A Step to Support Real-Time in Virtual Machine

Real-time is one of the unique requirements in embedded systems. In this paper, we perform a feasibility study on how to support real-time in an embedded virtual machine system. Firstly, we argue that the I/O model of the current virtual machine monitor like Xen is not suitable to support real-time applications because it lacks in predictability and it does not guarantee a deterministic I/O processing. We provide an alternative I/O model for virtualized embedded systems. Devices are categorized into four groups: dedicated, active, running, dynamic. Dedicated devices make a virtual machine simple because they do not need to be virtualized for isolation. However, dedication does not mean the performance isolation. Our experimental results with dedicated device show that traditional dedication cannot guarantee the timely responsiveness in heavy interrupt cases. Specifically, responsiveness of real-time OS degrades as interrupt load increases. Therefore, a proper interrupt control mechanism is required at virtual machine monitor level in order to support timely responsiveness. In addition, our result supports that (1) short and prioritized interrupt processing helps responsiveness in a virtual machine system; (2) smaller time quantum results in better responsiveness also.