Systems with Dynamic Real-Time Guarantees in Uncertain and Faulty Execution Environments

In many practical real-time systems, the physical environment and the system platform can impose uncertain execution behaviour to the system. For example, if transient faults are detected, the execution time of a task instance can be increased due to recovery operations. Such fault recovery routines make the system very vulnerable with respect to meeting hard real-time deadlines. In theory and in practical systems, this problem is often handled by aborting not so important tasks to guarantee the response time of the more important tasks. However, for most systems such faults occur rarely and the results of not so important tasks might still be useful, even if they are a bit late. This implicates to not abort these not so important tasks but keep them running even if faults occur, provided that the more important tasks still meet their hard real time properties. In this paper, we present Systems with Dynamic Real-Time Guarantees to model this behaviour and determine if the system can provide full timing guarantees or limited timing guarantees without any online adaptation after a fault occurred. We present a schedulability test, provide an algorithm for optimal priority assignment, determine the maximum interval length until the system will again provide full timing guarantees and explain how we can monitor the system state online. The approaches presented in this paper can also be applied to mixed criticality systems with dual criticality levels.