[petsc-users] GMRES for outer solver

[Matthew Knepley]

On Mon, May 2, 2022 at 1:56 PM Amneet Bhalla <mail2amneet at gmail.com> wrote:

> Thanks Matt and Barry. Since we are using MATSHELL, we printed max norm of
> || b || and || A*x ||, the two things that have all the physics of the
> problem. Both are coming out to be finite (no NANs). Perhaps there is a NaN
> in PCSHELL. But this is counter-intuitive because the true residual norm ||
> b - A*x || has a NaN where there is no PC application. We checked the
> velocity and pressure field in VisIt and they seem to be reasonable and
> matching the analytical solution well.  Need to try it in a debugger or
> -fp_trap next.
>

I would recommend -fp_trap. It will just fail at the first NaN so it is
really easy to find. You can also send the code and I can run it.


> The behavior is similar to what is discussed in this thread:
> https://www.mail-archive.com/petsc-users@mcs.anl.gov/msg34602.html
>

That turned out to be a bug in their code.

  Thanks,

    Matt


> On Mon, May 2, 2022 at 7:56 AM Barry Smith <bsmith at petsc.dev> wrote:
>
>>
>>
>> On May 2, 2022, at 8:12 AM, Matthew Knepley <knepley at gmail.com> wrote:
>>
>> On Mon, May 2, 2022 at 12:23 AM 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
>>>
>>
>> NaN can always be tracked back. I recommend tracing it back to the first
>> NaN produced. My guess is that your equation of state if producing a NaN.
>>
>>
>>    You can run in the debugger with -ksp_error_if_not_converged or
>> -fp_trap to see when the Nan first appears. If the problem does not appear
>> on one rank or you need to use mpiexec to start the program you can use the
>> option -start_in_debugger to have the program started up in the debugger
>> https://petsc.org/main/docs/manualpages/Sys/PetscAttachDebugger.html
>>
>>
>>
>>
>>
>>
>> Also, we have an example of low Mach flow in TS ex76.
>>
>>   Thanks,
>>
>>     Matt
>>
>>
>>> 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
>>>>>
>>>>>
>>>>>
>>>>>
>>
>> --
>> What most experimenters take for granted before they begin their
>> experiments is infinitely more interesting than any results to which their
>> experiments lead.
>> -- Norbert Wiener
>>
>> https://www.cse.buffalo.edu/~knepley/
>> <http://www.cse.buffalo.edu/~knepley/>
>>
>>
>>
>
> --
> --Amneet
>
>
>
>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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