[petsc-users] PCKSP

[Margarita Satraki]

I'll give it a go. Thanks!

Margarita

On 19 June 2012 19:16, Matthew Knepley <knepley at gmail.com> wrote:

> On Tue, Jun 19, 2012 at 12:13 PM, Margarita Satraki <
> margarita.satraki at gmail.com> wrote:
>
>> For my problem (incompressible nonlinear elasticity) pcksp seemed to work
>> ok with gmres and even better with fgmres.
>> The best option was to use LU as PC but this requires a lot of memory.
>>
>
> You can just use
>
>   -ksp_type preonly
>   -pc_type lu
>
> to get a direct solution. Its not clear why you would want PCKSP.
>
>
>> Why do you think that would not be a good option?
>>
>
> Incompressible elasticity has a constraint, so it is a saddle point
> system. We would recommend using
> PC_FIELDSPLIT, splitting into blocks for the displacement and pressure.
> You can do this automatically
> using -pc_fieldsplit_detect_saddle_point.
>
> We would also proba bly recommend using AMG (ML, or GAMG, etc.) for the
> elastic block. You have to provide
> the near null modes (6 rot+trans), but there are examples of this from
> Mark Adams and me.
>
>     Matt
>
>
>> Margarita
>>
>>
>> On 19 June 2012 19:00, Matthew Knepley <knepley at gmail.com> wrote:
>>
>>> On Tue, Jun 19, 2012 at 11:54 AM, Margarita Satraki <
>>> margarita.satraki at gmail.com> wrote:
>>>
>>>> Thank you very much Matt.
>>>>
>>>
>>> I will just point out that this is unlikely to ever be what you want. Do
>>> you have a particular solver
>>> configuration in mind?
>>>
>>>     Matt
>>>
>>>
>>>> Margarita
>>>>
>>>> On 19 June 2012 18:44, Matthew Knepley <knepley at gmail.com> wrote:
>>>>
>>>>> On Tue, Jun 19, 2012 at 11:41 AM, Margarita Satraki <
>>>>> margarita.satraki at gmail.com> wrote:
>>>>>
>>>>>> If I understand correctly:
>>>>>>
>>>>>> For a system:
>>>>>> M^{-1} A x = M^{-1} b
>>>>>> we don't need to multiply M^{-1} A explicitly, but we solve M w = v
>>>>>> whenever needed.
>>>>>>
>>>>>> So the Krylov method is used in order to solve that system, or
>>>>>> equivalently to compute the vector M^{-1} v?
>>>>>>
>>>>>
>>>>> Yes
>>>>>
>>>>>    Matt
>>>>>
>>>>>
>>>>>>
>>>>>> On 19 June 2012 17:54, Matthew Knepley <knepley at gmail.com> wrote:
>>>>>>
>>>>>>> On Tue, Jun 19, 2012 at 10:33 AM, Margarita Satraki <
>>>>>>> margarita.satraki at gmail.com> wrote:
>>>>>>>
>>>>>>>> Hello,
>>>>>>>>
>>>>>>>> I have difficulty understanding how PCKSP works.
>>>>>>>>
>>>>>>>> From:
>>>>>>>>
>>>>>>>> http://www.mcs.anl.gov/petsc/documentation/tutorials/Columbia04/DDandMultigrid.pdf
>>>>>>>> I understand that instead of using preconditioners, it uses Krylov
>>>>>>>> methods for the ''inner solvers''.
>>>>>>>>
>>>>>>>> What are the ''inner solvers''? Is there some kind of a subsystem
>>>>>>>> that is solved instead of applying a preconditioner?
>>>>>>>>
>>>>>>>
>>>>>>> Nope, its jsut like a PC:
>>>>>>>
>>>>>>>   M^{-1} A x = M^{-1} b
>>>>>>>
>>>>>>> where now M^{-1} instead of being an LU solve, for instance, is a
>>>>>>> Krylov solve.
>>>>>>>
>>>>>>>    Matt
>>>>>>>
>>>>>>>
>>>>>>>> Many thanks,
>>>>>>>>
>>>>>>>> Margarita
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> 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
>>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> 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
>>>>>
>>>>
>>>>
>>>
>>>
>>> --
>>> 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
>>>
>>
>>
>
>
> --
> 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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