Fully concurrent garbage collection of actors on many-core machines

Abstract

Disposal of dead actors in actor-model languages is as important as disposal of unreachable objects in object-oriented languages. In current practice, programmers are required to either manually terminate actors, or they have to rely on garbage collection systems that monitor actor mutation through write barriers, thread coordination through locks etc. These techniques, however, prevent the collector from being fully concurrent. We developed a protocol that allows garbage collection to run fully concurrently with all actors. The main challenges in concurrent garbage collection is the detection of cycles of sleeping actors in the actors graph, in the presence of concurrent mutation of this graph. Our protocol is solely built on message passing: it uses deferred direct reference counting, a dedicated actor for the detection of (cyclic) garbage, and a confirmation protocol (to deal with the mutation of the actor graph). We present our ideas informally through an example, and then present a formal model, prove soundness and argue completeness. We have implemented the protocol as part of a runtime library. As a preliminary performance evaluation, we discuss the performance of our approach as currently used at a financial institution, and use four benchmarks from the literature to compare our approach with other actor-model systems. These preliminary results indicate that the overhead of our approach is small.