Power Management

Power Management

When you're on the go, you have to carry your power with you. Notebook power management therefore receives a lot more attention than desktop power management, even though attention to these issues on the desktop can result in significant savings in electrical costs, system wear, and heat production.

For many years, power-management interfaces have been proprietary and required custom software supplied by the hardware vendor in order to function well (even when they purportedly adhered to industry standards). The situation is slowly improving, and Fedora contains good tools for power management on well-behaved systems.

How Do I Do That?

Fedora uses the Advanced Configuation and Power Interface (ACPI) specification to monitor and manage the current power configuration. This approach requires support from the motherboard and CPU as well as the operating system; fortunately, most systems built in the last decade have some level of ACPI support, though many BIOS implementations are nonstandard.

Using gnome-power-manager to conserve power

Fedora's main power-management tool is gnome-power-manager. You can access the gnome-power-manager configuration window using the menu option SystemPreferencesMore PreferencesPower Management.

Fedora Core does not include the KDE ACPI modules. However, you can use gnome-power-manager in KDE by starting it manually: press Alt-F2 or open a terminal, and type:

$ gnome-power-manager

A second power-management icon will appear in the KDE panel. You can disable KDE's icon through the Control Center menu option; go to Power ControlLaptop Battery and deselect the checkbox labeled "Show battery monitor."

gnome-power-manager will automatically be started next time you enter KDE.

The same program controls the power-management icon in the panel bar, which may or may not appear depending on the system configuration (it will usually appear by default on a system that has a battery, including most notebooks, but will not appear by default on a system with no battery). The symbol used for the icon will change according to the power supply: it will show a battery when discharging the battery, a battery plus a power cord when charging the battery, or just a power cord on a system with no battery. You can access the gnome-power-manager configuration window by right-clicking on the icon and selecting Preferences.

Regardless of how you access the configuration window, you will see the display shown in Figure.

Power Manager Preferences window

This window contains three tabs: one that configures power management when the system is running on AC or charging the battery, one that configures power management when the system is running on (and discharging) the battery, and one for general power-management settings.

The controls on first two tabs are almost identical:


Configures the conditions under which the display and the whole system is put to sleep. For the display, "sleep" is defined as a low-power mode; for the system, the definition of "sleep" is taken from the setting on the General tab. You can set either to a value between 11 and 60 minutes (in one-minute intervals), or you can disable sleep by moving the slider all the way to the right (Never).

When laptop lid is closed

Specifies the action to be taken when a lid closure event is signaled by ACPI. The options include: do nothing, which causes the system to continue to use full power, to be available the moment you open the screen; blank the screen, which is similar to doing nothing but will use slightly less power and delay system startup for a few seconds; or suspend or hibernate the system.

Configuring the screen to blank without suspending or hibernating the system whenever the lid is closed is ideal for listening to digital music.

Prefer power savings over performance

If you are performing a task that is not very demanding, such as editing a document, instruct gnome-power-manager to maximize the battery life (or, on AC, reduce power consumption) by selecting the checkbox. On the other hand, if you need optimal performance without regard to the power consumptionsuch as when you are giving an important presentationleave the checkbox for this option unselected.

When battery power critical

This button appears only on the "Running on Battery" tab, and it configures the action to be taken if the battery runs down to the point that it will power the system for only a few minutes. The options are to do nothing, to suspend or hibernate, or to shut down the system. For most users, doing nothing is a poor choice because the system will abruptly shut off within a few minutes; suspend continues to use power, although at a much lower rate than usual, so the battery will ultimately go dead in due course (causing the loss of any data in memory). Shutdown in an option, but the shutdown procedure itself may take a few minutes and uses a fair bit of power. Therefore, the best choice (if your system supports it) is to hibernate the system when the battery power becomes critical. Hibernation takes less than a minute to complete and even less time to resume, yet it uses no power during the time that the system is in hibernation.

Depending on the hardware installed on your system, you may see different combinations of controls on the "Running on AC" and "Running on Battery" tabs.

The third tab in this window controls general power-management options, as shown in Figure.

