Utility Accrual Scheduling of Distributable Threads: The Tempus Approach

Dynamic, adaptive, real-time embedded control systems are present at any level(s) of an enterprise— e.g., devices in the defense domain such as multi-mode phased array radars and battle management. These embedded systems often include“soft”as well as“hard” time constraints. Jensen’s time/utility functions [4] (or TUFs) allow the semantics of soft time constraints to be precisely specified. A TUF specifies the utility to the system, resulting from the completion of an activity, as a function of the its completion time. Figure 1 shows examples of TUF time constraints. TUFs have been successfully used in two significant, real-time applications, including an AWACS (Airborne WArning and Control System) surveillance mode tracker system built by MITRE and Open Group, and a coastal air defense system built by CMU and General Dynamics. When timing constraints are expressed with TUFs, the scheduling optimality criteria are based on factors in terms of maximizing accrued utility from those activities—e.g., maximizing the sum, or the expected sum, of the activities’ attained utilities. Such criteria are called Utility Accrual (or UA) criteria, and sequencing (scheduling, dispatching) algorithms that consider UA criteria are called UA sequencing algorithms. Motivated by the need for dynamic scheduling, e.g., UA scheduling, in real-time distributed systems, the Object Management Group recently adopted the Real-Time CORBA 2.0 (Dynamic Scheduling) standard [7] (abbreviated here as RTC2). RTC2 specifies distributable threads (or DT s) as a programming and scheduling abstraction for system-wide, end-to-end scheduling in real-time distributed systems. The distributable thread model is a subset of the distributed thread model that was created in Jensen’s Archons Project [3]. A DT is a single thread of execution with a globally unique identifier that transparently extends and retracts through an arbitrary number of local and remote objects. Concurrency is at the DT-level.