[petsc-users] Inquiry about the preconditioner setup of KSP.

[Jed Brown]

Is it possible that Julia is transferring the matrix to PETSc as a dense
matrix (storing the nonzeros) instead of preserving sparsity?  If you
store the zeros, then ILU will be allowed to fill those entries, thereby
becoming LU.

Xiaodong Liu <xliu29 at ncsu.edu> writes:

> I am trying to use Julia to call Petsc.
> 1) First, I run the built-in example ex2.c
> https://www.mcs.anl.gov/petsc/petsc-current/src/ksp/pc/examples/tutorials/ex2.c.html
>
> For this case,  I tried KSPGMRES , initial zero solution and different PC
> (LU, ILU, ICC, JACOBI). And they work as expected.
>
>  75: KSPSetType(ksp,KSPGMRES);
>  76: KSPSetInitialGuessNonzero(ksp,PETSC_FALSE);
>  87: KSPGetPC(ksp,&pc);
>  88: PCSetType(pc,PCICC);
>
> 2) Second, I tried to call KSP from Julia using the same matrix and right
> hand side as ex2.c . A wrapper has been written to call Petsc code from
> Julia. After I transfer matrix to from Julia to Petsc, I checked the matrix
> and preconditioner matrix in the context of Petsc. These two matrices are
> right. For no preconditioner,Jacobi precontioner and LU, the residual for
> Julia version is same as that of original Petsc one for every iteration.
> However, for ILU preconditioner, the residual for Julia is alwaysthe same
> as the LU one. This is not expected. For both Julia and original Petsc
> version, I checked the PC type inside the subroutine PCSetType and the
> types are correct, namely ilu.I am trying to dig into the source code to
> check how the preconditioner is interacting with GMRES.
>
> Where is the subroutine for the GMRES solver, namely,
>  ierr=(*KSP->ops->solve)(ksp);
>
> Or do you have any suggestions?
>
> Thanks,
>
> Xiaodong Liu, PhD
> X: Computational Physics Division
> Los Alamos National Laboratory
> P.O. Box 1663,
> Los Alamos, NM 87544
> 505-709-0534