> On Mar 9, 2017, at 1:49 AM, Klaij, Christiaan <[email protected]> wrote:
>
> Barry,
>
> I came across the same problem and decided to use KSPSetNormType
> instead of KSPSetPCSide. Do I understand correctly that CG with
> KSP_NORM_UNPRECONDITIONED would be as efficient as with
> KSP_NORM_PRECONDITIONED?
Yes, it as efficient.
> Since PC_RIGHT is not supported, I was
> under the impression that the former would basically be the
> latter with an additional true residual evaluation for the
> convergence monitor, which would be less efficient.
It is true for GMRES with left preconditioning (but not right) that
computing the unpreconditioned residual norm adds a good amount of additional
work but it is not true for CG, this is because CG, by its nature tracks both y
= A x and B y so either is equally available to compute its norm. GMRES with
left preconditioner computes B A something at each iteration but doesn't
directly compute the residual (either preconditioned or not) it uses a
recursive formula for the residual norm.
>
> Chris
>
>> On Mar 8, 2017, at 10:47 AM, Kong, Fande <[email protected]> wrote:
>>
>> Thanks Barry,
>>
>> We are using "KSPSetPCSide(ksp, pcside)" in the code. I just tried
>> "-ksp_pc_side right", and petsc did not error out.
>>
>> I like to understand why CG does not work with right preconditioning?
>> Mathematically, the right preconditioning does not make sense?
>
> No, mathematically it makes sense to do it on the right. It is just that
> the PETSc code was never written to support it on the right. One reason is
> that CG is interesting that you can run with the true residual or the
> preconditioned residual with left preconditioning, hence less incentive to
> ever bother writing it to support right preconditioning. For completeness we
> should support right as well as symmetric.
>
> Barry
>
>
> dr. ir. Christiaan Klaij | Senior Researcher | Research & Development
> MARIN | T +31 317 49 33 44 | mailto:[email protected] | http://www.marin.nl
>
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