Augmented CPU Reservations: Towards Predictable Execution on General-Purpose Operating Systems

One problem with performing soft real-time computations on general-purpose operating systems is that these OSs may spend significant amounts of time in the kernel instead of performing work on behalf of the application that is nominally scheduled: the OS effectively steals time from the running application. Stolen time can be a significant obstacle to predictable program execution on real-time versions of Linux and Windows 2000, where it can cause applications to miss essentially all of their deadlines. We propose augmented CPU reservations, a novel mechanism for using fine-grained accounting information about the amount of stolen time to help the scheduler allow applications to meet their deadlines. We have designed and implemented Rez-C and Rez-FB, two schedulers that provide augmented reservations, and we have tested them in Windows 2000, showing that they can increase the predictability of CPU reservations. We also experimentally quantify the severity of stolen time caused by a variety of devices such as hard disk controllers, a network interface, and a software modem under real-time versions of Windows 2000 and Linux.