General tab on Power Management Preferences window

The options in this window control how the system is put to sleep when the inactivity period set in the other tabs is reached (do nothing, suspend, or hibernate). It also configures the times that the the power-management icon is displayed in the Notification Area; the default is to display it only when the battery is present, so if you want to easily access the power-management controls on an AC-only system, set this to "Always display icon."

It's important to understand the difference between the suspend and hibernate options: in both cases, the system is effectively off, but in suspend mode the memory is still operating (and consuming power), while in hibernate mode the memory is transferred to swapspace. This means that suspend state will eventually run your battery down, and then the information in RAM will be lostbut until that time, the system will be able to quickly resume its operation. On the other hand, your system can be in hibernate state forever without any power and still resume successfully, but it will take a little longer to do so.

In order to hibernate or suspend successfully, the drivers for all of the devices attached to the system must be able to save and restore the device state. There's no simple way to predict if this will be the case, so it's best to try suspend and resume operations when you have nothing critical happening on the system. For hibernation, it's also necessary to have a swap partition instead of (or in addition to) swapspace on a logical volume.

To manually provoke a suspend or hibernate operation, right-click on the power-management icon in the status bar and use the menu options that appear. To resume from a suspend, use the wake-up key defined for your system (you may have to consult the system documentation or experiment to find this). To resume from hibernation, turn your system power on with the power button, then let it start a normal boot cycle. The kernel will recognize the hibernation state and attempt to resume; if this fails, a normal boot will take place (but you may lose changes to any files that were opened when you hibernated your computer, so it's a good idea to save changes before you hibernate).

A password is required to unlock the screen after a resume if passwords are configured in the screensaver (which is the default).

Resuming from hibernation will fail if the kernel has been upgraded since the last boot.

Viewing power information

If your system has a battery, gnome-power-manager provides a number of interesting graphs showing your power state. To view them, right-click on the power management icon on the panel, then select Information. The window shown in Figure will be presented.

Power Information window

The tabs across the top provide access to the available information and graphs:

Device Information

Displays the current battery state. Clicking on More presents the battery technology, serial number, and a capacity rating showing the percentage of the original design capacity that the battery is now capable of holding (the higher the number, the better the battery condition).

Charge History

This graph shows the battery charge (as a percentage of its current maximum) over time. The graphs are automatically scaled horizontally.

Power History

Displays the battery charge and discharge rates over time, as shown in Figure. If the battery is fully charged and the system is on external power, the discharge rate will be shown as zero, but if the battery is charging, the system will show the rate at which it is charging. When running on battery power, this graph shows the rate of discharge. Power events are marked on this graph, including transitions to and from AC power, suspend and hibernate events, lid closures, and display idle periods. You can use the information in this graph to see the impact of your usage patterns on the system's power consumption.

Power History graph

Estimated Time History

Shows the history of the calculated time to charge the battery, when you're on AC power, or to discharge the battery, when you're running only on the battery. Power events are also shown on this log.

Event Log

Displays a more verbose listing of the same power events shown on the graphs.

On an AC-only system, the graphs will all be flat lines, but power events will still be displayed.

Changing the CPU speed

If CMOS semiconductors were perfect, they would consume power only when they changed state. They're not perfect, so they leak energy and consume power when idle, but that's a tiny fraction of the power they consume when changing state.

The system clock is a pulse generator that controls the speed at which the CPU changes state, and therefore controls the amount of energy used by the CPU and related system components. Therefore, there is a trade-off between performance and power consumption.

Fedora can balance power usage against performance automatically according to system workload. This feature is provided by the cpuspeed service and is enabled by default.

The default parameters used by this service work well in most cases, but can be adjusted by editing the file /etc/cpuspeed.conf, which looks like this:


# uncomment this and set to the name of your CPUFreq module

# Let background (nice) processes speed up the cpu

# Add your favorite options here
#OPTS="$OPTS -s 0 -i 10 -r"

# uncomment and modify this to check the state of the AC adapter
#OPTS="$OPTS -a /proc/acpi/ac_adapter/*/state"

# uncomment and modify this to check the system temperature
#OPTS="$OPTS -t /proc/acpi/thermal_zone/*/temperature 75"

Usually the DRIVER, VMAJOR, and VMINOR lines should not be changed.

