Capacity and lifetime of laptop batteries has improved much in the last years. Nevertheless modern processors consume much more energy than older ones and each laptop generation introduces more devices hungry for energy. That's why Power Management is more important than ever. Increasing battery run time doesn't necessarily mean buying another battery. Much can be achieved applying intelligent Power Management policies.
Please notice that this guide describes Power Management for
As this guide has become rather long, here's a short overview helping you to find your way through it.
The
Nearly every component can operate in different states - off, sleep, idle, active to name a few - consuming a different amount of energy. Major parts are consumed by the LCD display, CPU, chipset and hard drives. Often one is able to activate OS-independent Power Management in the BIOS, but an intelligent setup in the operating system adapting to different situations can achieve much more.
Before going into the details on making individual devices Power Management
aware, make sure certain requirements are met. After controlling the BIOS
settings, some kernel options want to be enabled - these are in short ACPI,
sleep states and CPU frequency scaling. As power saving most of the time comes
along with performance loss or increased latency, it should only be enabled
when running on batteries. That's where a new runlevel
First have a look into your BIOS Power Management settings. The best way is to combine BIOS and operating system policies, but for the moment it's better to disable most of the BIOS part. This makes sure it doesn't interfere with your policies. Don't forget to re-check BIOS settings after you configured everything else.
ACPI (Advanced Configuration and Power Interface) support in the kernel is still work in progress. Using a recent kernel will make sure you'll get the most out of it.
In kernel config, activate at least these options:
Power Management Options ---> [*] Power Management Support [ ] Software Suspend [ ] Suspend-to-Disk Support ACPI( Advanced Configuration and Power Interface ) Support ---> [*] ACPI Support [ ] Sleep States [*] AC Adapter [*] Battery <M> Button <M> Fan <M> Processor <M> Thermal Zone < > ASUS/Medion Laptop Extras < > Toshiba Laptop Extras [ ] Debug Statements CPU Frequency Scaling ---> [*] CPU Frequency scaling Default CPUFreq governor (userspace) <*> 'performance' governor <*> 'powersave' governor <*> 'ondemand' cpufreq policy governor <*> CPU frequency table helpers <M> ACPI Processor P-States driver <*> CPUFreq driver for your processor
Decide yourself whether you want to enable Software Suspend, Suspend-to-Disk and Sleep States (see below). If you own an ASUS, Medion or Toshiba laptop, enable the appropriate section.
The kernel has to know how to enable CPU frequency scaling on your processor. As
each type of CPU has a different interface, you've got to choose the right
driver for your processor. Be careful here - enabling
Compile your kernel, make sure the right modules get loaded at startup and boot
into your new ACPI-enabled kernel. Next run
# emerge sys-power/acpid # /etc/init.d/acpid start # rc-update add acpid default
The default policy will be to enable Power Management only when needed -
running on batteries. To make the switch between AC and battery convenient,
create a runlevel
# cd /etc/runlevels # cp -a default battery
Finished. Your new runlevel
Typical ACPI events are closing the lid, changing the power source or pressing
the sleep button. An important event is changing the power source, which should
cause a runlevel switch. Create the following files to switch between
#!/bin/bash# BEGIN configuration RUNLEVEL_AC="default" RUNLEVEL_BATTERY="battery"# END configuration if [ ! -d "/etc/runlevels/${RUNLEVEL_AC}" ] then logger "${0}: Runlevel ${RUNLEVEL_AC} does not exist. Aborting." exit 1 fi if [ ! -d "/etc/runlevels/${RUNLEVEL_BATTERY}" ] then logger "${0}: Runlevel ${RUNLEVEL_BATTERY} does not exist. Aborting." exit 1 fi if on_ac_power then if [[ "$(cat /var/lib/init.d/softlevel)" != "${RUNLEVEL_AC}" ]] then logger "Switching to ${RUNLEVEL_AC} runlevel" /sbin/rc ${RUNLEVEL_AC} fi elif [[ "$(cat /var/lib/init.d/softlevel)" != "${RUNLEVEL_BATTERY}" ]] then logger "Switching to ${RUNLEVEL_BATTERY} runlevel" /sbin/rc ${RUNLEVEL_BATTERY} fi
# replace "ac_adapter" below with the event generated on your laptop # See /var/log/acpid event=ac_adapter.* action=/etc/acpi/actions/pmg_switch_runlevel.sh %e
# replace "battery" below with the event generated on your laptop # See /var/log/acpid event=battery.* action=/etc/acpi/actions/pmg_switch_runlevel.sh %e
Additionally you need the package sys-power/powermgmt-base which contains
the
# emerge powermgmt-base # chmod +x /etc/acpi/actions/pmg_switch_runlevel.sh # /etc/init.d/acpid restart
Give it a try: Plug AC in and out and watch syslog for the "Switching to AC mode" or "Switching to battery mode" messages. See the Troubleshooting section if the script is not able to detect the power source correctly.
