A Simple On-Chip Optical Interconnection for Improving Performance of Coherency Traffic in CMPs

Nanophotonic interconnection is a promising solution for inter-core communication in future chip multiprocessors (CMPs). Main benefits derive from its intrinsic low-latency and high-bandwidth, especially when employing wavelength division multiplexing (WDM), as well as reduced power requirements when compared to electronic NoCs. Existing works on optical NoCs (ONoC) mainly concentrate on relatively complex proposals needed to host the whole CMP traffic. In some proposals complexity is increased also from the need of an electronic network for preliminary pathsetup in the optical one. This paper proposes to enhance a conventional NoC with only a simple photonic structure, a ring, and aims at investigating its suitability to support the low-latency transmission of small latency-critical coherency control messages as to improve performance of multithreaded applications. In particular, our proposed scheme supports fast multicast transmission of invalidation messages. We have simulated Parsec benchmarks on an 8 core full-system CMP. Results show that a careful selection of coherency control messages to be forwarded to the photonic ring allows improving execution time up to 19%, with an average of 6% across all considered benchmarks. We discuss how different selections of messages, i.e. related to read and/or write operations, affect results and single out the most profitable set. Moreover, we show that the sharing behavior of benchmarks has a central role in the final performance.