Flexible snooping: Adaptive forwarding and filtering of snoops in embedded-ring multiprocessors

A simple and low-cost approach to supporting snoopy cache coherence is to logically embed a unidirectional ring in the network of a multiprocessor, and use it to transfer snoop messages. Other messages can use any link in the network. While this scheme works for any network topology, a naive implementation may result in long response times or in many snoop messages and snoop operations. To address this problem, this paper proposes Flexible Snooping algorithms, a family of adaptive forwarding and filtering snooping algorithms. In these algorithms, a node receiving a snoop request may either forward it to another node and then perform the snoop, or snoop and then forward it, or simply forward it without snooping. The resulting design space offers trade-offs in number of snoop operations and messages, response time, and energy consumption. Our analysis using SPLASH-2, SPECjbb, and SPECweb workloads finds several snooping algorithms that are more cost-effective than current ones. Specifically, our choice for a high-performance snooping algorithm is faster than the currently fastest algorithm while consuming 9-17% less energy; our choice for an energy-efficient algorithm is only 3-6% slower than the previous one while consuming 36-42% less energy.