Optical on-chip communication is considered a promising candidate to overcome latency and energy bottle-necks of electrical interconnects. Although recently proposed hybrid Networks-on-chip (NoCs), which implement both electrical and optical links, improve power eciency, they often fail to combine these two interconnect technologies eciently and suer from considerablelaser 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 andin turn laser power. Moreover, when optimally combined with electrical links for short distances, this can be done without trading o latency. We present the effectiveness of this concept with Lego, our hybrid, mesh-based NoC that provides high power eciency by utilizing electrical links for local trac, andlow-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 re-sources. Our routing algorithm always chooses the link that oers the lowest latency and energy. Compared to state-of-the-art proposals, Lego increases throughput-per-watt by at least 40%, and lowers latency by 35% onaverage for synthetic trac. On SPLASH-2/PARSEC workloads, Lego improves power eciency by at least 37% (up to 3.5).
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