Tail Contagion: Sub-microsecond Time Protection in Shared Software Network Datapaths

TL;DR

Virtuoso introduces a time-based protection mechanism for shared software datapaths, achieving strong tail latency isolation with low overhead and microsecond-scale latency in datacenter networks.

Contribution

It presents Virtuoso, a novel time protection approach that enforces per-tenant CPU-time budgets to improve tail latency isolation without preemption.

Findings

7.8X reduction in victim tail latency under interference

3X efficiency improvement over siloed datapaths

Maintains throughput within 5% of unmodified TAS

Abstract

Shared software datapaths underpin modern datacentre networking. They implement mechanisms such as virtual switching, network virtualisation tunneling, or reliable transport, and enforce policies, such as tenant rate limits, virtual network isolation, or congestion control. However, because multiple applications, containers, or VMs share them, often across tenants, they pose a tail latency isolation challenge. Current isolation approaches either sacrifice efficiency via coarse-grained core partitioning or provide weak tail latency isolation when sharing cores with basic rate limits. This paper presents Virtuoso, a time protection mechanism for shared software datapaths that provides strong cross-tenant tail latency isolation while preserving low overhead and microsecond-scale latency. Our key insight is that tail latency is fundamentally a time metric, so byte or packet throughput is the wrong metric for controlling interference when packet processing costs vary. Our design instead enforces isolation through per-tenant CPU-time budgets at datapath intervention points within run-to-completion loops, without relying on preemption. In a case study, we instantiate Virtuoso in the TAS TCP stack and demonstrate a 7.8X reduction in victim tail latency under adversarial interference while keeping throughput within 5% of unmodified TAS. We also observe a 3X per-core efficiency improvement compared to siloed datapaths under bursty workloads.