A High-Performance Hierarchical Ring On-Chip Interconnect with Low-Cost Routers

Energy consumption of routers in commonly used mesh-based on-chip networks for chip multiprocessors is an increasingly important concern: these routers consist of a crossbar and complex control logic and can require signifi- cant buffers, hence high energy and area consumption. In contrast, an alternative design uses ring-based networks to connect network nodes with small and simple routers. Rings have been used in recent commercial designs, and are well-suited to smaller core counts. However, rings do not scale as efficiently as meshes. In this paper, we propose an energy-efficient yet high performance alternative to traditional mesh-based and ringbased on-chip networks. We aim to attain the scalability of meshes with the router simplicity and efficiency of rings. Our design is a hierarchical ring topology which consists of small local rings connected via one or more global ring. Routing between rings is accomplished using bridge routers that have minimal buffering, and use deflection in place of buffered flow control for simplicity. We comprehensively explore new issues in the design of such a topology, including the design of the routers, livelock freedom, energy, performance and scalability. We propose new router microarchitectures and show that these routers are significantly simpler and more area and energy efficient than both buffered and bufferless mesh based routers. We develop new mechanisms to preserve livelock-free routing in our topology and router design. Our evaluations compare our proposal to a traditional ring network and conventional buffered and bufferless mesh based networks, showing that our proposal reduces average network power by 52.4% (30.4%) and router area footprint by 70.5% from a buffered mesh in 16-node (64-node) configurations, while also improving system performance by 0.6% (5.0%).