performance. In my experiments, I have found
>> out that *schur *is the best in terms of overall iteration count, but it
>> takes ages to converge! Using additive or multiplicative looks a better
>> call, but in some cases I get a very high number of iterations to converge
>> (500+).
>>
>> I attach here the logs (ksp_view and log_view)  for an example case of
>> the flow past a 90deg T-junction, 285k cells on 4 procs.
>>
>> GMRES + fieldsplit and schur take 90s to converge with 4 iters. Do you
>> see anything strange in the way ksp is set up?
>>
>
> 1. You are using A11 as the preconditioning matrix for the Schur
> complement. Do you expect this to be a good idea?
>
> 2. You are using BJacobi/ILU(0) for A00. Is this a good idea?
>
> 3. Do you know how many iterates you are using for the A00 solve and for
> the Schur complement? Note that A00 gets solved for each iteration of the
> Schur complement, so you would multiply those two together to get an idea
> of the time.
>
> Once we see the number of subiterates, I think we can say something more.
>
>   Thanks,
>
>     Matt
>
>
>> Thank you for the support as 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
>
> https://www.cse.buffalo.edu/~knepley/
> <http://www.cse.buffalo.edu/~knepley/>
>
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