Ride, Raise, or Wake: A Theory of Light-Thread Placement on Multi-Tier Processors

Overview & Synopsis

hones and laptops now mix several kinds of cores, and each kind sits in its own voltage and clock domain. When a light thread arrives, should it run beside heavy threads on a fast cluster that is already awake, or wake a cluster of efficient cores? We answer with three costs. A thread can \emph{ride} an awake domain at the voltage other threads hold, \emph{raise} that voltage for everyone, or \emph{wake} a sleeping domain and pay its fixed power. From these costs follows a simple rule: ride while the thread's demand is below $r^*=Φ/(e_h-e_l)$, the fixed power of the cluster it would wake over the gap in energy per operation; otherwise wake the smallest tier that can serve it. The best tier is not monotone in demand: tiny threads ride high, medium threads wake a low tier, heavy threads go high again. Two common designs err on opposite sides of the rule: grouping a light thread with the heavy threads of its application overpays above the threshold, and an energy model without idle costs overpays below it. In a model with five tiers, grouping costs 80 to 145\% more energy than the rule, and more as tiers are added. Measurements on an Apple M6 anchor the parameters.