[petsc-users] the specific preconditioner

[Barry Smith]

  You need to call KSPGetPC(ksp,&pc); before setting the pc

  Note that since you call KSPSetFromOptions(ksp); you can control the KSP's and PC's from the command line and don't need to edit the code and recompile it for each change to the KSP or PC.  For example

   -ksp_type bcgs -pc_type jacobi

  Barry

> On Nov 21, 2014, at 8:18 AM, huaibao zhang <paulhuaizhang at gmail.com> wrote:
> 
> Hi Jed,
> 
> I have a specific question regarding the attached my initialization function.  Previously I used  KSPSetType(ksp,KSPFGMRES) solver, and I would like to see if there is some improvement using the other choices like KSPBCGS. It seems hoverer, the PC         //PCSetType(pc,PCBJACOBI)  does  not work with me. And I am not quite sure why. 
> 
> By the way, can you please give some suggestions on my code? I am not quite sure if I am doing the things rightly. 
> 
> 
> Many thanks,
> Paul
> 
> 
> 
> 
> void petsc_init(void) {
> 
>         vector<Cell>::iterator cit;
>         vector<int>::iterator it;
> 
>         //Create nonlinear solver context
>         KSPCreate(PETSC_COMM_WORLD,&ksp);
> 
>         VecCreateMPI(PETSC_COMM_WORLD,grid[gid].cellCount*nVars,grid[gid].globalCellCount*nVars,&RHS);
>         VecSetFromOptions(RHS);
>         VecDuplicate(RHS,&deltaX);
> 
> 
>         VecSet(RHS,0.);
>         VecSet(deltaX,0.);
> 
>         vector<int> diagonal_nonzeros, off_diagonal_nonzeros;
>         int nextCellCount;
> 
>         // Calculate space necessary for matrix memory allocation
>         for (cit=grid[gid].cell.begin();cit!=grid[gid].cell.end();cit++) {
>                 nextCellCount=0;
>                 for (it=(*cit).faces.begin();it!=(*cit).faces.end();it++) {
>                         if (grid[gid].face[*it].bc==INTERNAL_FACE) {
>                                 nextCellCount++;
>                         }
>                 }
>                 for (int i=0;i<nVars;++i) {
>                         diagonal_nonzeros.push_back( (nextCellCount+1)*nVars);
>                         off_diagonal_nonzeros.push_back( ((*cit).ghosts.size())*nVars);
>                 }
>         }
> 
>         MatCreateMPIAIJ(
>                         PETSC_COMM_WORLD,
>                         grid[gid].cellCount*nVars,
>                         grid[gid].cellCount*nVars,
>                         grid[gid].globalCellCount*nVars,
>                         grid[gid].globalCellCount*nVars,
>                         0,&diagonal_nonzeros[0],
>                         0,&off_diagonal_nonzeros[0],
>                         &impOP);
> 
>         KSPSetOperators(ksp,impOP,impOP,SAME_NONZERO_PATTERN);
>                                                                                                                                                                    
>         KSPSetTolerances(ksp,rtol,abstol,1.e15,maxits);
>         KSPSetType(ksp, KSPBCGS);
>         //PCSetType(pc,PCBJACOBI);
>         KSPGMRESSetRestart(ksp,30);
>         KSPSetFromOptions(ksp);
> 
>         return;
> }