Highly-Concurrent Doubly-Linked Lists

TL;DR

This paper introduces 'adlist', a highly concurrent doubly-linked list that enables parallel insertions and deletions, significantly improving scalability in multi-core file systems while maintaining simplicity and consistency.

Contribution

The paper presents a novel concurrent doubly-linked list design that allows parallel operations on non-overlapping nodes, enhancing scalability in multi-threaded environments.

Findings

Enables parallel insert/delete on non-overlapping nodes

Maintains local consistency during concurrent operations

Used in Data Domain File System for improved scalability

Abstract

As file systems are increasingly being deployed on ever larger systems with many cores and multi-gigabytes of memory, scaling the internal data structures of file systems has taken greater importance and urgency. A doubly-linked list is a simple and very commonly used data structure in file systems but it is not very friendly to multi-threaded use. While special cases of lists, such as queues and stacks, have lock-free versions that scale reasonably well, the general form of a doubly-linked list offers no such solution. Using a mutex to serialize all operations remains the de-facto method of maintaining a doubly linked list. This severely limits the scalability of the list and developers must resort to ad-hoc workarounds that involve using multiple smaller lists (with individual locks) and deal with the resulting complexity of the system. In this paper, we present an approach to building highly concurrent data structures, with special focus on the implementation of highly concurrent doubly-linked lists. Dubbed "advanced doubly-linked list" or "adlist" for short, our list allows iteration in any direction, and insert/delete operations over non-overlapping nodes to execute in parallel. Operations with common nodes get serialized so as to always present a locally consistent view to the callers. An adlist node needs an additional 8 bytes of space for keeping synchronization information. The Data Domain File System makes extensive use of adlists which has allowed for significant scaling of the system without sacrificing simplicity.