# [petsc-users] How to use multigrid?

Matthew Knepley knepley at gmail.com
Sat Nov 3 12:52:13 CDT 2012

```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
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
> -- Norbert Wiener
>
>
>
>

--
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their