[petsc-users] General Questions

[Junchao Zhang]

On Fri, Aug 8, 2025 at 3:29 PM Matthew Knepley <knepley at gmail.com> wrote:

> 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.
>

The experimental results were here,
https://urldefense.us/v3/__https://gitlab.com/petsc/petsc/-/merge_requests/6580*note_1477784906__;Iw!!G_uCfscf7eWS!ZtYn-pBESeQYKuVZWdDAExVVX8TAYzfmL3zhMHnUbQcF73fi0TYJOnIs2Kufu58HSzDbtYoMUGGF-E7vzU_1s4L9-tBB$ 


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