History-Based Arbitration for Fairness in Processor-Interconnect of NUMA Servers

Abstract

NUMA (non-uniform memory access) servers are commonly used in high-performance computing and datacenters. Within each server, a processor-interconnect (e.g., Intel QPI, AMD HyperTransport) is used to communicate between the different sockets or nodes. In this work, we explore the impact of the processor-interconnect on overall performance -- in particular, the performance un- fairness caused by processor-interconnect arbitration. It is well known that locally-fair arbitration does not guarantee globally-fair bandwidth sharing as closer nodes receive more bandwidth in a multi-hop network. However, this work demonstrates that the opposite can occur in a commodity NUMA server where remote nodes receive higher bandwidth (and perform better). We analyze this problem and iden- tify that this occurs because of external concentration used in router micro-architectures for processor-interconnects without globally-aware arbitration. While accessing remote memory can occur in any NUMA system, performance un- fairness (or performance variation) is more critical in cloud computing and virtual machines with shared resources. We demonstrate how this unfairness creates significant performance variation when a workload is executed on the Xen virtualization platform. We then provide analysis using synthetic workloads to better understand the source of unfair- ness and eliminate the impact of other shared resources, including the shared last-level cache and main memory. To provide fairness, we propose a novel, history-based arbitration that tracks the history of arbitration grants made in the previous history window. A weighted arbitration is done based on the history to provide global fairness. Through simulations, we show our proposed history-based arbitration can provide global fairness and minimize the processor- interconnect performance unfairness at low cost.