Durable Algorithms for Writable LL/SC and CAS with Dynamic Joining

TL;DR

This paper introduces durable, writable implementations of CAS and LLSC primitives that support dynamic joining, have constant time operations, and adapt space complexity based on active processes, advancing the state-of-the-art in durability and efficiency.

Contribution

It presents the first durable CAS algorithms supporting dynamic joining and the first durable LLSC objects, with adaptive space complexities.

Findings

DuraCAS supports $O(m + n)$ space for $m$ objects and $n$ processes.

DuraLL requires $O(m + n + C)$ space, where $C$ is total contexts.

EC Writable-LLSC is ABA-free with $O(m + n)$ space.

Abstract

We present durable implementations for two well known universal primitives -- CAS (compare-and-swap), and its ABA-free counter-part LLSC (load-linked, store-conditional). All our implementations are: writable, meaning they support a Write() operation; have constant time complexity per operation; allow for dynamic joining, meaning newly created processes (a.k.a. threads) of arbitrary names can join a protocol and access our implementations; and have adaptive space complexities, meaning the space use scales in the number of processes $n$ that actually use the objects, as opposed to previous protocols which are designed for a maximum number of processes $N$. Our durable Writable-CAS implementation, DuraCAS, requires $O(m + n)$ space to support $m$ objects that get accessed by $n$ processes, improving on the state-of-the-art $O(m + N^2)$. By definition, LLSC objects must store "contexts" in addition to object values. Our Writable-LLSC implementation, DuraLL, requires $O(m + n + C)$ space, where $C$ is the number of "contexts" stored across all the objects. While LLSC has an advantage over CAS due to being ABA-free, the object definition seems to require additional space usage. To address this trade-off, we define an External Context (EC) variant of LLSC. Our EC Writable-LLSC implementation is ABA-free and has a space complexity of just $O(m + n)$. To our knowledge, we are the first to present durable CAS algorithms that allow for dynamic joining, and our algorithms are the first to exhibit adaptive space complexities. To our knowledge, we are the first to implement any type of durable LLSC objects.