Don't be fat: Towards efficient online flow scheduling in data center networks

Abstract

Flow scheduling is one of the primary issues in data centers. The efficiency of flow scheduling affects the user experience significantly, since the service latency is determined by the flow completion time (FCT). Current transport protocols are based on the Processor Sharing (PS) policy by dividing the link bandwidth equally. As short flows may be blocked by long flows under PS policy, these protocols could not meet latency requirements. The Shortest Remaining Processing Time (SRPT) policy provides a near-optimal solution in term of reducing average FCT. However, this flow scheduling policy may cause long flows suffering unfair delays. In this paper, we propose MERP (Minimizing Expansion Ratio Protocol) for flow scheduling, which aims to navigate the tradeoff between fairness and the average FCT. We propose expansion ratio as a new metric to describe the gap between reality and ideal in terms of acquired link resources by a flow. Larger expansion ratio indicates that the flow obtains fewer bottleneck link resources. We strive to make the flow scheduling relatively fair by reducing the largest expansion ratio of all the flows. And we further demonstrate that reducing expansion ratio is more reasonable than reducing average FCT for services with the Partition/Aggregate pattern. And to meet the scalability requirements, we implement distributed MERP through flow preemption and explicit rate control. The experiments indicate that MERP could significantly reduce the completion time for 99.9th percentile flows under high load.