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 <M> AC Adapter <M> 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 <*> 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.
Compile your kernel, make sure the right modules get loaded at startup and boot
into your new ACPI-enabled kernel. Next run
# emerge sys-apps/acpid # modprobe button # /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. Every acpi event recognized by the kernel is catched by acpid
which calls
#!/bin/sh set $* group=${1/\/*/} action=${1/*\//}# runlevel to use in AC mode RLVL_AC="default"# runlevel to use in battery mode RLVL_BATTERY="battery"# file indicating the AC state AC_STATE="/proc/acpi/ac_adapter/AC/state"# this string means running on AC AC_ON="on-line"# this string means running on batteries AC_OFF="off-line" function SwitchRunlevel() { if [[ "$(grep ${AC_OFF} ${AC_STATE})" != "" && "$(cat /var/lib/init.d/softlevel)" != "${RLVL_BATTERY}" ]] then logger "Switching to ${RLVL_BATTERY} runlevel" /sbin/rc ${RLVL_BATTERY} elif [[ "$(grep ${AC_ON} ${AC_STATE})" != "" && "$(cat /var/lib/init.d/softlevel)" != "${RLVL_AC}" ]] then logger "Switching to ${RLVL_AC} runlevel" /sbin/rc ${RLVL_AC} fi } case "$group" in battery) case "$action" in battery) SwitchRunlevel ;; *) logger "ACPI group battery / action $action is not defined" ;; esac ;; ac_adapter) case "$action" in ac_adapter) SwitchRunlevel ;; *) logger "ACPI group ac_adapter / action $action is not defined" ;; esac ;; *) logger "ACPI group $group / action $action is not defined" ;; esac
Give it a try: Plug AC in and out and watch syslog for the "Switching to AC mode" or "Switching to battery mode" messages.
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/default.sh "battery/battery"
Prepared like this you can activate Power Management policies for individual devices.
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. To get comfortable with the interface to the kernel, first do some manual speed modifications. To set another CPU speed, use:
(Get current frequency) # cat /proc/cpuinfo | grep "cpu MHz"(Lists supported frequencies. This might fail.) # cd /sys/devices/system/cpu/cpu0/cpufreq/ # cat scaling_available_frequencies(Change frequency to 1 GHz (1000000 KHz) Replace with a frequency your laptop supports.) # echo -n userspace > scaling_governor # echo -n 1000000 > scaling_setspeed(Verify frequency was changed) # cat /proc/cpuinfo | grep "cpu MHz"
If you are getting error messages, please refer to the
You can also write to
The above is quite nice, but not doable in daily life. Better let your system set the appropriate frequency automatically. A couple of user space programs like to do it for you. The following table gives a quick overview to help you decide on one of them.
Name | Pro | Con |
---|---|---|
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, first try
# emerge speedfreq # rc-update add speedfreq battery
# /etc/init.d/speedfreq start
Setting up cpufreq is a little bit more complicated.
# emerge cpufreqd # rc-update add cpufreqd battery
[General] pidfile=/var/run/cpufreqd.pid poll_interval=2 pm_type=acpi# Uncomment the following line to enable ACPI workaround (see cpufreqd.conf(5)) # acpi_workaround=1 verbosity=4#(if you want a minimal logging set to 5) # Full performance [Profile] name=ac minfreq=600000 maxfreq=1400000 policy=performance# Maximum power saving [Profile] name=battery minfreq=600000 maxfreq=900000 policy=powersave# Constant frequency [Profile] name=dvd minfreq=900000 maxfreq=1100000 policy=powersave# Full performance when running on AC [Rule] name=ac_on ac=on profile=ac# Compiling should be fast if battery state is ok [Rule] name=compiling ac=off battery_interval=30-100 programs=emerge,make,gcc,cpp cpu_interval=0-100 profile=ac# watching DVD's gets sluggish with slow CPU frequency # Can also be used for games etc. [Rule] name=dvd_watching ac=off battery_interval=15-100 programs=xine,mplayer,avidemux,kaffeine,kmplayer cpu_interval=0-100 profile=dvd# If above doesn't apply, maximise power saving [Rule] name=battery_on ac=off battery_interval=0-100 cpu_interval=0-100 profile=battery
The last thing to check is that your new policies do a good job. An easy way to do so is monitoring the CPU speed while working with your laptop:
# watch -n 1 cat /proc/cpuinfo | grep "cpu MHz"
If
# watch -n 1 x86info -mhz
Depending on your setup, CPU speed should increase on heavy load, decrease on no activity or just stay at the same level.
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
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 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
# /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
To start and stop laptop-mode, create a script /etc/init.d/laptop-mode. You can
take the one included in
# rc-update add laptop-mode battery
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
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/statestandby # echo -n mem > /sys/power/statesleep (swsusp) # echo 4 > /proc/acpi/sleephibernate (Suspend-to-Disk) # echo -n disk > /sys/power/statehibernate (swsusp2) # echo > /proc/swsusp/activate
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 to patch the kernel with the patches provided at
$ 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.