Due to the nature of the event mechanism, your laptop will boot into runlevel
# Fake acpi event to switch runlevel if running on batteries /etc/acpi/actions/pmg_switch_runlevel.sh "battery/battery"
Prepared like this you can activate Power Management policies for individual devices.
CPU frequency scaling brings up some technical terms that might be unknown to you. Here's a quick introduction.
First of all, the kernel has to be able to change the processor's frequency. The
This doesn't sound like dynamic frequency changes yet and in fact it isn't.
Dynamics however can be accomplished with various approaches. For example,
the
Decreasing CPU speed and voltage has two advantages: On the one hand less energy is consumed, on the other hand there is thermal improvement as your system doesn't get as hot as running on full speed. The main disadvantage is obviously the loss of performance. Decreasing processor speed is a trade off between performance loss and energy saving.
It's time to test whether CPU frequency changing works. Let's install another
tool which is very handy for debugging purposes:
# emerge cpufrequtils # cpufreq-info
Here is an example output:
cpufrequtils 0.2: cpufreq-info (C) Dominik Brodowski 2004 Report errors and bugs to linux@brodo.de, please. analyzing CPU 0: driver: centrino CPUs which need to switch frequency at the same time: 0 hardware limits: 600 MHz - 1.40 GHz available frequency steps: 600 MHz, 800 MHz, 1000 MHz, 1.20 GHz, 1.40 GHz available cpufreq governors: ondemand, powersave, userspace, performance current policy: frequency should be within 924 MHz and 1.40 GHz. The governor "performance" may decide which speed to use within this range. current CPU frequency is 1.40 GHz (asserted by call to hardware).
Now play around with
The above is quite nice, but not doable in daily life. Better let your system
set the appropriate frequency automatically. There are many different approaches
to do this. The following table gives a quick overview to help you decide on one
of them. It's roughly seperated in three categories
Name | Category | Switch decision | Kernel governors | Further governors | Comments |
---|---|---|---|---|---|
While adjusting the frequency to the current load looks simple on the first view, it's not such a trivial task. A bad algorithm can cause switching between two frequencies all the time or wasting energy when setting frequency to an unnecessary high level.
Which one to choose? If you have no idea about it, try
# emerge cpufreqd
[General] pidfile=/var/run/cpufreqd.pid poll_interval=2 pm_type=acpi verbosity=5 [Profile] name=ondemand minfreq=0% maxfreq=100% policy=ondemand [Profile] name=powersave minfreq=0% maxfreq=100% policy=powersave [Profile] name=performance minfreq=0% maxfreq=100% policy=performance [Rule] name=battery ac=off profile=ondemand [Rule] name=battery_low ac=off battery_interval=0-10 profile=powersave [Rule] name=ac ac=on profile=performance
You can't use a percentage value like above for min_freq and max_freq if you
are using kernel 2.6 with the sysfs interface (you probably do). Replace it
with the lowest and highest frequency as reported by
minfreq=600000 maxfreq=1400000
Last not least start the daemon.
