On Thu, Jul 19, 2012 at 1:19 PM, Barry Smith <[email protected]> wrote:

Why not implement it using PetscObjectQuery()/PetscObjectCompose()? and not have to introduce a whole new datatype DMNamedVecLink linked list and other crap?

The only reason this thing has to exist, and it really has to exist in some form, is that we have a reference loop. For example, the SNES Vec X is obtained from the DM (thus X references the DM), but must be made available to the KSP callback that uses the DM. If we attached X to the DM, then we would have a loop X -> DM -> X. Same for the TS vector needed in SNESComputeFunction. The general form of PetscObjectCompose allowing reference loops is terribly dangerous and imprecise (you end up implementing a sort of limited garbage collector that periodically looks for loops, but that is super dangerous in parallel because you need determinism in destruction).