1.2. Power Management Basics

Effective power management is built on the following principles:
An idle CPU should only wake up when needed

Since Red Hat Enterprise Linux 6, the kernel runs tickless which means the previous periodic timer interrupts have been replaced with on-demand interrupts. Therefore, idle CPUs are allowed to remain idle until a new task is queued for processing, and CPUs that have entered lower power states can remain in these states longer. 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 or mouse movement are examples of such events.

Red Hat Enterprise Linux 7 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 Red Hat Enterprise Linux 7. 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:

  • SpeedStep
  • PowerNow!
  • Cool'n'Quiet
  • ACPI (C state)
  • Smart
If your hardware has support for these features and they are enabled in the BIOS, Red Hat Enterprise Linux 7 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:

  • Sleep (C-states)
  • Frequency and voltage (P-states)
    P-state describes frequency of a processor and its voltage operating point, which are both scaled as the P-state increases.
  • Heat output (T-states or "thermal states")
A CPU running on the lowest sleep state possible 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 Red Hat Enterprise Linux 7.