Speculative Concurrent Processing with Transactional Memory in the Actor Model

Abstract

The actor model has been successfully used for scalable computing in distributed systems. Actors are objects with a local state, which can only be modified by the exchange of messages. One of the fundamental principles of actor models is to guarantee sequential message processing, which avoids typical concurrency hazards, but limits the achievable message throughput. Preserving the sequential semantics of the actor model is, however, necessary for program correctness.

In this paper, we propose to add support for speculative concurrent execution in actors using transactional memory (TM). Our approach is designed to operate with message passing and shared memory, and can thus take advantage of parallelism available on distributed and multi-core systems. The processing of each message is wrapped in a transaction executed atomically and in isolation, but concurrently with other messages. This allows us (1) to scale while keeping the dependability guarantees ensured by sequential message processing, and (2) to further increase robustness of the actor model against threats due to the rollback ability that comes for free with transactional processing of messages. We validate our design within the Scala programming language and the Akka framework. We show that the overhead of using transactions is hidden by the improved message processing throughput, thus leading to an overall performance gain.