CPS-Xen: A Virtual Execution Environment for Cyber-Physical Applications

Abstract

The range of applications for virtualization technology grows continually. The possibility of workload consolidation, the facilitated system administration, the fault-tolerance properties and cost reduction are what renders this technique so interesting. Thus, it stands to reason to expand its field of application to the domain of Cyber-Physical Systems (CPSs). Unfortunately, the integration of multiple CPS on a single server by means of virtualization is not a straightforward task. In this domain, real-time constraints of critical tasks have to be satisfied, in order to avoid damage or even a catastrophe, and virtualization was initially not designed to cope with such requirements. In this article we present CPS-Xen, a platform for executing virtualized safety-critical CPS applications. CPS-Xen is based upon the Xen-Hypervisor - a popular open-source Virtual Machine Monitor (VMM). We extend the VMM with a real-time scheduler implementing the rate-monotonic (RM) scheduling policy and show that optimizing the VMM-scheduler alone is not enough, as the I/O-scheduling introduces delays and priority inversion in the scheduling of the VMs. In order to solve this issue, we propose an architecture that synergizes the work of both schedulers. Finally, throughout an extensive set of experiments the proposed architecture is shown to fulfill – even under high CPU load and up to 36 concurrent VMs – the hard real-time requirements of CPS applications.

This work has been carried out in the course of research unit 1511 Protection and control systems for reliable and secure operations of electrical transmission systems funded by the German Research Foundation (DFG). The authors would like to thank the DFG for funding and all the project partners for the helpful discussions.