WiP: Scheduling Multi-Threaded Tasks to Reduce Intra-Task Cache Contention

Research on hard real-time systems and their models has predominately focused upon single-threaded tasks. When multi-threaded tasks are introduced to the classical real-time model the individual threads are treated as distinct tasks, one for each thread. These artificial tasks share the deadline, period, and worst case execution time of their parent task. In the presence of instruction and data caches this model is overly pessimistic, failing to account for the execution time benefit of cache hits when multiple threads of execution share a memory address space. This work takes a new perspective on instruction caches. Treating the cache as a benefit to schedulability for a single task with m threads. To realize the “inter-thread cache benefit” a new scheduling algorithm and accompanying worst-case execution time (WCET) calculation method are proposed. The scheduling algorithm permits threads to execute across conflict free regions, and blocks those threads that would create an unnecessary cache conflict. The WCET bound is determined for the entire set of m threads, rather than treating each thread as a distinct task. Both the scheduler and WCET method rely on the calculation of conflict free regions which are found by a static analysis method that relies on no external information from the system designer. By virtue of this perspective the system’s total execution execution time is reduced and is reflected in a tighter WCET bound compared to the techniques applied to the classical model. Obtaining this tighter bound requires the integration of three typically independent areas: WCET, schedulability, and cacherelated preemption delay analysis.