Overrun and Skipping in Hierarchically Scheduled Real-Time Systems

Recently, two SRP-based synchronization protocols for hierarchically scheduled real-time systems based on fixed priority preemptive scheduling (FPPS) have been presented, i.e., HSRP and SIRAP. Preventing depletion of budget during global resource access, the former implements an overrun mechanism, while the later exploits a skipping mechanism. A theoretical comparison of the performance of these mechanisms revealed that none of them was superior to the other, as their performance is heavily dependent on the system´s parameters. To better understand the relative strengths and weaknesses of these mechanisms, this paper presents a comparative evaluation of the depletion prevention mechanisms overrun (with or without payback) and skipping. These mechanisms are investigated in detail and the corresponding system load imposed by these mechanisms is explored in a simulation study. The mechanisms are evaluated assuming FPPS and a periodic resource model. The periodic resource model is selected as it supports locality of schedulability analysis, allowing for a truthful comparison of the mechanisms. Given system characteristics, guiding the design of hierarchically scheduled real-time systems, the results of this paper indicate when one mechanism is better than the other and how a system should be configured in order to operate efficiently.