Challenging the Two-Core Assumption: Deterministic Single-Core Zephyr Virtualization with PCIe NIC Passthrough
Industrial edge platforms increasingly consolidate real-time control and general-purpose workloads on a single system-on-chip (SoC) to reduce costs, power, and complexity. Conventional real-time virtual machine (RTVM) setups, however, typically rely on PREEMPT_RT Linux and commonly reserve at least two CPU cores to isolate real-time tasks from housekeeping, which hinders scalability on resource-constrained edge platforms. This paper investigates whether low-latency Ethernet networking with tight observed tail latency can be sustained in a single-core RTVM configuration. We implement a single-core Zephyr-based RTVM on ACRN and compare it with a single-core PREEMPT_RT Linux RTVM under the same VM topology, using the same directly assigned Intel i226-LM PCIe Ethernet NIC via passthrough and an identical UDP echo workload. Latency measurements across 30 million packets at a traffic rate of 8 thousand packets per second (8 kpps) characterize both averagecase and extreme-tail behavior. PREEMPT_RT Linux shows severe tail amplification even without interference (99.999th percentile: $\mathbf{2 1 6 3} \boldsymbol{\mu} \mathbf{s}$; max: $\mathbf{6 8 7 2} \boldsymbol{\mu} \mathbf{s})$, while Zephyr maintains a tightly concentrated latency distribution (99.999th percentile: $79 \mu \mathrm{s}$; max: $82 \mu \mathrm{s})$. Under full-system noisy-neighbor load, Zephyr preserves sub- $\mathbf{1 0 0}-\boldsymbol{\mu} \mathbf{s}$ observed tail latency, whereas Linux degrades further. These empirical findings provide evidence that a specialized RTOS-based RTVM can sustain tight tail-latency performance and low delay variation on a single core, challenging the conventional two-core provisioning strategy for real-time edge systems.