On Fri, Feb 10, 2012 at 5:24 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> > On Feb 10, 2012, at 5:14 PM, Jed Brown wrote: > > > On Fri, Feb 10, 2012 at 17:08, Dmitry Karpeev <karpeev at mcs.anl.gov> > wrote: > > I think it's completely natural for a DM to assemble two operators -- > the discretizations for the two are likely to be related anyway -- as soon > as we decide that it's natural for KSP to take in two matrices and, more > importantly, for the callback set with DMSetJacobian() to compute two > matrices: if a DM knows how to compute two "Jacobians", > > DM doesn't/shouldn't know how to compute two Jacobians. Where did you > get that from? > Here's the current declaration of DMSetJacobian: PetscErrorCode DMSetJacobian(DM dm,PetscErrorCode (*f)(DM,Vec,Mat,Mat,MatStructure*)) This attaches (to a single DM) a callback that computes *two* matrices, just like SNES/KSP would want. Dmitry. > > > why wouldn't it know how to create/preallocate the two corresponding > matrices? > > > The reason I don't like having the single DM doing this is that one > could use very different beasties to do the true Jacobian and > preconditioner (not just a simple stencil change) and shoving that stuff > into a single DM is unnatural. For example true problem is on an > unstructured grid, preconditioner problem on a simplier structured grid. > > > > > > Lots of functions get messy if the DM has multiple ways to do something. > Should DMCreateLocalVector() use the full stencil or the preconditioning > stencil, what should DMGlobalToLocalBegin() be updating, etc. > > > > Barry's solution of having separate DMs sounds cleaner to me, at least > modulo needing conventions about which DM on which to PetscObjectCompose() > things needed by certain callbacks (e.g. in the FAS with TS stuff I'm > doing). > > Clean that up and provide a formal way to do those callbacks that don't > use kludgy PetscObjectCompose() or PetscObjectComposeFunction() > > Barry > > Yes, it will be slightly painful to pass two DM into the KSP and PC and > track them in multigrid but we do do that with the two operators already so > using two DM seems very natural. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20120210/4a60359d/attachment.html>