Chapter 33. Power Management

Contents

33.1. Power Saving Functions
33.2. APM
33.3. ACPI
33.4. Rest for the Hard Disk
33.5. The powersave Package
33.6. The YaST Power Management Module

Abstract

Power management is especially important on laptop computers, but is also useful on other systems. Two technologies are available: APM (advanced power management) and ACPI (advanced configuration and power interface). In addition to these, it is also possible to control CPU frequency scaling to save power or decrease noise. These options can be configured manually or using a special YaST module.

Unlike APM, which was previously used on laptops for power management only, the hardware information and configuration tool ACPI is available on all modern computers (laptops, desktops, and servers). All power management technologies require suitable hardware and BIOS routines. Most laptops and many modern desktops and servers meet these requirements.

APM had been used in many older computers. Because APM largely consists of a function set implemented in the BIOS, the level of APM support may vary depending on the hardware. This is even more true of ACPI, which is even more complex. For this reason, it is virtually impossible to recommend one over the other. Simply test the various procedures on your hardware then select the technology that is best supported.

[Important]Power Management for AMD64 Processors

AMD64 processors with a 64-bit kernel only support ACPI.


33.1. Power Saving Functions

Power saving functions are not only significant for the mobile use of laptops, but also for desktop systems. The main functions and their use in the power management systems APM and ACPI are:

Standby

This operating mode turns off the display. On some computers, the processor performance is throttled. This function is not available in all APM implementations. This function corresponds to the ACPI state S1 or S2.

Suspend (to memory)

This mode writes the entire system state to the RAM. Subsequently, the entire system except the RAM is put to sleep. In this state, the computer consumes very little power. The advantage of this state is the possibility of resuming work at the same point within a few seconds without having to boot and restart applications. Devices using APM can usually be suspended by closing the lid and activated by opening it. This function corresponds to the ACPI state S3. The support of this state is still under development and therefore largely depends on the hardware.

Hibernation (suspend to disk)

In this operating mode, the entire system state is written to the hard disk and the system is powered off. There must be a swap partition at least as big as the RAM to write all the active data. Reactivation from this state takes about 30 to 90 seconds. The state prior to the suspend is restored. Some manufacturers offer useful hybrid variants of this mode, such as RediSafe in IBM Thinkpads. The corresponding ACPI state is S4. In Linux, suspend to disk is performed by kernel routines that are independent from APM and ACPI.

Battery Monitor

ACPI and APM check the battery charge status and provide information about it. Additionally, both systems coordinate actions to perform when a critical charge status is reached.

Automatic Power-Off

Following a shutdown, the computer is powered off. This is especially important when an automatic shutdown is performed shortly before the battery is empty.

Shutdown of System Components

Switching off the hard disk is the greatest single aspect of the power saving potential of the overall system. Depending on the reliability of the overall system, the hard disk can be put to sleep for some time. However, the risk of losing data increases with the duration of the sleep periods. Other components, like PCI devices that can be put into a special power saving mode, can be deactivated with ACPI (at least theoretically) or permanently disabled in the BIOS setup.

Processor Speed Control

In connection with the CPU, energy can be saved in three different ways: frequency and voltage scaling (also known as PowerNow! or Speedstep), throttling, and putting the processor to sleep (C states). Depending on the operating mode of the computer, these methods can also be combined.