Aurora: A Cross-Layer Solution for Thermally Resilient Photonic Network-on-Chip

With silicon optical technology moving toward maturity, the use of photonic networks-on-chip (NoCs) for global chip communication is emerging as a promising solution to the communication requirements of future many core processors. It is expected that photonic NoCs will play an important role in alleviating current power, latency, and bandwidth constraints. However, photonic NoCs are sensitive to ambient temperature variations because their basic constituents, ring resonators, are themselves sensitive to those variations. Since ring resonators are basic building blocks for photonic modulators, switches, multiplexers, and demultiplexers, variations of on-chip temperature pose serious challenges to the proper operation of photonic NoCs. Proposed methods that mitigate the effects of temperature at the device level are either difficult to use in CMOS processes or not suitable for large scale implementation. In this paper, we propose Aurora, a thermally resilient photonic NoC architecture design that supports reliable and low bit error rate (BER) on-chip communications in the presence of large temperature variations. Our proposed architecture leverages cross-layer solutions at the device, architecture, and operating system (OS) layers that individually provide considerable improvements and synergistically provide even more significant improvements. To compensate for small temperature variations, our design varies the bias current through ring resonators. For larger temperature variations, we propose architecture-level techniques to reroute messages away from hot regions, and through cooler regions, to their destinations. We also propose a thermal/congestion-aware coscheduling algorithm at the OS level to further lower BER by reorganizing the thermal profile of the chip. Our simulation results show that Aurora provides a robust architectural solution to handle temperature variation effects on future photonic NoCs. For instance, average BER and message error rate are reduced by 96% and 85%, respectively, when the combined thermal optimization scheme [shortest path first + OS] is applied. From the perspective of power efficiency, Aurora is also superior to conventional photonic NoC architectures by as much as 37%.