Fast Wait-Free Construction for Pool-Like Objects with Weakened Internal Order: Stacks as an Example

Abstract

This paper focuses on a large class of concurrent data structures that we call pool-like objects (e.g., stack, double-ended queue, and queue). Performance and progress guarantee are two important characteristics for concurrent data structures. In the aspect of performance, weakening the internal order in a pool-like object is an effective technique to reduce the synchronization cost among threads accessing the object, but no objects with weakened internal order provide a progress guarantee as strong as wait-freedom. Meanwhile, wait-free algorithms tend to be inefficient, which is mainly attributed to the helping mechanisms. Based on the philosophy of existing helping mechanisms, a wait-free pool-like object with weakened internal order would suffer from unnecessary process of getting the latest object state and synchronization. This paper takes a state-of-the-art implementation of stacks with weakened internal order as an example, and transforms it into a highly-efficient wait-free stack named WF-TS-Stack. The transformation method includes a helping mechanism with state reuse and a relaxed removal scheme. In addition, we use a simple and effective scheme to further improve the performance of WF-TS-Stack in Non-Uniform Memory Access (NUMA) architectures. Our evaluation with representative benchmarks shows that WF-TS-Stack outperforms its original building blocks by up to 1.45× at maximum concurrency. We also discuss how to yield an efficient double-ended queue (deque) variant of WF-TS-Stack, because deque is a more generalized pool-like object.