On Tue, Mar 15, 2011 at 21:39, Matthew Knepley <knepley at gmail.com> wrote:
> Here is the scenario. I have a FieldSplit solve with 4 splits. For the 4th > split, I have > a good preconditioner matrix. In the linear case, KSP, I am just doing > this: > > err = KSPSetUp(_ksp); > err = KSPGetPC(_ksp, &pc); > err = PCFieldSplitGetSubKSP(pc, &num, &ksps); > > MatStructure flag; > err = KSPGetOperators(ksps[num-1], &A, PETSC_NULL, &flag); > err = PetscObjectReference((PetscObject) A); > err = KSPSetOperators(ksps[num-1], A, _precondMatrix, flag); > err = PetscFree(ksps); > Yuck. > which works fine. However, if we embed this in a nonlinear solve, so that I > now call > > err = SNESGetKSP(_snes, &ksp); > <same code> > > then the PC matrix for the 4th split is ignored. This happens because > SNESSolve_LS() > calls KSPSetOperators() right before the solve. This causes FS to pull out > the same > matrices again and overwrite my special PC matrix. > The preconditioning matrix for this piece should be put into the global preconditioning matrix, perhaps stored as a MatNest. Fieldsplit should take that matrix apart for preconditioning and it'll find the preconditioning matrix for this part. More tricky is dealing with auxiliary operators such as appear in approximate-commutator Schur complement preconditioners. The current method is punting, you PetscObjectCompose the auxiliary matrix. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20110315/c62c27a6/attachment.html>
