Using ML and options

[Hong Zhang]

Ben,
>
> I tried to test ml using "-pc_type ml" on my problem. It was working ok
> using LU or hypre/boomeramg. However I got the warning:
>
> Gen_Prolongator warning : max eigen <= 0.0
> Gen_Prolongator warning : max eigen <= 0.0
> Gen_Prolongator warning : max eigen <= 0.0
> Gen_Prolongator warning : max eigen <= 0.0
> Gen_Prolongator warning : max eigen <= 0.0

I got the same warning on using ML for my application, which results
in unacceptable computational
result in my case. I've sent a request to ML developer for its
interpretation yesterday.

> When I use hypre, I used "-pc_type hypre -pc_hypre_type boomeramg which
> gives good and fast result for solving my poisson eqn. Is there a
> recommendation for ml as well?
>
> Moreover, although I got answers from ml, my answer using ml differs
> from LU and hypre from the 3 sig. fig. When I run the example ex2f, I
> get different norms of error:
>
> LU: Norm of error 0.1192E-05 iterations     4
>
> hypre: Norm of error < 1.e-12,iterations     1
>
> ml: Norm of error 0.2098E-03 iterations     2
You may test other available options (see -help),
in particular, find out the smoother and MG cycle used by hypre
and apply the same to ML.

Good luck,

Hong

>
> It seems that norm of error for ml is a bit too high. How can I make it
> lower?
>
> Thank you.
>
>
>
> Hong Zhang wrote:
> > On Thu, 16 Aug 2007, Ben Tay wrote:
> >
> >
> >> Btw, is the file to download
> >> ftp://ftp.mcs.anl.gov/pub/petsc/externalpackages/ml-5.0.tar.gz?
> >>
> >
> > Yes. You can simply use '--download-ml=1' during configuration.
> >
> > Run your program with '-help' to see all options for using ml.
> > See ~petsc/src/ksp/ksp/examples/tests/ex26.c on how to use ML.
> >
> > Hong
> >
> >
> >> Thanks
> >>
> >> Matthew Knepley wrote:
> >>
> >>> The multigrid PC needs information about the grid and discretization. If you
> >>> cannot provide this, use AMG through Hypre or ML. Or if you have a
> >>> structured grid, consider using DMMG.
> >>>
> >>>   Thanks,
> >>>
> >>>     Matt
> >>>
> >>> On 8/15/07, Ben Tay <zonexo at gmail.com> wrote:
> >>>
> >>>
> >>>> Hi,
> >>>>
> >>>> I found that there's a multigrid option in the manual which can be used
> >>>> by adding -pctype mg -pcmgtype PC_MG_MULTIPLICATIVE.
> >>>>
> >>>> However, it seems that I am actually just using LU since with or without
> >>>> this option, I got exactly the same answer. Am I using multigrid the
> >>>> wrong way?
> >>>>
> >>>> Thanks
> >>>>
> >>>>
> >>>>
> >>>>
> >>>
> >>>
> >>
> >
> >
> >
>
>