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1.2. Power Management Basics

Effective power management is built on the following principles:
An idle CPU should only wake up when needed
The kernel used in releases of Fedora prior to Fedora 18 used a periodic timer for each CPU. This timer prevents the CPU from truly going idle, as it requires the CPU to process each timer event (which would happen every few milliseconds, depending on the setting), regardless of whether any process was running or not. A large part of effective power management involves reducing the frequency at which CPU wakeups are made.
Because of this, the Linux kernel in Fedora 20 eliminates the periodic timer: as a result, the idle state of a CPU is now tickless. This prevents the CPU from consuming unnecessary power when it is idle. However, benefits from this feature can be offset if your system has applications that create unnecessary timer events. Polling events (such as checks for volume changes, mouse movement, and the like) are examples of such events.
Fedora 20 includes tools with which you can identify and audit applications on the basis of their CPU usage. Refer to Chapter 2, Power Management Auditing and Analysis for details.
Unused hardware and devices should be disabled completely
This is especially true for devices that have moving parts (for example, hard disks). In addition to this, some applications may leave an unused but enabled device "open"; when this occurs, the kernel assumes that the device is in use, which can prevent the device from going into a power saving state.
Low activity should translate to low wattage
In many cases, however, this depends on modern hardware and correct BIOS configuration. Older system components often do not have support for some of the new features that we now can support in Fedora 20. Make sure that you are using the latest official firmware for your systems and that in the power management or device configuration sections of the BIOS the power management features are enabled. Some features to look for include:
If your hardware has support for these features and they are enabled in the BIOS, Fedora 20 will use them by default.
Different forms of CPU states and their effects
Modern CPUs together with Advanced Configuration and Power Interface (ACPI) provide different power states. The three different states are:
A CPU running on the lowest sleep state possible (indicated by the highest C-state number) consumes the least amount of watts, but it also takes considerably more time to wake it up from that state when needed. In very rare cases this can lead to the CPU having to wake up immediately every time it just went to sleep. This situation results in an effectively permanently busy CPU and loses some of the potential power saving if another state had been used.
A turned off machine uses the least amount of power
As obvious as this might sound, one of the best ways to actually save power is to turn off systems. For example, your company can develop a corporate culture focused on "green IT" awareness with a guideline to turn of machines during lunch break or when going home. You also might consolidate several physical servers into one bigger server and virtualize them using the virtualization technology we ship with Fedora 20.