[petsc-users] KSP convergence problem

[Mehrdad H Farahani]

I am solving a Poisson equation with Neumann boundary conditions applied
along an arbitrary curve using a ghost fluid treatment. The matrix is
non-symmetric and the null space contains more than the constant vector. I
have implemented a solve for the null space itself and have previously
supplied it using MatSetNullSpace. Unfortunately the null space solve is
extremely expensive and does not play well with GAMG or ML. We've checked
that the answers we get without supplying this null space are very close to
the answers we get when we do include it. The matrix for the two solves in
the previous attachment are identical. The problem also seems to be
converging and then suddenly diverges. Is this common behavior for a
singular preconditioner?

Thanks,


>
> From: Jed Brown <jedbrown at mcs.anl.gov>
> Date: Wed, Mar 20, 2013 at 2:03 PM
> Subject: Re: [petsc-users] KSP convergence problem
> To: PETSc users list <petsc-users at mcs.anl.gov>
>
>
>  On Wed, Mar 20, 2013 at 2:00 PM, Mark F. Adams <mark.adams at columbia.edu>wrote:
>
>>
>> On Mar 20, 2013, at 2:12 PM, Mehrdad H Farahani <mh.farahani at gmail.com>
>> wrote:
>>
>> > I'm experience a weird convergence problem with bcgsl with both ILU and
>> GAMG preconditioning. The problem converges normally (at least in the
>> preconditioned norm), but then kicks out with converged reason = -4. I've
>> attached the convergence history of the previous time-step which worked and
>> the timestep which shows problems. Could someone give me an idea on how to
>> proceed?
>> >
>>
>> Did you run with -ksp_converged_reason ?
>>
>> I don't know were this -4 comes from.  Could this be from your code?
>>
>
> Some people call KSPGetConvergedReason(), but then don't compare to the
> enums and don't convert to a string using KSPConvergedReasons[reason].
>
>
> http://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/KSP/KSP_DIVERGED_DTOL.html
>
> The problem is likely that your preconditioner is singular. What sort of
> problem are you solving?
>
>


-- 
Mehrdad
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