Keep it slow and in time: Online DVFS with hard real-time workloads

To handle hot spots or power shortages, modern multicore processors are equipped with a supervisory dynamic thermal and power management (DTPM) system. When necessary, the DTPM system autonomously adapts the capacity of the cooling system or throttles the speed of core-local clocks via dynamic voltage and frequency scaling (DVFS) techniques. Opposed to best-effort scenarios, online DVFS with real-time workloads also needs to consider completion times of computations. Whereas execution times can be bounded adequately with worst-case estimates, arrival times of computation requests are potentially unknown. A deadline for completing a computation can easily be missed, if workloads suddenly peak and past clock speed assignments have built-up a non-negligible backlog of computations. To overcome this problem, we introduce an online DVFS management scheme which is history-aware. It operates a core at higher speed levels only if the future workload has the potential to result in timing violations, if not anticipated by rising clock speed assignments. We present an implementation of the scheme running on the Gem5 hardware simulator.