[petsc-users] General Questions

[Matthew Knepley]

On Fri, Aug 8, 2025 at 12:22 PM Frank Bramkamp <bramkamp at nsc.liu.se> wrote:

> Dear PETSc Team,
>
> I have the following general questions.
> When using GMRES with classic gram schmidt, you have an optional
> refinement.
>
> The default is not to use any refinement, is that correct ?


Yes. It has been this way at least since 2013 from checking the log.


> So you use the standard classic gram schmidt
> without any additional refinement step.
>
> I originally thought that you use one refinement step if needed as
> default. Or was that somehow changed one day
> and I remember it from older settings ?!
>

It is available, but we changed it since refinement was almost never
happening.


> With refinement one also needs about twice the MPI communication, if
> refinement is done in each iteration,
> which can be a bit more costly due to the MPI_ALLREDUCE for the dot
> products.
>

It is reduction that is expensive on large runs.


> I solve the compressible navier stokes equations with turbulence. In which
> cases do you have the experience one should
> better use a refinement step ?!  So far I typically used the default
> options, which seemed to work well for me so far.
>

If it is converging, everything is likely fine. You can always check the
true residual to detect breakdown, or turn on refinement if necessary with
the option.


> for classic gram schmidt one also has to compute multiple dot products
> which is VecMDot in PETSC if I see this correct.
>

Yes.


> Do you by standard use blas GEMV to compute multiple dot products at once
> or do you typically prefer your
> custom implementation. I think you also have a version where you unroll
> manual loops by a factor of 4 or so.
> I am not quite sure what is the typical default for this, or in which
> situation one would not use GEMV.
>

It turns out that Junchao did extensive tests on this last year (I
believe). By default, we use GEMV for MDot, but not for MAXPY (since
performance here was worse with vendor GEMV). It is difficult (or
impossible) to tell which algorithm will be better without testing, so
there are options to select each one.

  Thanks,

     Matt


> Thanks a lot. I wish you a nice weekend, Frank
>
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-- 
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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