Using ML and options

[Barry Smith]

On Fri, 17 Aug 2007, Ben Tay wrote:

> Hi,
> 
> 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
> 
> 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

  Huh? Why is this not 1.e-14, since LU is a direct solver, do you mean ILU?

> 
> hypre: Norm of error < 1.e-12,iterations     1
> 
> ml: Norm of error 0.2098E-03 iterations     2

   You may need to play around with the -ksp_rtol <rtol> value or options
in ml to (run with -pc_type ml -help to see ML options). Run with 
-ksp_monitor_true_residual and send us the results, We don't have
a lot of experience with ML on real problems.

  Barry

> 
> 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
> > > > > 
> > > > > 
> > > > > 
> > > > >         
> > > > 
> > > >       
> > >     
> > 
> > 
> >   
> 
>