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

[Barry Smith]

> 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




> It seems to have really reduced the robustness of
> PCBJACOBI.
> 
>> Message: 2
>> Date: Fri, 11 Aug 2017 12:28:43 -0500
>> From: Barry Smith <bsmith at mcs.anl.gov>
>> To: Gaetan Kenway <gaetank at gmail.com>
>> Cc: petsc-users <petsc-users at mcs.anl.gov>
>> Subject: Re: [petsc-users] Petsc ILU PC Change between 3.6.4 and
>>        3.7.x?
>> Message-ID: <E09F4C23-527D-4F48-A2E9-BA6B338D36CE at mcs.anl.gov>
>> Content-Type: text/plain; charset="us-ascii"
>> 
>> 
>>   Run with the additional option
>> 
>> -sub_pc_factor_shift_type nonzero
>> 
>> does this resolve the problem. We changed the default behavior for ILU when it detects "zero" pivots.
>> 
>> Please let us know if this resolves the problem and we'll update the changes file.
>> 
>>   Barry
> 
> 
> 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/News-items/Improved-modelling-of-sheet-cavitation-dynamics-on-Delft-Twistll-Hydrofoil-1.htm
>