Near-Ideal Networks-on-Chip for Servers

Abstract

Server workloads benefit from execution on many-core processors due to their massive request-level parallelism. A key characteristic of server workloads is the large instruction footprints. While a shared last-level cache (LLC) captures the footprints, it necessitates a low-latency network-on-chip (NOC) to minimize the core stall time on accesses serviced by the LLC. As strict quality-of-service requirements preclude the use of lean cores in server processors, we observe that even state-of-the-art single-cycle multi-hop NOCs are far from ideal because they impose significant NOC-induced delays on the LLC access latency, and diminish performance. Most of the NOC delay is due to per-hop resource allocation. In this paper, we take advantage of proactive resource allocation (PRA) to eliminate per-hop resource allocation time in single-cycle multi-hop networks to reach a near-ideal network for servers. PRA is undertaken during (1) the time interval in which it is known that LLC has the requested data, but the data is not yet ready, and (2) the time interval in which a packet is stalled in a router because the required resources are dedicated to another packet. Through detailed evaluation targeting a 64-core processor and a set of server workloads, we show that our proposal improves system performance by 12% over the state-of-the-art single-cycle multi-hop mesh NOC.