[petsc-users] Petsc ILU PC Change between 3.6.4 and 3.7.x?

[Matthew Knepley]

On Thu, Aug 24, 2017 at 2:29 AM, Klaij, Christiaan <C.Klaij at marin.nl> wrote:

> Matt,
>
> Thanks, I can understand the lower condition number of P A, but
> what about r? Doesn't that change to P r and if so why can we
> assume that ||r|| and ||P r|| have the same order?
>
r and Pr are the things we can control. We make them whatever we want.

   Matt

> Chris
>
>
> dr. ir. Christiaan Klaij | Senior Researcher | Research & Development
> MARIN | T +31 317 49 33 44 <+31%20317%20493%20344> | C.Klaij at marin.nl |
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> ------------------------------
> *From:* Matthew Knepley <knepley at gmail.com>
> *Sent:* Wednesday, August 23, 2017 8:37 AM
> *To:* Barry Smith
> *Cc:* Klaij, Christiaan; petsc-users at mcs.anl.gov
> *Subject:* Re: [petsc-users] Petsc ILU PC Change between 3.6.4 and 3.7.x?
>
> On Wed, Aug 23, 2017 at 2:30 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>>
>>    Some argue that the preconditioned residual is "closer to" the norm of
>> the error than the unpreconditioned norm. I don't have a solid mathematical
>> reason to prefer left preconditioning with the preconditioned norm.
>
>
> Because you have || x - x_exact || < k(A) || r ||
>
> where r is the residual and k is the condition number of A. If instead of
> A you use P A, which we assume has a lower condition number, then
> this bound is improved.
>
>   Thanks,
>
>      Matt
>
>
>>
>>    Barry
>>
>>
>>
>> > On Aug 22, 2017, at 11:27 PM, Klaij, Christiaan <C.Klaij at marin.nl>
>> wrote:
>> >
>> > Barry,
>> >
>> > Thanks for the explanation.
>> >
>> > We do have some rare cases that give false convergence, but
>> > decided to use
>> >
>> > CALL KSPSetNormType(ksp,KSP_NORM_UNPRECONDITIONED,ierr)
>> >
>> > so that convergence is always based on the true residual. Our
>> > results are much more consistent now. So that could have been
>> > your protection against the rare case as well, right? Why do you
>> > prefer left preconditioning?
>> >
>> > Chris
>> >
>> >
>> >
>> > dr. ir. Christiaan Klaij  | Senior Researcher | Research & Development
>> > MARIN | T +31 317 49 33 44 | mailto:C.Klaij at marin.nl |
>> http://www.marin.nl
>> >
>> > MARIN news: http://www.marin.nl/web/News/N
>> ews-items/BlueWeek-October-911-Rostock.htm
>> >
>> > ________________________________________
>> > From: Barry Smith <bsmith at mcs.anl.gov>
>> > Sent: Tuesday, August 22, 2017 6:25 PM
>> > To: Klaij, Christiaan
>> > Cc: petsc-users at mcs.anl.gov
>> > Subject: Re: [petsc-users] Petsc ILU PC Change between 3.6.4 and 3.7.x?
>> >
>> >> On Aug 22, 2017, at 6:49 AM, Klaij, Christiaan <C.Klaij at marin.nl>
>> wrote:
>> >>
>> >> We also faced this problem in our code. So I've added:
>> >>
>> >> CALL PetscOptionsSetValue(PETSC_NULL_OBJECT,"-sub_pc_factor_shift
>> _type","nonzero",ierr)
>> >>
>> >> since there seems to be no setter function for this (correct me
>> >> if I'm wrong). Then everythings fine again.
>> >>
>> >> Out of curiosity, what was the reason to change the default
>> >> behaviour?
>> >
>> >   The reason we changed this is that we would rather have a failure
>> that makes the user aware of a serious problem then to produce "garbage"
>> results. In some rare cases the shift can cause a huge jump in the
>> preconditioned residual which then decreases rapidly while the true
>> residual does not improve. This results in the KSP thinking it has
>> converged while in fact it has essentially garbage for an answer. Under the
>> previous model, where we shifted by default, users would in this rare case
>> think they had reasonable solutions when they did not.
>> >
>> >   For many users, such as yourself, the previous default behavior was
>> fine because you didn't have the "rare case" but we decided it was best to
>> protect against the rare case even though it would require other users such
>> as yourself to add the option.
>> >
>> >   Barry
>>
>>
>
>
> --
> 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
>
> http://www.caam.rice.edu/~mk51/
>
>


-- 
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

http://www.caam.rice.edu/~mk51/
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