[petsc-users] Command line option to set number iterations for Chebyshev eigenvalue estimation?

[Dave May]

Great

To be honest, I think the only way to "find" these options is to either do
one of the following:

1) Run with -ksp_view and very carefully go through the hierarchy of the
multi-level preconditioner. This immediately shows the prefixes of nested
objects (like "est") and then you can see how to configure those nested
objects.

2) When I have no idea what prefixes might be defined in a petsc code, I
run with -help and trawl through it. Once I have isolated a prefix of
interest, I use -help and grep to narrow down all the options which are
connected with that particular solver.

These are the only ways I know how to explore very complicated solver
configurations. As you can see, I miss spell option names from time to
time. Thus when debugging solver configuration, I also always use
  -options_left 1
to indicate whether I used an option that PETSc didn't recgonize.


Cheers,
  Dave




On 23 January 2014 16:18, Garth N. Wells <gnw20 at cam.ac.uk> wrote:

> On 2014-01-23 14:54, Dave May wrote:
>
>> If you don't have any prefix set on the outer KSP, you should be able
>> to simply do
>>
>> -mg_levels_ksp_type chebychev
>> -mg_levels_ksp_chebychev_estimate_eigenvalues 0,0.2,0,1.1
>> -mg_levels_est_ksp_type cg
>>  -mg_levels_est_ksp_max_it X
>>
>>
> Thanks. Last two are what I was looking for and I can confirm that they
> work as expected.
>
> It would be helpful is last two options could be (easily) found in the
> docs - I couldn't find them.
>
> Garth
>
>
>  Cheers,
>>   Dave
>>
>> On 23 January 2014 15:49, Garth N. Wells <gnw20 at cam.ac.uk> wrote:
>>
>>  On 2014-01-23 14:32, Matthew Knepley wrote:
>>> On Thu, Jan 23, 2014 at 5:35 AM, Garth N. Wells <gnw20 at cam.ac.uk>
>>> wrote:
>>>
>>> Is there are command line option to set the number of iterations
>>> used for the eigenvalue estimation inside the Chebyshev
>>> preconditioner?  "-gamg_est_ksp_max_it" does the trick with GAMG,
>>> but I'd also like to change the number of iterations when using
>>> Chebyshev smoothing with ML.
>>>
>>> In GAMG, we use PETSc itself to do the estimate, and give it the
>>> prefix gamg_est_. In ML, as far as I can see from the code, we use
>>> the
>>> internal
>>> estimator, and I don't think they give us this knob.
>>>
>>
>>  I'm not so interested in the what happens inside ML, but the
>> Chebyshev smoother that PETSc applies. With the default 10 GMRES
>> iterations, the eigenvalue estimate is not good enough for my problem,
>> which leads to the outer CG breaking down with an indefinite operator
>> error.
>>
>>  I can get it working by changing
>>
>>    -mg_levels_ksp_chebyshev_estimate_eigenvalues
>>
>>  but I'd rather do a few more iterations to get a better eigenvalue
>> estimate. It would also be nice to be able to switch from GMRES to CG.
>> I can do this with GAMG via "-gamg_est_ksp_type"
>>
>>  Garth
>>
>>     Matt
>>>
>>>
>>>  Garth
>>>>
>>>
>>> --
>>> 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
>>>
>>
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