P u b l i s h e d b y t h e I E E E C o m p u t e r S o c i e t y• circuit techniques such as disabling the clock signal to a processor's unused parts; • architectural techniques such as replacing complex uniprocessors with multiple simple cores; and • support for multiple low-power states in processors, memory, and disks.At the system level, the latter approach requires policies to intelligently exploit these low-power states for energy savings. Across multiple systems in a cluster or data center, these policies can involve dynamically adapting workload placement or power provisioning to meet specific energy or thermal goals. To facilitate these optimizations, we need metrics to define energy efficiency, which will help designers compare designs and identify promising energy-efficient technologies. We also need models to predict the effects of dynamic power management policies, particularly over many systems. Unlike the significant body of work on power management and optimization, there has been relatively little focus on metrics and models.We address the challenges in defining metrics for energy efficiency with a specific case study on JouleSort, which provides a complete, full-system benchmark for energy efficiency across a variety of system classes. Power consumption and energy efficiency are important factors in the initial design and day-to-day management of computer systems. Researchers and system designers need benchmarks that characterize energy efficiency to evaluate systems and identify promising new technologies.To predict the effects of new designs and configurations, they also need accurate methods of modeling power consumption.