Practical considerations for non-blocking concurrent objects

An important class of concurrent objects are those that are nonblocking, that is, whose operations are not contained within mutually exclusive critical sections. A nonblocking object can be accessed by many threads at a time, yet update protocols based on atomic compare-and-swap operations can be used to guarantee the object's consistency. The author examines the compare-and-swap operation in the content of contemporary bus-based shared memory multiprocessors, although the results generalize to distributed shared memory multiprocessors. He describes an operating system-based solution that permits the construction of a nonblocking compare-and-swap function on architectures that only support more primitive atomic primitives such as test-and-set or atomic exchange. Several locking strategies are evaluated that can be used to synthesize a compare-and-swap operation, and it is shown that the common techniques for reducing synchronization overhead in the presence of contention are inappropriate when used as the basis for nonblocking synchronization. A simple synchronization strategy is described that has good performance because it avoids much of the synchronization overhead that normally occurs when there is contention.<>