[petsc-users] reporting failing pcksp solves

[Stephan Kramer]

On 20/06/10 16:21, Stephan Kramer wrote:
> On 20/06/10 05:07, Barry Smith wrote:
>>
>>
>>     Hmm, this is a good question. There are tons of places where some sort of "inner solve" is imbedded in an "outer solver", in fact many levels of nesting. We should handle this in a systematic way which likely means carefully checks put in various places in the code.
>>
>>     We could instead (or in addition) add an option to have KSPSolve() generate an error on "not-convergence"  instead of returning with a negative converged reason. The drawback to this is that you cannot "handle" the error and continue running, it just ends. Would this be useful?
>>
>>      Barry
>
> I can see the logistical nightmare of having to check return values of every PCApply, MatMult, etc. in petsc code. I guess this is where exception handling would be a great thing to have.
>
> I think for our own purposes, we will follow Matt's suggestion of wrapping KSPDefaultConverged() and set a global flag that can be checked afterwards. Our code checks successful completion convergence
> of every solve afterwards. It always handles solver failures as a fatal error, but the the code continues for a bit (end of timestep) to ensure final diagnostics, mesh and fields are written out - it
> is an adaptive mesh model, so users may not have even seen the mesh that was used to solve on.
>
> For simpler applications a petsc error generated on the spot would be a useful option, yes, but a more robust general way of handling nested solver failures would be greatly appreciated. I was
> slightly surprised to be honest to discover petsc happily continuing its ksp solve even though the MatSchurComplement solves were silently failing. A drawback of the suggested workaround, is that even
> if the MatSchurComplement solve fails in the first iteration (for instance reaching max iterations), the outer solve will still continue, trying and failing to solve the MatSchurComplement inner solve
> each iteration, until it finally hits the maximum number of outer iterations.
>
> Cheers
> Stephan

Actually, looking a little closer, I see a lot of negative KSPConvergedReasons are not actually raised by KSPDefaultConvergence(). In particular I still won't be able to trap KSP_DIVERGED_ITS this 
way, except for me adding an extra n < ksp->max_it check of course, but that still leaves out a lot of krylov method-specific reasons...

>
>
>
>>
>> On Jun 18, 2010, at 11:35 AM, Stephan Kramer wrote:
>>
>>> Dear all,
>>>
>>> Is there a way in petsc, when performing inner solves like PCKSP or MatSchurComplementGetKSP, to make the outer solve stop immediately and report back a negative convergence reason? I find that often when such "inner solves" fail, the outer solve happily continues and sometimes falsely reports convergence due to the preconditioner becoming rank deficient. I'd like our code, using petsc, to be able to trap that sort of situations and give a suitable error message.
>>>
>>> Cheers
>>> Stephan
>>
>>
>


-- 
Stephan Kramer  <s.kramer at imperial.ac.uk>
Applied Modelling and Computation Group,
Department of Earth Science and Engineering,
Imperial College London