[petsc-users] How to use multigrid?

[A.L. Siahaan]

Is PCGAMG a combination of AMG and geometric multigrid ? Do we have to 
choose either pure AMG (when using -pc_type gamg -pc_gamg_agg_nsmooths 1) 
or pure geometric MG (type="geo") ? From my thread search in this mailing 
list about using PCGAMG, it is mostly advised to use pure AMG (-pc_type 
gamg -pc_gamg_agg_nsmooths 1).

Of all algebraic multigrid implemented or interfaced in PETSc, i.e. 
BoomerAMG (PCHypre), ML(PCML), and PCGAMG("agg"), what is the order of the 
cost (memory) to set up the preconditioner (setting nullspace etc) ?

Regards,
Antony



On Nov 9 2012, Mark F. Adams wrote:

>
>On Nov 9, 2012, at 10:05 AM, w_ang_temp <w_ang_temp at 163.com> wrote:
>
>> Hello,Jed
>>    
>>     Sorry again for the interruption.
>>     
>>     As you said, if I want to use Algebraic multigrid, I can just use 
>> '-pc_type gamg -pc_gamg_agg_nsmooths 1' in the command line. I think 
>> there are many other -pc_gamg_XXX parameters. How can I find all the 
>> -pc_gamg_XXX?
>
>-help
>
>>     
>>     Besides, I want to know more about AMG. Are there any details from 
>> PETSc, like manual, documentaion, FAQ, etc?
>
> I would recommend the book "Multigrid" by Trottenberg, et al, and the 
> classic "Multigrid Tutorial" is good to look at also.
>
>>     Also, from the above post, I want to know that if AMG can not be 
>> used in 3.2 by command line option, because now I have to use 3.3 for 
>> using AMG. I just donot want to change my version 3.2 now.
>
> You can use 3.2 if you like (free country) but this code was developed a 
> lot between 3.2 and 3.3. I forget what was in 3.2 exactly. If you have a 
> straight forward problem and 3.2 works then its is probably OK for your 
> current purposes.
>
>>     
>>     I am major in Civil Engineering and not good at math and 
>> programming. So please forgive my weakness.
>>     Thanks.
>>                                 Jim 
>>  
>> 
>> 
>> 
>> 
>> 
>> > At 2012-10-28 21:17:00,"Jed Brown" <jedbrown at mcs.anl.gov> wrote: 
>> > Algebraic multigrid can be used directly, -pc_type gamg 
>> > -pc_gamg_agg_nsmooths 1. Geometric either required that you use the 
>> > PCMG interface to set >interpolation (and provide a coarse operator 
>> > for non-Galerkin) or use a DM that provides coarsening capability.
>> 
>> >What kind of problem are you solving?
>> 
>> >>On Oct 28, 2012 6:09 AM, "w_ang_temp" <w_ang_temp at 163.com> wrote:
>> >>Hello,
>>  >>  I want to use the multigrid as a preconditioner. The introduction 
>>  >> about it in the manual is little.
>> >> So are there some typical examples or details about multigrid? Is it 
>> >> used just like other preconditioners like jacobi, sor, which can be 
>> >> simply used in the cammand line options?
>>  >>  Thanks.
>>  > >                                                                      
>>  > > Jim
>> 
>> 
>> 
>> 
>
>