Aequitas: admission control for performance-critical RPCs in datacenters

With the increasing popularity of disaggregated storage and microservice architectures, high fan-out and fan-in Remote Procedure Calls (RPCs) now generate most of the traffic in modern datacenters. While the network plays a crucial role in RPC performance, traditional traffic classification categories cannot sufficiently capture their importance due to wide variations in RPC characteristics. As a result, meeting service-level objectives (SLOs), especially for performance-critical (PC) RPCs, remains challenging. We present Aequitas, a distributed sender-driven admission control scheme that uses commodity Weighted-Fair Queuing (WFQ) to guarantee RPC-level SLOs. In the presence of network overloads, it enforces cluster-wide RPC latency SLOs by limiting the amount of traffic admitted into any given QoS and downgrading the rest. We show analytically and empirically that this simple scheme works well. When the network demand spikes beyond provisioned capacity, Aequitas achieves a latency SLO that is 3.8× lower than the state-of-art congestion control at the 99.9th-p and admits up to 2× more PC RPCs meeting SLO when compared with pFabric, Qjump, D3, PDQ, and Homa. Results in our fleetwide production deployment show a 10% latency improvement.