Cooperative scheduling of multiple resources

Abstract

Obtaining simultaneous and timely access to multiple resources is known to be an NP-complete problem. Complete resource decoupling is, therefore, often used for managing end-to-end delays in distributed real-time system where each processor is scheduled independent of the others. This decoupling approach unfortunately fails when multiple resources must be managed within a single node. Resources such as disk bandwidth and network bandwidth are available on a single node but must be managed by their host processor by means of device drivers, filesystem or protocol services. The host processor acting as a controlling resource, therefore, must play multiple roles. One, it is used by applications on that node. Two, it is used to control and manage other (time-shared) controlled resources including disk bandwidth and network bandwidth. These two roles, unfortunately can often be at odds with one another. In this paper we investigate the problem of co-scheduling controlling and controlled resources. We propose the use of a Cooperative Scheduling Server (CS S), which is a dedicated server that manages one specific controlled resource (like disk bandwidth, network bandwidth, inter-process communication, etc.) while using a controlling resource (like the processor). Two core ideas underlie our approach. First, a single (aperiodic) server is created on a controlling resource (such as a CPU) to handle all local requests for a controlled resource (such as disk bandwidth). This implies that conjuctive admission control must be carried out on both the controlling and controlled resources. Secondly, timing constraints at the application level are partitioned into multiple stages, each of which will be guaranteed to complete on a particular resource. RTFS is a real-time filesystem that provides disk bandwidth guarantees under light CPU loads. With a cooperative scheduling server (FS-CSS) for this disk-based filesystem, disk bandwidth guarantees can be obtained under both heavy CPU and disk workloads. We describe the design and implementation of FS-CSS for providing disk bandwidth guarantees. We conclude with a detailed performance evaluation of FS-CSS.