The Last-Level-Cache Interference in Guest Performance: a Case-Study with Zephyr OS

Embedded systems are increasingly relying on virtual machines (VMs) to ensure portability and composability of services. To achieve high-performance and resources' isolation, the VM can be mapped to a dedicated core. In shared-memory multi/many cores architectures, the last level cache (LLC) is shared among cores. The state-of-the-art shows that interference on LLC can be a bottleneck for VM's performance. Such interference can depend on several factors, including the design of the applications, guest and host OSes, the hypervisor, and the architecture. Therefore, it is expected that studies analyze in-depth each of these factors. The goal of this work is focusing on the interference that cannot be mitigated by cache isolation techniques supported by the hypervisor, for instance, those caused by host OS applications. Such interference is unpredictable and can jeopardize the performance of the guest. The contribution is a new perspective about how cache interference affects the latency of the guest application and guest OS, crossing different performance metrics in comprehensive plots. We run our experiments on Arm Cortex-A53 processor, and, as guest OS, we employ the state-of-the-art Zephyr OS. Thus, a side contribution is to show Zephyr performance facing cache interference. Our results show that interference on LLC can affect in up to +8 × to slow down the guest application and up to +2.8 ×the subset of kernel functions which are involved for the application execution. Results also show that the guest is mostly affected when its data can fit on the LLC: after this point, the LLC saturation occurs and the host interference becomes insignificant from a guest perspective.