Mending Fences with Self-Invalidation and Self-Downgrade

TL;DR

This paper introduces a formal model and analysis method for cache coherence protocols based on self-invalidation and self-downgrade, enabling optimal fence insertion to ensure program correctness and improve performance.

Contribution

It presents a novel formal model, a reachability analysis algorithm, and a counter-example guided fence insertion method for protocols with diverse fences, addressing correctness and optimization.

Findings

The method effectively checks program safety under self-invalidation/downgrade protocols.

Optimal fence insertion ensures program correctness with minimal cost.

Prototype implementation demonstrates practical effectiveness on various benchmarks.

Abstract

Cache coherence protocols based on self-invalidation and self-downgrade have recently seen increased popularity due to their simplicity, potential performance efficiency, and low energy consumption. However, such protocols result in memory instruction reordering, thus causing extra program behaviors that are often not intended by the programmers. We propose a novel formal model that captures the semantics of programs running under such protocols, and features a set of fences that interact with the coherence layer. Using the model, we design an algorithm to analyze the reachability and check whether a program satisfies a given safety property with the current set of fences. We describe a method for insertion of optimal sets of fences that ensure correctness of the program under such protocols. The method relies on a counter-example guided fence insertion procedure. One feature of our method is that it can handle a variety of fences (with different costs). This diversity makes optimization more difficult since one has to optimize the total cost of the inserted fences, rather than just their number. To demonstrate the strength of our approach, we have implemented a prototype and run it on a wide range of examples and benchmarks. We have also, using simulation, evaluated the performance of the resulting fenced programs.