How FIFO is your concurrent FIFO queue

Abstract

Abstract Designing and implementing high-performance concurrent data structures whose access performance scales on multicore hardware is difficult. Concurrent implementations of FIFO queues, for example, seem to require algorithms that efficiently increase the potential for parallel access by implementing semantically relaxed rather than strict FIFO queues where elements may be returned in some out-of-order fashion. However, we show experimentally that the on average shorter execution time of enqueue and dequeue operations of fast but relaxed implementations may offset the effect of semantical relaxations making them appear as behaving more FIFO than strict but slow implementations. Our key assumption is that ideal concurrent data structure operations should execute in zero time. We define two metrics, elementfairness and operation-fairness, to measure the degree of element and operation reordering, respectively, assuming operations take zero time. Element-fairness quantifies the deviation from FIFO queue semantics had all operations executed in zero time. With this metric even strict implementations of FIFO queues are not FIFO. Operation-fairness helps explaining element-fairness by quantifying operation reordering when considering the actual time operations took effect relative to their invocation time. In our experiments, the effect of poor operation-fairness of strict but slow implementations on element-fairness may outweigh the effect of semantical relaxation of fast but relaxed implementations. Categories and Subject Descriptors D.1.3 [Concurrent Programming]: Parallel Programming General Terms Measurement Keywords Zero-Time Linearization, Element-Fairness, Operation-Fairness * This work has been supported by the National Research Network RiSE on Rigorous Systems Engineering (Austrian Science Fund S11404-N23). Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee