Quantifying the Flexibility of Real-Time Systems

International audienceIn this paper we define the flexibility of a system as its capability to schedule a new task. We present an approach to quantify the flexibility of a system. More importantly, we show that it is possible under certain conditions to identify the task that will directly induce the limitations on a possible software update. If performed at design time, such a result can be used to adjust the system design by giving more slack to the limiting task. We illustrate how these results apply to a simple system

Incorporating scenarios and heuristics to improve flexibility in real-time embedded systems

Flexibility, the ability to adapt to change, is important for real-time systems. As in any type of system, changes arise from maintenance, enhancements and upgrades. These changes are only feasible if timing requirements imposed by the real-time nature of the system can still be met. A flexible design will allow tasks to be added without impinging on other tasks, causing them to miss deadlines. The design space for these systems consists of many configurations describing how tasks and messages are allocated to hardware and scheduled on a hardware platform. Heuristic search is a well recognised strategy for solving allocation and scheduling problems but most research is limited to finding any valid solution for a current set of requirements. The technique proposed here incorporates scenario based analysis into heuristic search strategies where the ability of a solution to meet a scenario is included as another heuristic for the changeability of a system. This allows future requirements to be taken into account when choosing a solution so that future changes can be accommodated with minimal alterations to the existing system

A Review of Priority Assignment in Real-Time Systems

It is over 40 years since the first seminal work on priority assignment for real-time systems using fixed priority scheduling. Since then, huge progress has been made in the field of real-time scheduling with more complex models and schedulability analysis techniques developed to better represent and analyse real systems. This tutorial style review provides an in-depth assessment of priority assignment techniques for hard real-time systems scheduled using fixed priorities. It examines the role and importance of priority in fixed priority scheduling in all of its guises, including: preemptive and non-pre-emptive scheduling; covering single- and multi-processor systems, and networks. A categorisation of optimal priority assignment techniques is given, along with the conditions on their applicability. We examine the extension of these techniques via sensitivity analysis to form robust priority assignment policies that can be used even when there is only partial information available about the system. The review covers priority assignment in a wide variety of settings including: mixed-criticality systems, systems with deferred pre-emption, and probabilistic real-time systems with worstcase execution times described by random variables. It concludes with a discussion of open problems in the area of priority assignment