Power Saving Functions - Novell LINUX ENTERPRISE SERVER 10 - INSTALLATION AND ADMINISTRATION 08-05-2008 Installation Manual

28.1 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 per-
formance is throttled. 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.
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
504 Installation and Administration