# rc-update add cpufreqd default battery # rc
The last thing to check is that your new policies do a good job. An easy way to do so is monitoring CPU speed while working with your laptop:
# watch grep \"cpu MHz\" /proc/cpuinfo
If
# watch x86info -mhz
Depending on your setup, CPU speed should increase on heavy load, decrease on
no activity or just stay at the same level. When using cpufreqd and verbosity
set to 5 or higher in
As you can see in
First thing to check is the standby/suspend/off timings of the display. As this
depends heavily on your windowmanager, I'll let you figure it out yourself.
Just two common places: Blanking the terminal can be done with
Section "ServerLayout" Identifier [...] [...] Option "BlankTime" "5"# Blank the screen after 5 minutes (Fake) Option "StandbyTime" "10"# Turn off screen after 10 minutes (DPMS) Option "SuspendTime" "20"# Full suspend after 20 minutes Option "OffTime" "30"# Turn off after half an hour [...] EndSection [...] Section "Monitor" Identifier [...] Option "DPMS" "true" [...] EndSection
This is the same for XFree86 and
Probably more important is the backlight dimming. If you have access to the
dimming settings via a tool, write a small script that dims the backlight in
battery mode and place it in your
To be able to set the brightness level, the ibm_acpi module has to be loaded with the experimental parameter.
(Please read the warnings above before doing this!) # emerge ibm-acpi # echo "options ibm_acpi experimental=1" >> /etc/modules.d/ibm_acpi # /sbin/modules-update # echo ibm_acpi >> /etc/modules.autoload.d/kernel-2.6 # modprobe ibm_acpi
This should work without error messages and a file
# See /proc/acpi/ibm/brightness for available values # Please read /usr/share/doc/ibm-acpi-*/README.gz # brigthness level in ac mode. Default is 7. BRIGHTNESS_AC=7# brightness level in battery mode. Default is 4. BRIGHTNESS_BATTERY=4
#!/sbin/runscript set_brightness() { if on_ac_power then LEVEL=${BRIGHTNESS_AC:-7} else LEVEL=${BRIGHTNESS_BATTERY:-4} fi if [ -f /proc/acpi/ibm/brightness ] then ebegin "Setting LCD brightness" echo "level ${LEVEL}" > /proc/acpi/ibm/brightness eend $? else ewarn "Setting LCD brightness is not supported." ewarn "Check that ibm_acpi is loaded into the kernel" fi } start() { set_brightness } stop () { set_brightness }
When done, make sure brightness is adjusted automatically by adding it to the battery runlevel.
# chmod +x /etc/init.d/lcd-brightness # rc-update add lcd-brightness battery # rc
Let's bring the hard disk to sleep as early as possible whenever it is not
needed. I'll show you two possibilities to do it. First
################################################ # DISK OPTIONS # (disabled by default) ################################################ # # Timeout to put the disk in standby mode if there was no # io during that period (in seconds) # TIMEOUT=60# # Specified disks to spindown (comma separated devices) # DISKS=/dev/hda
The second possibility is using a small script and hdparm. Create
#!/sbin/runscript depend() { after hdparm } start() { ebegin "Activating Power Management for Hard Drives" hdparm -q -S12 /dev/hda eend $? } stop () { ebegin "Deactivating Power Management for Hard Drives" hdparm -q -S253 /dev/hda eend $? }
See
# chmod +x /etc/init.d/pm.hda # /sbin/depscan.sh # rc-update add pm.hda battery
Recent kernels (2.6.6 and greater, recent 2.4 ones and others with patches)
include the so-called
# emerge laptop-mode-tools
Besides putting your disk to sleep state as early as possible, it is a good idea to minimize disk accesses. Have a look at processes that write to your disk frequently - the syslogd is a good candidate. You probably don't want to shut it down completely, but it's possible to modify the config file so that "unnecessary" things don't get logged and thus don't create disk traffic. Cups writes to disk periodically, so consider shutting it down and only enable it manually when needed.
