Silicon-photonic network architectures for scalable, power-efficient multi-chip systems

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Abstract
Scaling trends of logic, memories, and interconnect networks lead towards dense many-core chips. Unfortunately, process yields and reticle sizes limit the scalability of large single-chip systems. Multi-chip systems break free of these areal limits, but in turn require enormous chip-to-chip bandwidth. The "macrochip" concept presented here integrates multiple many-core processor chips in a single package with silicon-photonic interconnects. This design enables a multi-chip system to approach the performance of a single large die. In this paper we propose three silicon-photonic network designs that provide low-power, high-bandwidth inter-die communication: a static wavelength-routed point-to-point network, a "two-phase" arbitrated network, and a limited-connectivity point-to-point network. We also adapt two existing intra-chip silicon-photonic interconnects: a token-ring-based crossbar and a circuit-switched torus. We simulate a 64-die, 512-core cache-coherent macrochip using all of the above networks with synthetic kernels, and kernels from Splash-2 and PARSEC. We evaluate the networks on performance, optical power and complexity. Despite a narrow data-path width compared to the token-ring or torus, the point-to-point performs 3.3x and 3.9x better respectively. We show that the point-to-point is over 10x more power-efficient than the other networks. We also show that, contrary to electronic network designs, a point-to-point network has the lowest design complexity for an inter-chip silicon-photonic network.

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