Using ML and options

[Ben Tay]

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

hypre: Norm of error < 1.e-12,iterations     1

ml: Norm of error 0.2098E-03 iterations     2

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