Uncomment the OPTS lines that contain additional options you wish to use. The pre-configured lines have these meanings:


Allow processes that have been marked as low priority using the nice command to run at full speed. The default is to slow down the processor when only low-priority processes are running.

-s 0 -i 10 -r

Manages only CPU 0 (-s 0), making speed change decisions once a second instead of the default of once every two seconds (-i 10), and restore the original speed when cpuspeed exits (-r).

-a /proc/acpi/ac_adapter/*/state

Monitors the AC adapter and switches to minimum speed when AC power is removed. Adding -C will force the system to operate at maximum speed when AC power is connected.

-t /proc/acpi/thermal_zone/*/temperature 75

Watches the system temperature and slows down the CPU when the temperature exceeds 75 degrees Celsius (167 degrees Fahrenheit).

To see a list of all available options, run this command:

$ /usr/sbin/cpuspeed --help 2>&1|less

After editing /etc/cpuspeed, restart the cpuspeed service:

# service cpuspeed restart
Stopping cpuspeed:                                         [  OK  ]
Starting cpuspeed:                                         [  OK  ]

You can monitor the CPU speed by installing a monitor on your panel. Right-click on your GNOME panel, then select "Add to Panel." Choose the CPU Frequency Scaling Monitor and click Add. The icon shown in Figure will appear; the bar graph will rise and fall as the CPU clock speed is adjusted, and hovering your mouse cursor over the display will show the current frequency setting (the box that appears below the icon in Figure).

CPU Frequency Scaling Monitor

Managing power from the command line

If you are using your system through a text-mode console, you can still access the important power-management tools.

To suspend the system, execute the pm-suspend script:

$ pm-suspend

To hibernate, use the pm-hibernate script:

$ pm-hibernate

To view the battery status, access ACPI through the /proc filesystem:

$ cat /proc/acpi/battery/*/state
present:                 yes
capacity state:          ok
charging state:          charging
present rate:            3079 mA
remaining capacity:      2912 mAh
present voltage:         16273 mV

Battery details are also available from ACPI:

$ cat /proc/acpi/battery/*/info
present:                 yes
design capacity:         6450 mAh
last full capacity:      5154 mAh
battery technology:      rechargeable
design voltage:          14800 mV
design capacity warning: 515 mAh
design capacity low:     156 mAh
capacity granularity 1:  5 mAh
capacity granularity 2:  5 mAh
model number:            DELL 0017F
serial number:           14639
battery type:            LION
OEM info:                Sony

By dividing the last full capacity by the design capacity, you can determine the condition of the battery. In this case, the battery can presently store 5154/6450 mAh, or about 80 percent of its design capacity, indicating that it's in reasonably good condition.

Other ways to extend battery life

To further reduce power consumption:

  • Dim your screen as much as the ambient lighting conditions will allow. The backlight for the screen draws a huge amount of power; one of my systems draws 31 percent more power in total when the screen is at maximum brightness than when it is set to minimum brightness.

  • Turn off all unneeded services, reducing CPU and disk activity.

  • Use a lightweight desktop environment such as Xfce instead of GNOME or KDE.

Xfce can be easily installed (see Lab 5.3, "Using Repositories"). To select your desktop environment, use the Session menu on the graphical login screen.

  • Unplug any unneeded external devices, including USB devices, headphones, mice, and keyboards. Each USB device can consume up to 2.5 watts of power, increasing your power consumption by as much as 20 percent.

  • Turn off your wireless network radio when it is not in use. Most laptops have a wireless kill switch (sometimes labeled airplane or flight mode) that turns off the radio portion of the wireless card. This can cut your power consumption by up to 5 percent.

How Does It Work?

The ACPI specification enables a system's BIOS to supply data and program code to the operating system. The code is written in a unique, processor-independent pseudo-machine language called ACPI Machine Langauge (AML). The Linux kernel interpretively executes the AML code to access certain features of the host hardware system. You can think of the AML code as a type of device driver that is downloaded from the BIOS to the operating system.

The Linux kernel uses the ACPI data and code to collect data that is then exposed through the /proc/acpi directory. Information received through the /proc/acpi directorysuch as instructions to change the CPU frequencyis passed to the ACPI code.

