ABSTRACT
We discussed reference counting in Chapter 5. The two chief issues facing naïve reference counting were its inability to collect garbage cycles and the high cost of manipulating reference counts, particularly in the face of races between different mutator threads. The solution to cyclic garbage was trial deletion (partial tracing). We used deferred reference counting to avoid having mutators manipulate reference counts on local variables and coalescing to avoid having to make ‘redundant’ changes to reference counts that would be cancelled out by later mutations; a useful side-effect of coalescing is that it tolerates mutator races. All three solutions required stopping the world while the collector reconciled reference counts and reclaimed any garbage. In this chapter, we relax this requirement and consider the changes that need to be made in order to allow a reference counting collector thread to run concurrently with mutator threads.
