[petsc-users] multigrid implementation problem

[Jed Brown]

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.
>
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