Rainbow: A composable coherence protocol for multi‐chip servers

Abstract

SummaryThe use of multi‐chip modules (MCM) and/or multi‐socket boards is the most suitable approach to increase the computation density of servers while keep chip yield attained. This article introduces a new coherence protocol suitable, in terms of complexity and scalability, for this class of systems. The proposal uses two complementary ideas: (1) A mechanism that dissociates complexity from performance by means of colored‐token counting, (2) A construct that optimizes performance and cost by means of two functionally symmetrical structures working in the last level cache of each chip and each memory controller. The coordinated work of both structures minimizes the coherence‐related effects on the average memory latency perceived by the processor. Our proposal is able to improve on the performance of a HyperTransport‐like coherence protocol by from 25% to 60%.