Table of Contents
This chapter provides an overview of the various power management technologies in Linux. The configuration of all available APM (advanced power management), ACPI (advanced configuration and power interface), and CPU frequency scaling settings are described in detail.
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). On many types of modern hardware, the CPU frequency can be adapted to the situation, which helps save valuable battery time especially on mobile devices (CPU frequency scaling).
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.
|Power Management for AMD64 Processors|
AMD64 processors with a 64-bit kernel only support ACPI.
Power saving functions are not only significant for the mobile use of laptops, but also for desktop systems. The following paragraphs briefly introduce the main functions and their use in the power management systems APM and ACPI:
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.
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.
In this operating mode, the entire system state is written to the hard disk and the system is powered off. The reactivation from this state takes about thirty to ninety 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.
ACPI and APM check the battery charge status and provide information about the charge status. Additionally, both systems coordinate actions to perform when a critical charge status is reached.
Following a shutdown, the computer is powered off. This is especially important when an automatic shutdown is performed shortly before the battery is empty.
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 can be deactivated via ACPI (at least theoretically) or permanently in the BIOS setup.
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.