[petsc-users] malconfigured gamg

[Barry Smith]

> On Jan 11, 2017, at 3:51 PM, Jed Brown <jed at jedbrown.org> wrote:
> 
> Arne Morten Kvarving <arne.morten.kvarving at sintef.no> writes:
> 
>> hi,
>> 
>> first, this was an user error and i totally acknowledge this, but i 
>> wonder if this might be an oversight in your error checking: if you 
>> configure gamg with ilu/asm smoothing, and are stupid enough to have set 
>> the number of smoother cycles to 0, your program churns along and 
>> apparently converges just fine (towards garbage, but apparently 'sane' 
>> garbage (not 0, not nan, not inf))
> 
> My concern here is that skipping smoothing actually makes sense, e.g.,
> for Kaskade cycles (no pre-smoothing).  I would suggest checking the
> unpreconditioned (or true) residual in order to notice when a singular
> preconditioner causes stagnation (instead of misdiagnosing it as
> convergence due to the preconditioned residual dropping).

  Jed,

  Yeah but what about checking that the sum of the number of pre and post smooths >=1 ? 
> 
>> once i set sor as smoother, i got the error message
>> 
>> 'PETSC ERROR: Relaxation requires global its 0 positive' which pointed 
>> me to my stupid.
>> 
>> fixing this made both asm and sor work fine.
>> 
>> it's all wrapped up in a schur/fieldsplit (it's P2/P1 navier-stokes), 
>> constructed by hand due to "surrounding" reasons. but i don't think 
>> that's relevant as such. i've used 3.6.4 as the oldest and 3.7.4 as the 
>> newest version and behavior was the same. if you want logs et al don't 
>> hesitate to ask for them, but i do not think they would add much.
>> 
>> cheers
>> 
>> arnem