Applications such as cpuspeed and gnome-power-manager monitor the ACPI information and combine it with other information (such as current process load) and then make power management decisions. These decisions cause actions to be taken by various subsystems such as the ACPI, the X Window server, storage, and loadable modules.

The Fedora power-management system is in active development. To stay informed of the latest developments, subscribe to the fedora-laptop-list (see Lab 9.1, "Participating in the Fedora Mailing Lists").

What About...

...stopping the hard drive motors when the drives are not in use?

The hdparm program can be used to stop a drive immediately or to configure it to stop if it is idle for a certain length of time; this is called a spin-down (and restarting the drive is called a spin-up). This does save some power; however, the drive is rarely idle for very long and the length of time (and amount of energy) required to spin-up the drive is significant (and hard on some hardware), so opinion is divided on whether it makes sense to use this feature.

In the case of a two-drive system where the second drive is rarely used, an idle spin-down timeout is a good idea. Configure it with the hdparm command:

# hdparm -S 6 /dev/[hs]d[a-z]

The -S option configures the amount of time that the drive must be idle before spin-down is triggered. The scheme used to encode the timeout period is a bit convoluted (it is described in detail on the manpage for hdparm), but 0 means that spin-down is disabled, and a value from 1 to 240 sets the idle timeout in multiples of 5 seconds (5 seconds to 20 minutes). The value of 6 used here indicates a 30-second idle timeout.

...turning the computer on and off (or hibernating and resuming) at certain times automatically?

Some important operations are best performed when a system is not in normal use, such as backup, software updating, and data indexing. It's not necessary to keep a system running 24x7 in order to schedule nighttime tasks.

Most modern system BIOSes contain an alarm wake feature, which causes the system to turn on at a preset time. This feature can be used with the hibernate state or a full system shutdown to automate nocturnal activities.

The BIOS configuration utility varies from system to system; consult your system or motherboard documentation.

Some BIOS versions permit the wake time to be set through ACPI. To see if this is possible on your system, enable the alarm wake feature using your BIOS configuration utility, then boot Fedora and examine the /proc/acpi/alarm file:

$ cat /proc/acpi/alarm
2007-03-17 00:00:00

Attempt to set this to a time in the future (the date portion of the time may be ignored by your system). If your system clock is maintained in UTC, be sure to also specify this time in UTC:

# echo "2007-03-17 16:45:00" >/proc/acpi/alarm
$ cat /proc/acpi/alarm
2007-03-17 16:45:00

Shut down your system and see if it turns on at the specified time.

Regardless of whether you can set the alarm time using Linux, you can use the wake-up feature in conjunction with cron to configure automatic boot and shutdown.

Configure your system to turn on at a specified time using either the BIOS or Linux ACPI methods. Create a nocturnal script that performs the work you wish to do at boot time:

# /usr/local/bin/nocturnal :: script for nighttime processing

# Place whatever commands you wish to execute at night here
                  yum -y update

# Shut the system back off - you can use pm-hibernate here
# if it works on your system
shutdown -h now

Replace the last line with pm-hibernate if hibernation works on your system and you wish to reduce the startup time in the morning.

Edit your crontab:

# crontab -e

Add a line to execute the nocturnal script a few minutes after your preset wake-up time:

# Perform nighttime processing after the 4:30 wake-up
35 4 * * * /usr/local/bin/nocturnal

If you shut down the system at the end of your working day, it will wake up at night, perform the processing you have configured, and then shut down.

If your system supports changing the alarm time through ACPI, you can schedule multiple wake-up times: have your system start up in the middle of the night and perform the operations described above, and then have it schedule the next wake-up time before shutting down:

# /usr/local/bin/nocturnal :: script for nighttime processing

# Please whatever commands you wish to execute at night here
                  yum -y update 

# Schedule another wake-up
date "+%Y-%m-%d 
                     07:50:00 " >/proc/acpi/alarm

# Shut the system back off; you can use pm-hibernate here
# if it works on your system
shutdown -h now

This sets the next wake-up for 7:50 a.m. the same day, just in time to start work at 8 a.m. If your nocturnal processing takes place before midnight, schedule the wake-up for the following day:

# Schedule another wake-up
date +"%Y-%m-%d 07:50:00" -d tomorrow >/proc/acpi/alarm

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