[petsc-users] Help with fieldsplit performance

[Matthew Knepley]

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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