  Hierarchical ring networks, which hierarchically connect multiple levels of
rings, have been proposed in the past to improve the scalability of ring
interconnects, but past hierarchical ring designs sacrifice some of the key
benefits of rings by introducing more complex in-ring buffering and buffered
flow control. Our goal in this paper is to design a new hierarchical ring
interconnect that can maintain most of the simplicity of traditional ring
designs (no in-ring buffering or buffered flow control) while achieving high
scalability as more complex buffered hierarchical ring designs. Our design,
called HiRD (Hierarchical Rings with Deflection), includes features that allow
us to mostly maintain the simplicity of traditional simple ring topologies
while providing higher energy efficiency and scalability. First, HiRD does not
have any buffering or buffered flow control within individual rings, and
requires only a small amount of buffering between the ring hierarchy levels.
When inter-ring buffers are full, our design simply deflects flits so that they
circle the ring and try again, which eliminates the need for in-ring buffering.
Second, we introduce two simple mechanisms that provides an end-to-end delivery
guarantee within the entire network without impacting the critical path or
latency of the vast majority of network traffic. HiRD attains equal or better
performance at better energy efficiency than multiple versions of both a
previous hierarchical ring design and a traditional single ring design. We also
analyze our design's characteristics and injection and delivery guarantees. We
conclude that HiRD can be a compelling design point that allows higher energy
efficiency and scalability while retaining the simplicity and appeal of
conventional ring-based designs.
