The need for computation reuse (between J and J_pre, for example) might require a change in the SNES callback model. Right now each call -- FormFunction, FormJacobian -- is stateless. It takes a Vec X and computes the corresponding quantities -- residual, J, J_pre -- solely from X. Any callback that wanted to reuse any other callback's computation would have to be able to make sure the reuse is safe: am I being called with the same X as this previous callback that already precomputed a bunch of stuff for me? This is only possible with a costly, collective (and imprecise) vector comparison, or a fragile state tracker within X. If this is unacceptable, the callbacks must carry (and share) a state, and, thus, become objects.
That's probably undesirable within the basic SNES callback model, but may be natural within the DM-based callback model: a SNES sets its DM's Vec X and calls DMFunction, DMJacobian without any argument -- those calls pull out X and compute whatever they need, including some common data for others (e.g., PC DM) to reuse. A new call to DMSetVec() will reset these common data structures. This way the DM can also be viewed as a "factory". This would probably make my interaction with libMesh a bit easier, but isn't strictly necessary. Still, it would be good to know any thoughts on this. Dmitry. On Sun, Feb 12, 2012 at 10:32 AM, Dmitry Karpeev <karpeev at mcs.anl.gov>wrote: > > > On Sat, Feb 11, 2012 at 8:36 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote: > >> On Sat, Feb 11, 2012 at 08:32, Dmitry Karpeev <karpeev at mcs.anl.gov>wrote: >> >>> This sounds fine to me, but what's the API for doing this? You want to >>> do KSPSetDM() and PCSetDM(), >>> so that the user has to explicitly pull out the PC to set its DM? >>> >> >> The PC *always* owns both matrices anyway. >> > > I think there is a rather natural API for handling DM grouping, code reuse > and dealing with the issue of single matrix per DM: > DM should support DMGetKSPDM(), DMGetPCDM() -- similar to > DMDAGetCoordinateDA() -- each of which will compute the correct matrix. > These calls can return the original DM (or PETSC_NULL) by default. SNES > will pull out the KSP DM out to set it on the KSP, KSP will do the same for > the PC, etc. DM already has Vec x, so all of the DMs can share x and > compute their respective matrices at the same x and reuse whatever > computation is reusable. That's up to the implementation, though. > > Dmitry. > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20120217/ade9b955/attachment.html>