[petsc-users] Can you provide a basic example of using PCMG for multigrid applications?

[Shiyuan]

Is there any support in PETSC (with other packages) for finite element
multigrid on unstructured meshes? Any recommended way to do that? Any
tutorial or examples? Thanks.

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Message: 3
Date: Mon, 16 May 2011 14:08:12 -0500
From: Barry Smith <bsmith at mcs.anl.gov>
Subject: Re: [petsc-users] Can you provide a basic example of using
       PCMG for        multigrid applications?
To: PETSc users list <petsc-users at mcs.anl.gov>
Message-ID: <E727136F-395E-4E88-9D0A-6C8E07177DD4 at mcs.anl.gov>
Content-Type: text/plain; charset=us-ascii


On May 16, 2011, at 1:36 AM, Li, Zhisong (lizs) wrote:

>  Hi, Petsc Team,
>
> In my knowledge of Petsc, the PCMG object is the main tool for general
multigrid calculations, as DMMG only works for simple constant matrix and
RHS.

  This is not correct. DMMG can be used for non-constant matrix. DMMG is
restricted to structured grids only. But if your problem is on a two or
three dimensional structured grid you can use it and it is much easier than
coding the pieces directly.



>  But it's difficult to find any example code for using PCMG.  I checked
out the petsc-user mail archive on this topic but still not find a good
description on it.  I encounter similar errors in the practice as some
previous question-raisers did.
>
> Here is my simple code implementing the PCMG based on my understanding
from the user manual and tutorial.  It is supposed to have multigrid of only
two levels (finest and coarsest grid).  The error message indicates SEGV
segmentation violation because of this part of code.  Anything wrong with
this implementation?
>

   This code looks more or less correct. You need to run with
-start_in_debugger to track down what is trigger the SEGV

   src/ksp/ksp/examples/
tutorials/ex19.c is a simple example that runs on two levels.


  Barry


>
>   DAGetMatrix(da, MATAIJ, &M);
>
>   KSPCreate(PETSC_COMM_WORLD, &ksp);
>   KSPSetType(ksp, KSPGMRES);
>   KSPGetPC(ksp, &pcmg);
>   PCSetType(pcmg, PCMG);
>
>   PCMGSetLevels(pcmg, 2, &PETSC_COMM_WORLD);
>   PCMGSetType(pcmg, PC_MG_MULTIPLICATIVE);
>   PCMGSetCycleType(pcmg, PC_MG_CYCLE_W);
>   PCMGSetCyclesOnLevel(pcmg, 0, 1);
>   PCMGSetCyclesOnLevel(pcmg, 1, 1);
>
>   PCMGGetCoarseSolve(pcmg, &ksp);
>
>   PCMGGetSmoother(pcmg, 0, &ksp);
>   PCMGGetSmoother(pcmg, 1, &ksp);
>   PCMGSetInterpolation(pcmg, 1, M);
>   PCMGSetRestriction(pcmg, 1, M);
>
>   PCMGSetResidual(pcmg, 0, PCMGDefaultResidual, M);
>   PCMGSetResidual(pcmg, 1, PCMGDefaultResidual, M);
>
>
>  The mandatory remaining PCMGSetRhs( ), PCMGSetX( ) and PCMGSetR( )
functions should be generated by Petsc automatically.
>
> It is tough to learn some Petsc functions when no detailed example and few
guidance is provided.  I wish you can add some tutorials codes on PCMG usage
in the future version of Petsc.
>
>
> Thank you very much.
>
>
> Zhisong Li
>
>
>



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