Efficient Multi-word Compare and Swap

TL;DR

This paper introduces a new lock-free multi-word compare-and-swap algorithm that minimizes the number of CAS instructions needed, improving efficiency and durability in concurrent algorithms.

Contribution

It proves the lower bound for CAS usage in k-CAS and presents the first algorithm achieving near-optimal CAS count with practical implementation and durability features.

Findings

Outperforms existing MCAS implementations in benchmarks

Achieves durably linearizable MCAS with minimal persistence fences

Proves lower bound on CAS packing for k-CAS

Abstract

Atomic lock-free multi-word compare-and-swap (MCAS) is a powerful tool for designing concurrent algorithms. Yet, its widespread usage has been limited because lock-free implementations of MCAS make heavy use of expensive compare-and-swap (CAS) instructions. Existing MCAS implementations indeed use at least 2k+1 CASes per k-CAS. This leads to the natural desire to minimize the number of CASes required to implement MCAS. We first prove in this paper that it is impossible to "pack" the information required to perform a k-word CAS (k-CAS) in less than k locations to be CASed. Then we present the first algorithm that requires k+1 CASes per call to k-CAS in the common uncontended case. We implement our algorithm and show that it outperforms a state-of-the-art baseline in a variety of benchmarks in most considered workloads. We also present a durably linearizable (persistent memory friendly) version of our MCAS algorithm using only 2 persistence fences per call, while still only requiring k+1 CASes per k-CAS.