Transparent Synchronization Protocols for Compositional Real-Time Systems

Hierarchical scheduling frameworks (HSFs) provide means for composing complex real-time systems from well-defined, independently analyzed components. To support resource sharing in two-level HSFs, three synchronization protocols based on the stack resource policy (SRP) have recently been presented for single-processor execution platforms, i.e., HSRP, SIRAP, and BROE. This paper presents a transparent implementation of these three protocols side-by-side in an HSF-enabled real-time operating system. Transparent synchronization interfaces make it possible to select a protocol during integration time based on its relative strengths. A timing interface describes the required budget to execute a component on a shared platform and an accessor´s maximum critical-section execution time to global shared resources. These resources are arbitrated based on the available budget of the accessing task. We enable this explicit synchronization of virtual time with global time by means of a novel virtual-timer mechanism. Moreover, we investigate system overheads caused by each synchronization protocol, so that these can be included in the system analysis. Based on the analytical and implementation overheads of each protocol, we present guidelines for the selection of a synchronization protocol during system integration. Finally, we show that unknown task-arrival times considerably complicate an efficient implementation of SIRAP´s self-suspension mechanism. We briefly discuss the implementation complexity caused by these arrivals for bandwidth-preserving servers, e.g., deferrable servers and BROE.