Feedback - Based Scheduling : a Toolbox

1 Background and Introduction Fine-grain scheduling based on software feedback 3] was introduced in the Synthesis operating system 4, 2] to solve two problems: the dependency between jobs in a pipeline and the low-latency requirements of multimedia type applications. The performance level achieved and the adaptive-ness of applications running on Synthesis demonstrated the success of ne-grain scheduling based on software feedback. However, the Synthesis implementation of software feedback is specialized for that particular architecture and a particular application (pipelined process scheduling). Consequently , despite the proven success of ne-grain scheduling, it is not easy to port it to another operating system or to apply its lessons elsewhere, even within Synthesis. To address the problems of portability and extensibility of software feedback scheduling mechanisms , we have taken a toolbox approach in our current research. Instead of creating a specialized solution for each particular scheduling problem, we are developing a toolbox of standard, relatively simple components with well-deened performance and functionality characteristics. The goal is the ability to quickly implement sophisticated software feedback mechanisms by composing these basic toolbox components. The intended applications are primarily in the adaptive scheduling needed in multimedia and real-time domains, especially when input/output operations introduce a large variance in job completion time. The rest of this position paper is organized as follows. Section 2 contains a summary of the basic concepts of software feedback. Section 3 then describes a concrete example using our toolbox approach. Section 5 concludes the paper by qualitatively evaluating this approach and outlining our current research eeorts. By software feedback mechanism we mean a software implementation of feedback mechanisms similar to phase-locked loops used in control systems. An example of phase-locked loop is the FM radio receiver, which uses feedback to \track" frequency modulations in the radio broadcast. The goal of a feedback system is to adjust an output sequence according to the observation of input, essentially