Fedora Linux

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 3-2 .

Figure 3-2. 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.

3.1.1.2. 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 3-3 will be presented.

Figure 3-3. 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 3-4 . 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.

Figure 3-4. 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.

3.1.1.3. 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:

VMAJOR=1
VMINOR=1
# uncomment this and set to the name of your CPUFreq module
#DRIVER='powernow-k7'
# Let background (nice) processes speed up the cpu
OPTS='$OPTS -n'
# Add your favorite options here