[petsc-users] GMRES for outer solver

[Ramakrishnan Thirumalaisamy]

Thank you. I have a couple of questions. I am solving the low Mach
Navier-Stokes system using a projection preconditioner (pc_shell type) with
GMRES being the outer solver and Richardson being the Krylov
preconditioner. The solver diverges when ksp_pc_type is "right”:

Linear stokes_ solve did not converge due to DIVERGED_NANORINF iterations 0

and it converges when ksp_pc_type is "left":

Residual norms for stokes_ solve.
  0 KSP preconditioned resid norm 8.829128536017e+04 true resid norm
    -nan ||r(i)||/||b||           -nan
  1 KSP preconditioned resid norm 1.219313641627e+00 true resid norm
    -nan ||r(i)||/||b||           -nan
  2 KSP preconditioned resid norm 8.547033285706e-12 true resid norm
    -nan ||r(i)||/||b||           -nan
Linear stokes_ solve converged due to CONVERGED_RTOL iterations 2

 I am curious to know why this is happening. The solver also diverges with
"FGMRES" as the outer solver (which supports only right preconditioning).

2. Is it also possible to not get "-nan" when || b || = 0?


Regards,
Rama

On Sun, May 1, 2022 at 12:12 AM Dave May <dave.mayhem23 at gmail.com> wrote:

>
>
> On Sun 1. May 2022 at 07:03, Amneet Bhalla <mail2amneet at gmail.com> wrote:
>
>> How about using a fixed number of Richardson iterations as a Krylov
>> preconditioner to a GMRES solver?
>>
>
> That is fine.
>
> Would that lead to a linear operation?
>>
>
> Yes.
>
>
>
>> On Sat, Apr 30, 2022 at 8:21 PM Jed Brown <jed at jedbrown.org> wrote:
>>
>>> In general, no. A fixed number of Krylov iterations (CG, GMRES, etc.) is
>>> a nonlinear operation.
>>>
>>> A fixed number of iterations of a method with a fixed polynomial, such
>>> as Chebyshev, is a linear operation so you don't need a flexible outer
>>> method.
>>>
>>> Ramakrishnan Thirumalaisamy <rthirumalaisam1857 at sdsu.edu> writes:
>>>
>>> > Hi,
>>> >
>>> > I have a Krylov solver with a preconditioner that is also a Krylov
>>> solver.
>>> > I know I can use "fgmres" for the outer solver but can I use gmres for
>>> the
>>> > outer solver with a fixed number of iterations in the Krylov
>>> > preconditioners?
>>> >
>>> >
>>> > Thanks,
>>> > Rama
>>>
>> --
>> --Amneet
>>
>>
>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20220501/53e400e4/attachment.html>