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''Energy proportionality'' is a measure of the relationship between power consumed in a computer system, and the rate at which useful work is done (its utilization, which is one measure of performance). If the overall power consumption is a linear function of the computer's utilization, then the machine is said to be energy proportional.〔L. A. Barroso and U. Hölzle, “The Case for Energy-Proportional Computing,” Computer, vol. 40, no. 12, pp. 33–37, Dec. 2007. (). Available: http://www.computer.org/csdl/mags/co/2007/12/mco2007120033.html〕 Equivalently stated, for an idealized energy proportional computer, the overall energy per operation (a measure of energy efficiency) is constant for all possible workloads and operating conditions. The concept was first proposed in 2007 by Google engineers Luiz André Barroso and Urs Hölzle, who urged computer architects to design servers that would be much more energy efficient for the datacenter setting.〔 Energy proportional computing is currently an area of active research, and has been highlighted as an important design goal for cloud computing.〔M. Armbrust, I. Stoica, M. Zaharia, A. Fox, R. Griffith, A. D. Joseph, R. Katz, A. Konwinski, G. Lee, D. Patterson, and A. Rabkin, “A view of cloud computing,” Communications of the ACM, vol. 53, no. 4, p. 50, Apr. 2010. (). Available: http://dx.doi.org/10.1145/1721654.1721672〕 There are many technical challenges remaining in the design of energy proportional computers. Furthermore, the concept of energy proportionality is not inherently restricted to computing. Although countless energy efficiency advances have been made in non-computing disciplines, they have not been evaluated rigorously in terms of their energy proportionality. ==Background in energy sustainability== Sustainable energy is the ideal that society should serve its energy needs without negatively impacting future generations, and which various organizations, governments, and individuals have been advocating. To meet this ideal, efficiency improvements are required in three aspects of the energy ecosystem: * ''Energy generation'' * ''Energy storage'' * ''Energy consumption'' Since our need for energy generation and storage are driven by our demand, more efficient ways of consuming energy can drive large improvements in energy sustainability. Efforts in sustainable energy consumption can be classified at a high level by the three following categories: * ''Recycle'': Capture and recover wasted energy to do more work, that would otherwise be lost as heat. * ''Reuse'': Amortize the cost of energy generation, storage, and delivery by sharing energy and its infrastructure among different loads. * ''Reduce'': Reduce demand for energy by doing more work with less energy (improve consumption efficiency), or not doing the work at all by changing behavior. Many efforts in making energy consumption more sustainable are focused on the "reduce" theme for unpredictable and dynamic workloads (which are commonly encountered in computing). This can be considered as power management. These efforts can be lumped into two general approaches, which are not specific to computing, but commonly applied in that domain: * ''Idle power-down'': This technique exploits gaps in workload demand to shut off components that are idle. When shut down, components cannot do any useful work. The problems unique to this approach are: (1) it costs time and energy to transition between active and idle power-down states, (2) no work can be done in the off state, so power-up must be done to handle a request, and (3) predicting idle periods and adapting appropriately by choosing the right power state at any moment is difficult. * ''Active performance scaling'': Unlike idle-power down, this approach allows work to be done in any state, all of which are considered active, but with different power/performance tradeoffs. Usually, slower modes consume less power. The problems unique to this approach are: (1) it is difficult to determine which combination of states is the most energy efficient for an application, and (2) the energy efficiency improvements are usually not as lucrative as those from idle power-down modes. In practice, both types of approaches are used commonly and mixed together. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Energy proportional computing」の詳細全文を読む スポンサード リンク
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