The Impact of CPU Voltage Margins on Power-Constrained Execution

CPUs typically operate at a voltage which is higher than what is strictly required, using voltage margins to account for process variability and anticipate any combination of adverse operating conditions. However, these worst-case scenarios occur rarely, if ever, thus the operating voltage is overly pessimistic resulting in excessive power dissipation which leads to decreased performance under power capping. In this paper, we investigate the impact of reducing voltage margins beyond the nominal level on the efficiency of CPU power capping mechanisms, for three commercial systems, two Applied Micro ARMv8 micro-servers (X-Gene2 and X-Gene3) and an Intel x86-64 (Xeon E3). We show that CPU power capping at reduced voltage margins compared with Intel's RAPL and Dynamic Frequency Scaling (DFS) mechanisms results in performance improvement by up to 64 and 24 percent on average, respectively. In combination with state-of-the-art thread packing, the reduction of CPU voltage margins results in 36, 33 and 27 percent performance improvement compared with RAPL and DFS for the Xeon E3 and the X-Gene processors, respectively. Also, we validate the robustness of our approach with a set of long-running experiments and show that significant energy gains can be achieved even when considering the cost of checkpointing and recovery in large-scale systems.