Introduction to partial time composability for COTS multicores

In singlecore processors timing analysis involves a step of Execution Time Analysis at task level that yields an Execution Time Bound (ETB) for the task, and one of schedulability analysis, where the scheduling attributes of the individual tasks, including the ETB, are studied from a system level perspective. Response Time Analysis serves as a compositional bridge between those two steps by accounting for the contention that arises from resource sharing. In this paper, we show that the advent of multicore processors challenges the viability of this two-step approach. This stems from the fact that inter-task interference effects in a multicore are much more intricate in nature than what can be compositionally captured in response time analysis by widening the tasks' ETB with the time intervals during which tasks cannot progress while actually holding the CPU; we also show how contention in the access to hardware shared resources creates a circular dependence between the task ETB and its actual scheduling at run time. Finally, we show how various degrees of time composability can help breaking this knot.