Chapter 1. Overview Power management has been one of our focus points for improvements for Red Hat Enterprise Linux 7. Limiting the power used by computer systems is one of the most important aspects of green IT (environmentally friendly computing), a set of considerations that also encompasses the use of recyclable materials, the environmental impact of hardware production, and environmental awareness in the design and deployment of systems. In this document, we provide guidance and information regarding power management of your systems running Red Hat Enterprise Linux 7. 1.1. Importance of Power Management At the core of power management is an understanding of how to effectively optimize energy consumption of each system component. This entails studying the different tasks that your system performs, and configuring each component to ensure that its performance is just right for the job. The main motivator for power management is: reducing overall power consumption to save cost The proper use of power management results in: heat reduction for servers and computing centers reduced secondary costs, including cooling, space, cables, generators, and uninterruptible power supplies (UPS) extended battery life for laptops lower carbon dioxide output meeting government regulations or legal requirements regarding Green IT, for example Energy Star meeting company guidelines for new systems As a rule, lowering the power consumption of a specific component (or of the system as a whole) will lead to lower heat and naturally, performance. As such, you should thoroughly study and test the decrease in performance afforded by any configurations you make, especially for mission-critical systems. By studying the different tasks that your system performs, and configuring each component to ensure that its performance is just sufficient for the job, you can save energy, generate less heat, and optimize battery life for laptops. Many of the principles for analysis and tuning of a system in regard to power consumption are similar to those for performance tuning. To some degree, power management and performance tuning are opposite approaches to system configuration, because systems are usually optimized either towards performance or power. This manual describes the tools that Red Hat provides and the techniques we have developed to help you in this process. Red Hat Enterprise Linux 7 already comes with a lot of new power management features that are enabled by default. They were all selectively chosen to not impact the performance of a typical server or desktop use case. However, for very specific use cases where maximum throughput, lowest latency, or highest CPU performance is absolutely required, a review of those defaults might be necessary. To decide whether you should optimize your machines using the techniques described in this document, ask yourself a few questions: Q: Must I optimize? Q: How much do I need to optimize? Q: Will optimization reduce system performance to an unacceptable level? Q: Will the time and resources spent to optimize the system outweigh the gains achieved? Q: Must I optimize? A: The importance of power optimization depends on whether your company has guidelines that need to be followed or if there are any regulations that you have to fulfill. Q: How much do I need to optimize? A: Several of the techniques we present do not require you to go through the whole process of auditing and analyzing your machine in detail but instead offer a set of general optimizations that typically improve power usage. Those will of course typically not be as good as a manually audited and optimized system, but provide a good compromise. Q: Will optimization reduce system performance to an unacceptable level? A: Most of the techniques described in this document impact the performance of your system noticeably. If you choose to implement power management beyond the defaults already in place in Red Hat Enterprise Linux 7, you should monitor the performance of the system after power optimization and decide if the performance loss is acceptable. Q: Will the time and resources spent to optimize the system outweigh the gains achieved? A: Optimizing a single system manually following the whole process is typically not worth it as the time and cost spent doing so is far higher than the typical benefit you would get over the lifetime of a single machine. On the other hand if you for example roll out 10000 desktop systems to your offices all using the same configuration and setup then creating one optimized setup and applying that to all 10000 machines is most likely a good idea. The following sections will explain how optimal hardware performance benefits your system in terms of energy consumption.

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