PlugSMART: a pluggable open-source module to implement multihop bypass in Networks-on-Chip

Abstract

The integration of many processing elements per die makes it more difficult to provide low latency in the Network-on-Chip (NoC). Multihop bypass proposals, such as SMART, attack this problem by allowing flits to skip multiple routers in the path in a single cycle, drastically reducing latency while preserving a regular tiled layout. However, multihop bypass routers are more complex and relatively different from traditional NoC routers, since they rely on global broadcast signals and global allocation mechanisms. Additionally, the maximum number of nodes that can be bypassed within a single cycle is limited by the Critical Path Delay (CPD) of the NoC. Hence, a practical multihop bypass mechanism must also minimize this delay.

To simplify the design of multihop bypass mechanisms, this work introduces PlugSMART, an open-source pluggable Verilog module that extends a traditional router to support multihop bypass. PlugSMART follows a black box approach, requiring minimal modifications from the original router. As an application of PlugSMART, we introduce ProSMART, a multihop bypass extension of the efficient NoC router ProNoC. ProSMART is evaluated using simulations, FPGA, and ASIC synthesis. Results show that it is more performant and requires significantly fewer resources than previous open-source designs. The comparison with OpenSMART++, the most recent state-of-the-art SMART-based NoC, shows up to a 50% reduction in both area and CPD. Overall, PlugSMART constitutes a simple alternative for fast and efficient upgrading of existing NoC routers, allowing to implement multihop bypass and significantly improve performance while preserving the original characteristics of the router design.