Cost-Efficient Overclocking in Immersion-Cooled Datacenters

Cloud providers typically use air-based solutions for cooling servers in datacenters.  However, increasing transistor counts and the end of Dennard scaling will result in chips with thermal design power that exceeds the capabilities of air cooling in the near future.  Consequently, providers have started to explore liquid cooling solutions (e.g., cold plates, immersion cooling) for the most power-hungry workloads.  By keeping the servers cooler, these new solutions enable providers to operate server components beyond the normal frequency range (i.e., overclocking them) all the time.  Still, providers must tradeoff the increase in performance via overclocking with its higher power draw and any component reliability implications.  In this paper, we argue that two-phase immersion cooling (2PIC) is the most promising technology, and build three prototype 2PIC tanks.  Given the benefits of 2PIC, we characterize the impact of overclocking on performance, power, and reliability.  Moreover, we propose several new scenarios for taking advantage of overclocking in cloud platforms, including oversubscribing servers and virtual machine (VM) auto-scaling. For the auto-scaling scenario, we build a system that leverages overclocking for either hiding the latency of VM creation or postponing the VM creations in the hopes of not needing them.  Using realistic cloud workloads running on a tank prototype, we show that overclocking can improve performance by 20%, increase VM packing density by 20%, and improve tail latency in auto-scaling scenarios by 54%.  The combination of 2PIC and overclocking can reduce platform cost by up to 13% compared to air cooling.