Supporting efficient collective communication in NoCs

Across many architectures and parallel programming paradigms, collective communication plays a key role in performance and correctness. Hardware support is necessary to prevent important collective communication from becoming a system bottleneck. Support for multicast communication in Networks-on-Chip (NoCs) has achieved substantial throughput improvements and power savings. In this paper, we explore support for reduction or many-to-one communication operations. As a case study, we focus on acknowledgement messages (ACK) that must be collected in a directory protocol before a cache line may be upgraded to or installed in the modified state. This paper makes two primary contributions: an efficient framework to support the reduction of ACK packets and a novel Balanced, Adaptive Multicast (BAM) routing algorithm. The proposed message combination framework complements several multicast algorithms. By combining ACK packets during transmission, this framework not only reduces packet latency by 14.1% for low-to-medium network loads, but also improves the network saturation throughput by 9.6% with little overhead. The balanced buffer resource configuration of BAM improves the saturation throughput by an additional 13.8%. For the PARSEC benchmarks, our design offers an average speedup of 12.7% and a maximal speedup of 16.8%.