Lock-Free Hopscotch Hashing

TL;DR

This paper introduces a lock-free variant of Hopscotch Hashing that enhances speed, cache locality, and scalability, outperforming existing lock-free and blocking hashing algorithms in many cases.

Contribution

It presents the first lock-free implementation of Hopscotch Hashing, improving performance and progress guarantees over the original blocking version.

Findings

Performance surpasses existing hashing algorithms in many scenarios

Achieves better scalability and cache locality

Maintains features like physical deletion and probe-chain compression

Abstract

In this paper we present a lock-free version of Hopscotch Hashing. Hopscotch Hashing is an open addressing algorithm originally proposed by Herlihy, Shavit, and Tzafrir, which is known for fast performance and excellent cache locality. The algorithm allows users of the table to skip or jump over irrelevant entries, allowing quick search, insertion, and removal of entries. Unlike traditional linear probing, Hopscotch Hashing is capable of operating under a high load factor, as probe counts remain small. Our lock-free version improves on both speed, cache locality, and progress guarantees of the original, being a chimera of two concurrent hash tables. We compare our data structure to various other lock-free and blocking hashing algorithms and show that its performance is in many cases superior to existing strategies. The proposed lock-free version overcomes some of the drawbacks associated with the original blocking version, leading to a substantial boost in scalability while maintaining attractive features like physical deletion or probe-chain compression.