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

[Alp Kalpalp]

Thanks for the answers,

Jed, PCPG stands for Preconditioned Conjugate Projected Gradient. Since any
FETI literature suggests PCPG, I am planning to go stick with it.
My plan is to extend your KSPCG algorithm with optional application of
projection space and re-orthogonalizations.

Best regards,

On Mon, Dec 29, 2014 at 2:33 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
>   Take a look at KSPFischerGuessCreate() and the material it points to.
> From the command line you can run for example
>
> -ksp_fischer_guess 1,20
>
> This method "works" by saving information about Krylov directions and then
> projecting those directions out of the NEXT linear solve at the beginning
> of the new linear solve (constructing a "better" initial guess), hence it
> does not remove these directions out each KSP iteration, just for each new
> linear solve. It can be used with the preconditioned conjugate gradient
> method.  There is a tiny community of people who claim this helps
> significantly on their problems, we'd love to hear your experience.
>
>   Barry
>
>
> > On Dec 28, 2014, at 11:24 AM, Alp Kalpalp <alpkalpalp at gmail.com> wrote:
> >
> > 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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