[petsc-dev] change default KSP max iterations

[Barry Smith]

On Oct 13, 2014, at 10:30 PM, Jed Brown <jed at jedbrown.org> wrote:

> Barry Smith <bsmith at mcs.anl.gov> writes:
> 
>>  Does anyone object if I change the default KSP max iterations from 10,000 to 250? 
>> 
>>  The rational is that 10,000 is absurd and if you don’t see convergence by 250 it generally is pretty hopeless. Users, of course, can set a higher max it.
> 
> This depends a bit on the problem.  Iterative methods for high-frequency
> problems often need a lot of iterations, for example.
> 
> My concern is mostly for people that only every once in a while use more
> than 250 iterations, the solve fails (confusingly because it "used to
> work"), and they have to start over.  Also that just to demonstrate
> simple scaling behavior with an example like KSP ex2, you'll need to
> bump up the max iterations.
> 
> Does the marginal value of saving seconds/minutes(?)

   8+ hours is how long one of the (pretty small) nightly test examples that used inner outer iterations (when someone screwed up the code so the outer did not converge). Normally the example took < 10 iterations and ran very quickly. 




> for a solver to
> reach 10k iterations (when you forgot to enable enough monitoring to
> know what's happening) offset the confusion and alleged misbehavior for
> the people that want to keep iterating?