Critical Packet Prioritisation by Slack-Aware Re-routing in On-Chip Networks

Packet based Network-on-Chip (NoC) connect tens to hundreds of components in a multi-core system. The routing and arbitration policies employed in traditional NoCs treat all application packets equally. However, some packets are critical as they stall application execution whereas others are not. We differentiate packets based on a metric called slack that captures a packet's criticality. We observe that majority of NoC packets generated by standard application based benchmarks do not have slack and hence are critical. Prioritising these critical packets during routing and arbitration will reduce application stall and improve performance. We study the diversity and interference of packets to propose a policy that prioritises critical packets in NoC. This paper presents a slack-aware re-routing (SAR) technique that prioritises lower slack packets over higher slack packets and explores alternate minimal path when two no-slack packets compete for same output port. Experimental evaluation on a 64-core Tiled Chip Multi-Processor (TCMP) with 8x8 2D mesh NoC using both multiprogrammed and multithreaded workloads show that our proposed policy reduces application stall time by upto 22% over traditional round-robin policy and 18% over state-of-the-art slack-aware policy.