The Optimistic Readers Transformation

Abstract

Monitors, such as Java classes with synchronized methods, are a convenient and safe abstraction for designing and reasoning about multithreaded object-oriented programs. However, the straightforward implementation of monitors can be inefficient, particularly in programs in which the majority of calls are to read-only methods. We introduce the optimistic readers program transformation, which may be implemented either as a compiler optimization, or as a “design pattern.” This transformation produces an implementation whose observable behavior is equivalent to that of a monitor, but in which read-only methods do not acquire locks or perform any shared writes. As a result, programmers can reason about their programs as if each shared object were implemented using mutual exclusion, while achieving the performance benefits of unsynchronized reads. We present the program transformation using the platform-independent abstraction CRF. We then demonstrate the performance of this transformation as applied to benchmarks derived from the core module of a practical system — a Java-based publish-subscribe router. We compare the performance of the optimistic readers transformation to unoptimized synchronized methods and to reader and writer locks.