Efficient Control of Unscheduled Packets for Credit-based Proactive Transport

Proactive transport has been attractive in modern high-speed and shallow buffered datacenter networks. At its heart, the link capacity is proactively allocated as credit, and then following scheduled packets are triggered by active senders according to credits, providing (near) zero loss rate and extremely low latency. Despite being promising, when waiting for credits in the first RTT, called the “pre-credit phase a substantial amount of spare bandwidth is being underutilized. To bridge this gap, current practices send a Bandwidth-Delay-Product (BDP) worth of unscheduled packets with line rate in the pre-credit phase, however, degrading throughput or tail latency. One key insight is that they consider the spare bandwidth in the pre-credit phase as a fixed value but actually changes across time and space. In this paper, we present Schef, a novel switch-based bandwidth calculation mechanism to address the pre-credit phase challenge without introducing network congestion or throughput degradation. The main idea is to calculate the spare bandwidth and apply efficient control of unscheduled packets at switches. Schef is compatible with existing proactive transports. We integrate it into a representative proposal NDP and evaluate its performance through large-scale simulations. Compared with the state-of-art, Schef reduces the average and 99th flow completion time (FCT) of short flows by up to 23% and 31%, and achieves 13% higher goodput simultaneously.