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

[Barry Smith]

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
> 
> 
>