[petsc-users] no decrease in iteration counts of KSPCG during time stepping

[Alp Kalpalp]

Thanks for the answers,

Please forgive me, I forgot to say that my stiffness matrix is not changing
during time steps. I could not remember directly but just after a google
search..I just hit this

http://web.stanford.edu/group/frg/publications/recent/FETI-stoch.pdf

please look around eq37

My problem is not related to this random paper I found. But, I think I can
find several others that shows the enhancing power of orthogonalization
with successive directions when the system's behaviour is not changing
rapidly. In my current sample case a gradually increasing force is applied
to a linear system.

Since I use FETIDP, preconditioned conjugate projected gradient (PCPG) is
crucial in order to select any generalized inverse for the system.

So, any suggestions on how to complete these tasks?

For example anyway of obtaining search direction from KSPCG?

or

how to implement a projection space?

Is it posible or too difficuly to code a variant of a KSPCG that meets my
requirements?

On Sun, Dec 28, 2014 at 7:08 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Sun, Dec 28, 2014 at 11:02 AM, Umut Tabak <u.tabak at tudelft.nl> wrote:
>
>>  On 12/28/2014 05:54 PM, Alp Kalpalp wrote:
>>
>>    Hi,
>>
>>  Thank you Mark.
>>
>>  Let me clarify my questions;
>>
>>  1-)How to implement or activate a Reorthogonalization procedure for
>> KSPCG..
>>  As you know, search directions can be found more rapidly (with less
>> numer of iterations) by using previous successive directions
>>
>> Without answering the PETSc related questions, interesting discussion,
>>
>> indeed, but at the cost of purging the previous directions(which means
>> explicit orthogonalizations with respect to these vectors also), so I am
>> not sure if you can gain something with this, cost wise...
>>
>
> This has been proposed many times, but it has never been shown to work. I
> have tried every variant I could
> find and it did not work. You can try LGMRES, which is the closest one to
> working in my opinion. There is
> definitely no theoretical relation between Krylov directions from
> subsequent solves unless the operator is
> identical.
>
>    Matt
>
>  2-) How to implement or activate a projection space over CG. A sample
>> projection can be;
>> P = I - G*((G'*G)\G').
>>  I need to insert project,scale,precondition,re-scae,re-project steps
>> during each KSPCG iteration. How can I utilize this?
>>
>>   Just a side note, I had previous experience on this that these kinds
>> of practice increase the cost more...
>> BR,
>> Umut
>>
>
>
>
> --
> 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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