PathCAS: An Efficient Middle Ground for Concurrent Search Data Structures

TL;DR

PathCAS introduces a middle-ground primitive for concurrent search data structures, balancing simplicity and performance, and enabling the creation of efficient, easier-to-understand concurrent trees that outperform many handcrafted designs.

Contribution

We present PathCAS, a novel primitive combining ideas from multi-word CAS and transactional memory, simplifying concurrent search structure implementation while maintaining high performance.

Findings

PathCAS-based search trees outperform many handcrafted concurrent trees.

The approach rivals the fastest known concurrent binary trees in search-heavy workloads.

PathCAS enables easier construction and verification of concurrent data structures.

Abstract

To maximize the performance of concurrent data structures, researchers have often turned to highly complex fine-grained techniques, resulting in efficient and elegant algorithms, which can however be often difficult to understand and prove correct. While simpler techniques exist, such as transactional memory, they can have limited performance or portability relative to their fine-grained counterparts. Approaches at both ends of this complexity-performance spectrum have been extensively explored, but relatively less is known about the middle ground: approaches that are willing to sacrifice some performance for simplicity, while remaining competitive with state-of-the-art handcrafted designs. In this paper, we explore this middle ground, and present PathCAS, a primitive that combines ideas from multi-word CAS (KCAS) and transactional memory approaches, while carefully avoiding overhead. We show how PathCAS can be used to implement efficient search data structures relatively simply, using an internal binary search tree as an example, then extending this to an AVL tree. Our best implementations outperform many handcrafted search trees: in search-heavy workloads, it rivals the BCCO tree [5], the fastest known concurrent binary tree in terms of search performance [3]. Our results suggest that PathCAS can yield concurrent data structures that are relatively easy to build and prove correct, while offering surprisingly high performance.