[petsc-users] How to use multigrid?

[Mark F. Adams]

Just to add to Jed and Matt's comments:

1) What are your equations (I really don't care what physics you are modeling, its the equations that we see).  Is is a scalar div (alpha(x) grad ) u?

2) Try using hyper (-pc_type hyper -pc_hypre_type boomeramg).  Look at "PCApply              448", this should be like 10-20.  Configure your systems with --download-hyper=1 if you do not have it.

3) Why do you say it is ill conditioned?  Stretched grids? large jumps in material coefficients?  Why do you think it is illoconditioned?

On Nov 3, 2012, at 1:52 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Sat, Nov 3, 2012 at 1:38 PM, w_ang_temp <w_ang_temp at 163.com> wrote:
> Hello, Matthew
>  
>         I just mean that the problem that I am resolving is a finite element problem. The linear system of it  is true elliptic equations.
> I heared that AMG was an efficient solver, so I just want to have a try about AMG and find that if it is efficient.
> 
> And I meant it when I said, you MUST look it up .Next time you ask us what AMG can do, please include
> a reference for a paper in which they are attacking this problem with it and we can help.
> 
>  
>        By the way, I want to confirm a conception. In my view, AMG itself can be a solver like gmres. It can also be used as a preconditioner
> like jacobi and is used by combining with other solver. Is it right? If it is right, how use AMG solver?
> 
> This is true of almost all KSP and PC objects. These are all jsut approximate solvers.
> 
>   Matt
>  
>  
>         Thanks.
>  
>                                                                                                                         Jim
> 
> 
> >在 2012-11-04 01:21:59，"Matthew Knepley" <knepley at gmail.com> 写道：
> ><On Sat, Nov 3, 2012 at 1:17 PM, w_ang_temp <w_ang_temp at 163.com> wrote:
> >At 2012-11-04 01:08:26,"Jed Brown" <jedbrown at mcs.anl.gov> wrote:
> >1. What kind of equation are you solving? AMG is not working well if it takes that many iterations.
> 
> >I just deal with the typical soil-water coupled geotechnical problems. It is a typical finite element equation. The matrix is 30000X30000 and ill-conditioned.
> 
> 
> >We are now at the root of your problem. Solvers do not work on discretizations, they work on equations. No
> >solver is designed for "finite elements", and there is no typical finite element problem.
> 
> >Multigrid works best on elliptic equations with smooth coefficients. Without that, you have to do special things.
> 
> >I can tell from the above discussion that you have not spent a lot of time researching successful preconditioning
> >strategies for your problem in the literature. This is always the first step to building a high performance solver.
> 
>  > Thanks,
> 
> >     Matt
>  
> >2.
> 
> 
>       ##########################################################
>       #                                                        #
>       #                          WARNING!!!                    #
>       #                                                        #
>       #   This code was compiled with a debugging option,      #
>       #   To get timing results run ./configure                #
>       #   using --with-debugging=no, the performance will      #
>       #   be generally two or three times faster.              #
>       #                                                        #
>       ##########################################################
> >It is true a debugging version. And I used the same version dealing with the same problem, one preconditioner is asm
> >and the other is amg. The time with amg is about 3 times as with asm. I do not know the reason. And I also do not know the
> >meaning of '[0]PCSetData_AGG bs=1 MM=7601'.
> 
> 
> 
> 
> 
> 
> -- 
> 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
> 
> 
> 
> 
> 
> -- 
> 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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