[petsc-dev] DMGetNamedGlobalVector() implementation?

[Jed Brown]

On Thu, Jul 19, 2012 at 1:19 PM, Barry Smith <bsmith at mcs.anl.gov> 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).
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