The Open-Source BlackParrot-BedRock Cache Coherence System

TL;DR

This paper presents the open-source BedRock cache coherence protocol and three implementations within the BlackParrot RISC-V multicore processor, demonstrating the feasibility and challenges of integrating programmable coherence engines in modern shared-memory systems.

Contribution

It introduces the BedRock protocol with simplified states and provides three diverse cache coherence directory implementations, including a programmable engine, for performance and flexibility analysis.

Findings

BedRock reduces implementation complexity by eliminating transient states.

Programmable coherence directories are feasible within multicore processors.

Hybrid designs offer a balance between performance and flexibility.

Abstract

This dissertation revisits the topic of programmable cache coherence engines in the context of modern shared-memory multicore processors. First, the open-source BedRock cache coherence protocol is described. BedRock employs the canonical MOESIF coherence states and reduces implementation burden by eliminating transient coherence states from the protocol. The protocol's design complexity, concurrency, and verification effort are analyzed and compared to a canonical directory-based invalidate coherence protocol. Second, the architecture and microarchitecture of three separate cache coherence directories implementing the BedRock protocol within the BlackParrot 64-bit RISC-V multicore processor, collectively called BlackParrot-BedRock (BP-BedRock), are described. A fixed-function coherence directory engine implementation provides a baseline design for performance and area comparisons. A microcode-programmable coherence directory implementation demonstrates the feasibility of implementing a programmable coherence engine capable of maintaining sufficient protocol processing performance. A hybrid fixed-function and programmable coherence directory blends the protocol processing performance of the fixed-function design with the programmable flexibility of the microcode-programmable design. Collectively, the BedRock coherence protocol and its three BP-BedRock implementations demonstrate the feasibility and challenges of including programmable logic within the coherence system of modern shared-memory multicore processors, paving the way for future research into the application- and system-level benefits of programmable coherence engines.