[petsc-users] multigrid implementation problem

[Nakib Haider Protik]

Reason for two ksp objects:

Multigrid and Domain Decomposition in PETSc
Barry Smith
PETSc Developer

pg 28:

//Use same pre and post smoother
MGGetSmoother(pc,int level,KSP *ksp);


//Use different pre and post smoother
MGGetSmootherDown(pc,int level,KSP *dksp);
MGGetSmootherUp(pc,int level,KSP *uksp);

I was trying to use the second in my code. I would be greatly helped if
you could tell me where exactly my code goes wrong.

As for the "accidental" correctness of the 1d code, here's where I found
the prototype:

http://lists.mcs.anl.gov/pipermail/petsc-users/2011-May/008793.html

If my question does not abide by the regulations of this mailing list, or
if you are not obliged to answers questions of this kind, please let me
know. I am not looking for free lessons on c, but thanks for the link.
Despite the rude reply, thank you for taking the time.

> On Sun, Jun 10, 2012 at 3:49 PM, Nakib Haider Protik
> <nprot048 at uottawa.ca>wrote:
>
>> Thank you very much for the reply. Here's a change that works for the 2D
>> case:
>>
>> ////////////////////MG Solver Stuff////////////////////
>>    KSPCreate(PETSC_COMM_WORLD, &ksp);
>>    KSPSetType(ksp, KSPGMRES);
>>
>>    KSPCreate(PETSC_COMM_WORLD, &kspu);
>>    KSPSetType(kspu, KSPGMRES);
>>
>
> Why do you have two KSPs here?
>
>
>>
>>    KSPGetPC(ksp, &pc);
>>    KSPGetPC(kspu, &pc);
>>
>
> Why?
>
>
>>
>>    PCSetType(pc, PCMG);
>>    PCMGSetLevels(pc, 2, PETSC_NULL);
>>    PCMGSetType(pc, PC_MG_MULTIPLICATIVE);
>>    PCMGSetCycleType(pc, PC_MG_CYCLE_V);
>>    MatDuplicate(A, MAT_COPY_VALUES, &P);
>>    PCMGSetCyclesOnLevel(pc, 0, 1);
>>    PCMGSetCyclesOnLevel(pc, 1, 1);
>>    PCMGGetCoarseSolve(pc, &ksp);
>>     PCMGGetSmootherDown(pc, 0, &ksp);
>>    PCMGGetSmootherUp(pc, 1, &kspu);//(*)//
>>
>
> Okay, clearly you need to learn how variables work in C. Here is a free
> resource.
>
> http://c.learncodethehardway.org/book/
>
> (Any other resource will also do.) When you are comfortable with the
> language, we can revisit your code.
>
>
>>     PCMGSetInterpolation(pc, 1, P);
>>    PCMGSetRestriction(pc, 1, P);
>>     PCMGSetResidual(pc, 0, PCMGDefaultResidual, P);
>>    PCMGSetResidual(pc, 1, PCMGDefaultResidual, P);
>>    KSPSetOperators(ksp, A, P, SAME_NONZERO_PATTERN);
>>     KSPSetFromOptions(ksp);
>>    KSPSolve(ksp, breal, xreal_harm);
>>    KSPSolve(ksp, bimag, ximag_harm);
>>     //////////////////////////////////////////////////////////
>>
>> Do you think this is working again because of some accident? Without the
>> step marked with a //(*)//, the code works too.
>>
>


-- 
Nakib