A Nonblocking Algorithm for Shared Queues Using Compare-and-Swap

Nonblocking algorithms for concurrent objects guar- antee that an object is always accessible, in contrast to blocking algorithms in which a slow or halted process can render part or all of the data structure inaccessible to other processes. A number of algorithms have been proposed for shared FIFO queues, but nonblocking implementations are few and either limit the concurrency or provide inefficient solutions. In this paper we present a simple and efficient nonblocking shared FIFO queue algorithm with O(n) system latency, no additional memory requirements, and enqueuing and dequeuing times independent of the size of the queue. We use the compare&swap &swap operation as the basic synchronization primitive. We model our algorithm analytically and with a simulation, and compare its performance with that of a blocking FIFO queue. We find that the nonblocking queue has better performance if processors are occasionally slow, but worse performance if some processors are always slower than others.