In the Heat of Conflict: On the Synchronisation of Critical Sections

Advances in semiconductor technology greatly extend the scope of special-purpose applications as multi-core processors find the way into embedded systems. The increasing number of processor cores makes it more important than ever to have real-time operating systems process parallel threads in the most efficient way. In doing so, they have to pursue multiple (often conflicting) goals: namely being predictable as to time and energy demand. In shared-memory multi-core systems, contention at critical sections makes it inevitable for the operating system to execute competing threads with highly efficient synchronisation methods. Related research has primarily focussed on timing aspects of synchronisation methods, while the energy efficiency of the latter is an unexplored field, yet. In this paper, we implement and evaluate five distinct synchronisation methods and analyse their run-time characteristics (i.e. time, energy) in-depth. We evaluate the overall demand at application level, and empirically prove that contention increases the energy demand significantly even when competing processes are temporarily suspended. Furthermore, the evaluation reveals that choosing the right synchronisation method can decrease the energy demand by more than a factor of 5. We come to the conclusion that it is mandatory to consider the effects of process synchronisation for energy analysis and energy-efficiency optimisations.