FreewayNoC: A DDR NoC with Pipeline Bypassing

Paper in proceedings, 2018

This paper introduces FreewayNoC, a Network-on-chip that routes packets at Dual Data Rate (DDR) and allows pipeline bypassing. Based on the observation that routers datapath is faster than control, a recent NoC design allowed flits to be routed at DDR improving throughput to rates defined solely by the switch and link traversal, rather than by the control. However, such a DDR NoC suffers from high packet latency as flits require multiple cycles per hop. A common way to reduce latency at low traffic load is pipeline bypassing, then, flits that find a contention-free way to the output port can directly traverse the switch. Existing Single Data Rate (SDR) NoC routers support it, but applying pipeline bypassing to a DDR router is more challenging. It would need additional bypassing logic which would add to the cycle time compromising the DDR NoC throughput advantage. FreewayNoC design restricts pipeline bypassing on a DDR router to only flits that go straight simplifying its logic. Thereby, it offers low packet latency without affecting DDR router cycle time and throughput. Then, at low traffic loads, besides the few turns that a flit would take on its way from source to destination, all other hops could potentially offer minimum latency equal to the delay of the switch and link traversal. Post place and route results in 28 nm technology confirm the above and also show that zero-load latency scales to the hop count better than current state-of-the-art NoCs.