Weak Ref Processing

[Aleksey Shipilev]

On 4/1/19 5:12 PM, Simone Bordet wrote:
> I'm preparing for a conference session about Shenandoah (and ZGC), and
> I would like to confirm how Shenandoah does Weak (et al.) Reference
> Processing.
> 
> As far as I understand, weak reference processing is still a STW
> operation during the final mark phase, so GC parallel but not GC
> concurrent. Is that correct?

Depends on what "processing" means. The performance model for weakrefs is a bit complicated.

Discovery (figuring out the references that exist in heap) is still done concurrently during
concurrent mark. Precleaning (purging the references that have definitely alive referents) is done
concurrently after mark. Processing (figuring out what to do with references that have dead
referents, for example, clearing them or marking through the reachable referent) is done in
final-mark STW pause. Enqueueuing (putting references on associated ReferenceQueue, if any), is done
in final-mark STW pause as well.

So, the pause time depends on the weak references _churn_. For references+referents that stay alive
there is little to no overhead (because precleaning takes care of them). For references that die
along with referents, the cost is zero (because discovery never finds them). For references that
have always clear referents (i.e. everything except finalizable), the processing cost does not
involve marking through the suddenly reachable subgraph, so the cost is also low, but not as low as
you might want, hence the desire to have concurrent reference processing.

Reference processing is performed by multiple parallel workers. When in pause, it is guided by
ParallelGCThreads. In concurrent mode, by ConcGCThreads, and mostly piggybacking on the actual GC
threads that do the marking.

> Is there any plan to make it concurrent, if not already?

Roman had a prototype, I would let him talk about it.

-Aleksey