[petsc-users] questions about the multigrid framework

[Jed Brown]

On Wed, Feb 9, 2011 at 17:44, Peter Wang <pengxwang at hotmail.com> wrote:

> Did you mean All the Krylov methods alone will get worse with increasing
> grid number?
>

Yes, the number of Krylov iterations for second order elliptic problems with
no preconditioner scales proportional to the number of grid points in any
direction. You need a spectrally equivalent preconditioner, usually
multigrid of some sort, to prevent this.


> Since the finer grid has smaller size and more number of grid.
>
>    Since I am a new user of PETSc, the easiest way for me is still keep in
> KSP solver. However, if the solver cannot satisfy the speed reqirement. I am
> thinking to use MG method. However, I don't have any experience
> on multigrid. Could you please give me some suggestion on it?
>
>  1, Since I  have built the Matrix and the vector for finite difference
> scheme in KSP solver, where should I start from to transfer to multigrid?  I
> studied the example in: src/ksp/ksp/examples/tutorials/ex22f.F. Is it a good
> prototype to be based on to create my own code?  Is DMMG is the best tool
> for my problem?
>

Assuming you currently assemble a matrix, just configure PETSc with
--download-ml and --download-hypre, then try running your code with -pc_type
ml or -pc_type hypre. You can use geometric multigrid later to improve the
constants or handle cases where algebraic multigrid (ML or BoomerAMG from
Hypre) are having trouble.

You need to tell us what equations you are solving if you want useful
suggestions.
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