# rc-update del cupsd battery
Another possibility is to deactivate swap in battery mode. Before writing a swapon/swapoff switcher, make sure there is enough RAM and swap isn't used heavily, otherwise you'll be in big problems.
If you don't want to use laptop-mode, it's still possible to minimize disk
access by mounting certain directories as
none /tmp tmpfs size=32m 0 0
Wireless LAN cards consume quite a few energy. Put them in Power Management mode in analogy to the pm.hda script.
#!/sbin/runscript start() { ebegin "Activating Power Management for Wireless LAN" iwconfig wlan0 power on power max period 3 eend $? } stop () { ebegin "Deactivating Power Management for Wireless LAN" iwconfig wlan0 power off eend $? }
Starting this script will put wlan0 in Power Management mode, going to sleep at
the latest three seconds after no traffic.
Save it as
# chmod +x /etc/init.d/pm.wlan0 # /sbin/depscan.sh # rc-update add pm.wlan0 battery
There are two problems with USB devices regarding energy consumption: First, devices like USB mice, digital cameras or USB sticks consume energy while plugged in. You cannot avoid this (nevertheless remove them in case they're not needed). Second, when there are USB devices plugged in, the USB host controller periodically accesses the bus which in turn prevents the CPU from going into C3/4 sleep mode. The OS answer to this problem is the so called "USB selective suspend", which has not yet been implemented in the kernel. USB selective suspend only allows bus accesses in case the device is in use. The cruel workaround until it's implemented is as following: Compile USB support and devices as modules and remove them via a script while they are not in use (e.g. when closing the lid).
ACPI defines different sleep states. The more important ones are
They can be called whenever the system is not in use, but a shutdown is not wanted due to the long boot time.
The ACPI support for these sleep states is marked as experimental for good reason. APM sleep states seem to be more stable, however you can't use APM and ACPI together.
There are currently three implementations for S4. The original one is swsusp, then there is swsusp2 which has the nicest interface (including bootsplash support), but requires manual kernel patching. Last not least we have Suspend-to-Disk, a fork of swsusp.
If this confused you, have a look at a
The kernel part for this is as following:
Power Management Options --->(sleep and standby) ACPI( Advanced Configuration and Power Interface ) Support ---> [*] ACPI Support [*] Sleep States(hibernate with swsusp) [*] Software Suspend (EXPERIMENTAL)(hibernate with swsusp2) Software Suspend 2 --- Image Storage (you need at least one writer) [*] Swap Writer --- Page Transformers [*] LZF image compression (/dev/"your-swap-here") Default resume device name(hibernate with Suspend-to-Disk) [*] Suspend-to-Disk Suport (/dev/"your-swap-here") Default resume partition
Compile your kernel with the appropriate options enabled and issue
To put your system in one of the sleep states, use
(kernel 2.4 series) # echo 1 > /proc/acpi/sleep(standby) # echo 3 > /proc/acpi/sleep(sleep) (kernel 2.6 series) # echo -n standby > /sys/power/state(standby) # echo -n mem > /sys/power/state(sleep) (swsusp) # echo 4 > /proc/acpi/sleep(hibernate) (Suspend-to-Disk) # echo -n disk > /sys/power/state(hibernate) (swsusp2) # /usr/sbin/hibernate(hibernate, see below)
If you experience kernel panics due to uhci or similar, try to compile USB support as module and unload the modules before sending your laptop to sleep mode.
While the above should be sufficient to get swsusp and Suspend-to-Disk running
(I didn't say working), swsusp2 needs special care.
The first thing to do is patching the kernel with the patches provided at
# emerge hibernate-script # $EDITOR /etc/hibernate/hibernate.conf(Last chance to backup any data) # hibernate
$ grep capacity /proc/acpi/battery/BAT0/info design capacity: 47520 mWh last full capacity: 41830 mWh
If the "last full capacity" differs significantly from the design capacity, your battery is probably broken. Try to claim your warranty.