[petsc-users] How to use multigrid?

[Matthew Knepley]

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
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