Light speed arbitration and flow control for nanophotonic interconnects

By providing high bandwidth chip-wide communication at low latency and low power, on-chip optics can improve many-core performance dramatically. Optical channels that connect many nodes and allow for single cycle cache-line transmissions will require fast, high bandwidth arbitration. We exploit CMOS nanophotonic devices to create arbiters that meet the demands of on-chip optical interconnects. We accomplish this by exploiting a unique property of optical devices that allows arbitration to scale with latency bounded by the time of flight of light through a silicon waveguide that passes all requesters. We explore two classes of distributed token-based arbitration, channel based and slot based, and tailor them to optics. Channel based protocols allocate an entire waveguide to one requester at a time, whereas slot based protocols allocate fixed sized slots in the waveguide. Simple optical protocols suffer from a fixed prioritization of users and can starve those with low priority; we correct this with new schemes that vary the priorities dynamically to ensure fairness. On a 64-node optical interconnect under uniform random single-cycle traffic, our fair slot protocol achieves 74% channel utilization, while our fair channel protocol achieves 45%. Ours are the first arbitration protocols that exploit optics to simultaneously achieve low latency, high utilization, and fairness.