Less is More: Trading a Little Bandwidth for Ultra-Low Latency in the Data Center

Traditional measures of network goodness - goodput, quality of service, fairness - are expressed in terms of bandwidth. Network latency has rarely been a primary concern because delivering the highest level of bandwidth essentially entails driving up latency - at the mean and, especially, at the tail. Recently, however, there has been renewed interest in latency as a primary metric for mainstream applications. In this paper, the authors present the HULL (High-bandwidth Ultra-Low Latency) architecture to balance two seemingly contradictory goals: near baseline fabric latency and high bandwidth utilization. HULL leaves 'Bandwidth headroom' using Phantom Queues that deliver congestion signals before network links are fully utilized and queues form at switches.