Predictable and configurable component-based scheduling in the Composite OS
ACM Transactions on Embedded Computing Systems (TECS) - Special Section on ESTIMedia'10
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This paper focuses on a technique to empower commercial-off-the-shelf (COTS) systems with an execution environment, and corresponding services, to support realtime and embedded applications. By leveraging COTS systems, we are able to reduce the potentially expensive maintenance and development costs of proprietary solutions. We describe a system called "Hijack" that enables user-level services to take control of features such as CPU scheduling, interrupt handling and synchronization. In contrast to other approaches that support real-time tasks within the kernel of commodity systems such as Linux, Hijack provides the basis for predictable thread execution at user-level. No changes to the kernel source code are required to support this approach. Instead, Hijack works by using a combination of kernel module support and an interposed execution environment between traditional process address spaces and the kernel. This technique enables system calls and hardware interrupts to be intercepted with bounded latencies via the kernel module, that passes control to a user-level real-time executive. From within the executive, system-wide services and policies can be deployed to over-ride certain features of the underlying kernel, while still leveraging base kernel services where appropriate. Using this technique, we show how a vanilla Linux system can be hijacked to support predictable service execution using a series of user-defined policies. In particular, we show how to deliver and process asynchronous events with bounded latency, using interposition agents within a Hijack execution environment. Results show that for real-time streaming applications, Hijack is able to receive and process packets with significantly lower loss rates and jitter compared to using alternative application-level processes for the same task.