[petsc-users] GMRES for outer solver

[Ramakrishnan Thirumalaisamy]

As given in my previous email, when ksp_pc_type is "left", the residual
norms are:

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

where true resid norm is -nan. Is this related to left preconditioning? The
following thread suggests to use right preconditioning to avoid "-nan"

https://www.mail-archive.com/petsc-users@mcs.anl.gov/msg34602.html


Regards,
Rama

On Sun, May 1, 2022 at 9:29 PM Ramakrishnan Thirumalaisamy <
rthirumalaisam1857 at sdsu.edu> wrote:

> Sorry. There is a typo in my previous email. It should be ksp_pc_side.
>
>
> Regards,
> Rama
>
> On Sun, May 1, 2022 at 9:23 PM Ramakrishnan Thirumalaisamy <
> rthirumalaisam1857 at sdsu.edu> wrote:
>
>> 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
>>>>
>>>>
>>>>
>>>>
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