Atomic Instructions in Java

Atomic instructions atomically access and update one or more memory locations. Because they do not incur the overhead of lock acquisition or suspend the executing thread during contention, they may allow higher levels of concurrency on multiprocessors than lock-based synchronization. Wait-free data structures are an important application of atomic instructions, and extend these performance benefits to higher level abstractions such as queues. In type-unsafe languages such as C, atomic instructions can be expressed in terms of operations on memory addresses. However, type-safe languages such as Java do not allow manipulation of arbitrary memory locations. Adding support for atomic instructions to Java is an interesting but important challenge. In this paper we consider several ways to support atomic instructions in Java. Each technique has advantages and disadvantages. We propose idiom recognition as the technique we feel has the best combination of expressiveness and simplicity. We describe techniques for recognizing instances of atomic operation idioms in the compiler of a Java Virtual Machine, and converting such instances into code utilizing atomic machine instructions. In addition, we describe a runtime technique which ensures that the semantics of multithreaded Java[11] are preserved when atomic instructions and blocking synchronization are used in the same program. Finally, we present benchmark results showing that for concurrent queues, a wait-free algorithm implemented using atomic compareand-swap instructions yields better scalability on a large multiprocessor than a queue implemented with lock-based synchronization.