Achieving lightweight multicast in asynchronous NoCs using a continuous-time multi-way read buffer

Multicast communication (1-to-many) is common in parallel architectures and emerging areas such as neuromorphic computing. However, there is very limited research in supporting multicast in asynchronous NoCs. This paper proposes a new parallel multicast asynchronous NoC with 2D-mesh topology. To the best of our knowledge, this is the first general-purpose asynchronous NoC to support multicast in 2D meshes. A critical feature of this NoC is the use of a new continuous-time replication strategy, where the flits of a multicast packet are routed through the distinct outputs of the router according to each output's own rate, in parallel and in continuous time. This unique asynchronous continuous-time replication, not discretized to clock cycles, can handle subtle variations in network congestion and exploit “sub-cycle” differentials in operating speeds. A new continuous-time multi-way read (CMR) buffer is proposed to enable this replication strategy. Only a single CMR buffer is used per input port, with multiple independent read pointers, which are accessed by different outputs. For diverse multicast benchmarks, the new parallel multicast network achieved significant latency and throughput gains over a serial baseline. Moderate energy overhead was seen for one benchmark with a small multicast portion, but major reductions were achieved for higher amounts of multicast. Interestingly, consistent latency improvements were observed for unicast, in spite of the extra instrumentation. Experiments on isolated multicast packet transmissions also showed over an order-of-magnitude improvement in delivery time.