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Increasing fault rates in current and future technology nodes coupled with on-chip components in the hundreds calls for robust and fault-tolerant Network-on-Chip (NoC) designs. Given the central role of NoCs in today's many-core chips, permanent faults impeding their original functionality may significantly influence performance, energy consumption, and correct operation of the entire system. As a result, fault-tolerant NoC design gained much attention in recent years. In this article, we review the vast research efforts regarding a NoC's components, namely, topology, routing algorithm, router microarchitecture, as well as system-level approaches combined with reconfiguration; discuss the proposed architectures; and identify outstanding research questions.

Optical on-chip communication is considered a promising candidate to overcome latency and energy bottlenecks of electrical interconnects. Although recently proposed hybrid Networks-on-chip (NoCs), which implement both electrical and optical links, improve power efficiency, they often fail to combine these two interconnect technologies efficiently and suffer from considerable laser power overheads caused by high-bandwidth optical links. We argue that these overheads can be avoided by inserting a higher quantity of low-bandwidth optical links in a topology, as this yields lower optical loss and in turn laser power. Moreover, when optimally combined with electrical links for short distances, this can be done without trading off latency. We present the effectiveness of this concept with Lego, our hybrid, mesh-based NoC that provides high power efficiency by utilizing electrical links for local traffic, and low-bandwidth optical links for long distances. Electrical links are placed systematically to outweigh the serialization delay introduced by the optical links, simplify router microarchitecture, and allow to save optical resources. Our routing algorithm always chooses the link that offers the lowest latency and energy. Compared to state-of-the-art proposals, Lego increases throughputper-watt by at least 40%, and lowers latency by 35% on average for synthetic traffic. On SPLASH-2/PARSEC workloads, Lego improves power efficiency by at least 37% (up to 3.5×).

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Abstract-Optical Networks-on-chip constitute a promising approach to tackle the power wall problem present in largescale electrical NoC design. In order to enable their adoption and ensure scalability, oNoCs have to be carefully designed with power and energy consumption in mind. In this paper, we propose Amon, an all-optical NoC design based on passive microrings and Wavelength Division Multiplexing, including the switch architecture and a contention-free routing algorithm. The goal is to obtain a design that minimizes the total number of wavelengths and microrings, the wiring complexity and the diameter. An analytical comparison with state-of-the-art design proposals of all-optical NoCs shows that the proposed design can substantially improve the most important performance metrics: hop counts, chip area, energy and power consumption. Our experimental work confirms that this improvement translates into higher performance and efficiency. Finally, our design provides a tile-based structure which should facilitate VLSI integration when compared with recent ring-like solutions. In general we show it provides a more scalable solution than previous designs.