[petsc-users] malconfigured gamg

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:

>> 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 ? 

Usually fine, but what one potential use case is if someone wants to
test a more aggressive coarsening strategy.  For example, using zero
smooths on odd levels would be double-rate coarsening and might be more
convenient to implement than the direct operators.  (In the
strong-scaling limit, it might also be a good communication pattern for
reducing the process set.)
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