[petsc-users] Reusing the preconditioner when using KSPSetComputeOperators

[Barry Smith]

On Aug 29, 2014, at 9:35 AM, Åsmund Ervik <asmund.ervik at ntnu.no> wrote:

> > On 28. aug. 2014 20:52, Barry Smith wrote:
> > > 
> > > On Aug 28, 2014, at 4:34 AM, Åsmund Ervik <asmund.ervik at ntnu.no> wrote:
> > > 
> > >> Hello,
> > >>
> > >> I am solving a pressure Poisson equation with KSP, where the initial
> > >> guess, RHS and matrix are computed by functions that I've hooked into
> > >> KSPSetComputeXXX. (I'm also using DMDA for my domain decomposition.)
> > >>
> > >> For (single-phase|two-phase) I would like to (reuse|not reuse) the
> > >> preconditioner. How do I specify that when using this way of setting the
> > >> operator? Is it toggled by whether or not I call KSPSetOperators before
> > >> each KSPSolve? (The manual does not mention KSPSetComputeXXX.)
> > > 
> > >    You should call KSPSetOperators() before each KSPSolve() (otherwise the function you  provide to compute the matrix won’t be triggered).
> > > 
> > >    With PETSc 3.5 after the call to KSPSetOperators() call KSPSetReusePreconditioner() to tell KSP wether to reuse the preconditioner or build a new one.
> > >    With PETSc 3.4 and earlier, the final argument to KSPSetOperators() would be MAT_SAME_PRECONDITIONER to reuse the preconditioner or MAT_SAME_NONZERO_PATTERN to construct a new preconditioner
> > >
> 
> Thanks Barry for the clarification. Is there an example somewhere that
> does this? All the ones I can find which use KSPSetComputeOperators()
> have no calls to KSPSetOperators(). I guess this is because they are
> only doing one linear solve?
> 
> Furthermore, what should I pass in for Amat and Pmat to the KSPSetOperators() call? PetscNullObject, or do I get the Amat from the KSP somehow?

You better call KSPGetOperators() to get them. Yes this is kind of silly.



> 
> Åsmund
>