Per-bucket concurrent rehashing algorithms

TL;DR

This paper introduces a lock-free, concurrent rehashing algorithm for extensible hash tables that improves scalability and performance by separating resizing from rehashing and using minimal synchronization.

Contribution

The paper presents a novel concurrent rehashing algorithm that separates resizing and rehashing, reducing synchronization and improving performance over lock-based methods.

Findings

Reduces synchronization on hot paths, enhancing scalability.

Allows race conditions with detection and restart, improving concurrency.

Compatible with cache-friendly layouts, potential for higher performance.

Abstract

This paper describes a generic algorithm for concurrent resizing and on-demand per-bucket rehashing for an extensible hash table. In contrast to known lock-based hash table algorithms, the proposed algorithm separates the resizing and rehashing stages so that they neither invalidate existing buckets nor block any concurrent operations. Instead, the rehashing work is deferred and split across subsequent operations with the table. The rehashing operation uses bucket-level synchronization only and therefore allows a race condition between lookup and moving operations running in different threads. Instead of using explicit synchronization, the algorithm detects the race condition and restarts the lookup operation. In comparison with other lock-based algorithms, the proposed algorithm reduces high-level synchronization on the hot path, improving performance, concurrency, and scalability of the table. The response time of the operations is also more predictable. The algorithm is compatible with cache friendly data layouts for buckets and does not depend on any memory reclamation techniques thus potentially achieving additional performance gain with corresponding implementations.