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

[Matthew Knepley]

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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