SSP: Speeding up Small Flows for Proactive Transport in Datacenters

Proactive transports nowadays have drawn much attention because of fast convergence, near-zero queueing and low latency. Proactive protocols, however, need an extra RTT to allocate ideal sending rate for new flows. To solve this, some studies, such as pHost, Homa, send unscheduled packets with line rate in the first RTT, which will causes severe network congestion. To avoid the queue buildup, Aeolus directly drops unscheduled packets when congestion occurs. Nevertheless, based on our experiment, a considerable part of small flows (0-100 KB) will be completed in the first RTT under 100 Gbps network, so that dropping unscheduled packets will severely affect performance of the small flows. In this paper we propose SSP, a new scheme aimed to eliminate the extra RTT delay and improve the flow completion time (FCT) of small flows under the proactive mechanism. Like pHost and Homa, SSP sends unscheduled packets at line rate when new flow arrives. Different from Aeolus, SSP selectively drops scheduled packets once queue buildup happens in the switch, thus protecting unscheduled packets which are more likely belong to small flows. Besides, based on the short-job-first (SJF) principle, we give relative higher priorities for small flows at the sender. Our simulation results with realistic workloads show that SSP can improve the FCT of small flows significantly. Specifically, under Web Search workload, SSP facilitates nearly 63% of 0-100 KB flows to complete one RTT faster. Also, SSP reduces the tail FCT by 56.8% at the 99th percentile compared with Expresspass and 29.2% compared with Aeolus while not leads to large queue buildup.