Difference between revisions of "Power saving"

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Limiting disk writes can help when the hard disk is in power save mode; generally though this thought is considered unrealistic: tracking all programs, and how and when they write to disk can be laborious.  Laptop-mode-tools has a program that can display when and what programs write to disk if willing to try.  Also little things like setting {{ic|relatime}} or {{ic|noatime}} (see [[Fstab]]) can help (though {{ic|relatime}} is the default kernel option now).  If enough RAM is available consider disabling (or limiting [[Swap|swapiness]]) as it has the possibility to limit a good number of disk writes.
Limiting disk writes can help when the hard disk is in power save mode; generally though this thought is considered unrealistic: tracking all programs, and how and when they write to disk can be laborious.  Laptop-mode-tools has a program that can display when and what programs write to disk if willing to try.  Also little things like setting {{ic|relatime}} or {{ic|noatime}} (see [[Fstab]]) can help (though {{ic|relatime}} is the default kernel option now).  If enough RAM is available consider disabling (or limiting [[Swap|swapiness]]) as it has the possibility to limit a good number of disk writes.
=== Mount options ===
You might want to use the {{ic|noatime}} option, see [[Fstab#atime options]] for more information.
=== CD/DVD spin down ===
=== CD/DVD spin down ===

Revision as of 14:30, 22 October 2014

This article covers the configuration needed to turn on power saving features. Almost all of the features listed here are worth using whether or not the computer is on AC or battery power. Most have negligible performance impact and are just not enabled by default because of commonly broken hardware/drivers. Reducing power usage means reducing heat, which can even lead to higher performance on a modern Intel or AMD CPU, thanks to dynamic overclocking.


If you would like to create your own scripts and power saving settings such as by udev rules you can take the following settings as a reference.

Note: Most rules described below are also managed by tools like TLP and it is unwise to use multigoverning.


By default, audio power saving is turned off by most drivers. It can be enabled by setting the power_save parameter; a time (in seconds) to go into idle mode. To idle the audio card after one second, create

options snd_hda_intel power_save=1

for Intel, or use

options snd_ac97_codec power_save=1

for ac97.

Note: Toggling the audio card's power state can cause a popping sound or noticeable latency on some broken hardware.


When system starts, screen backlight is set to maximum by default. This can be fixed by specifying backlight level in the following udev rule:

# set backlight level
SUBSYSTEM=="backlight", ACTION=="add", KERNEL=="acpi_video0", ATTR{brightness}="1"

See Backlight for more information.


Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: The device should likely be disabled with hciconfig first. (Discuss in Talk:Power saving#)

To disable bluetooth completely, blacklist the btusb and bluetooth modules.

To turn off bluetooth only temporarily, use rfkill:

# rfkill block bluetooth

Or with udev rule:

# disable bluetooth
SUBSYSTEM=="rfkill", ATTR{type}=="bluetooth", ATTR{state}="0"

Or just enable the instantiated rfkill-block@bluetooth.service provided by the rfkill package.

Web camera

If you will not use integrated web camera then blacklist the uvcvideo module.

Kernel parameters

This section uses configs in /etc/sysctl.d/, which is "a drop-in directory for kernel sysctl parameters." See The New Configuration Files and more specifically systemd's sysctl.d man page for more information.

Disabling NMI watchdog

The NMI watchdog is a debugging feature to catch hardware hangs that cause a kernel panic. On some systems it can generate a lot of interrupts, causing a noticeable increase in power usage.

kernel.nmi_watchdog = 0

or add nmi_watchdog=0 to the kernel line to disable it completely from early boot.

Writeback Time

Increasing the VM dirty writeback time helps to aggregate disk I/O together, thus reducing spanned disk writes, and increasing power saving. Several articles (needs citation) mention that there isn't much power save value after two minutes. For every minute (default 15 seconds):

vm.dirty_writeback_centisecs = 6000

To do the same for journal commits with supported filesystems (e.g. ext4, btrfs...), use commit=60 as a option in fstab or with the rootflags=commit=60 kernel parameter.

Laptop Mode

See the kernel documentation on the laptop mode 'knob.' "A sensible value for the knob is 5 seconds."

vm.laptop_mode = 5

Network interfaces

Wake-on-LAN can be a useful feature, but if you are not making use of it then it is simply draining extra power waiting for a magic packet while in suspend. Disabling for all Ethernet interfaces:

ACTION=="add", SUBSYSTEM=="net", KERNEL=="eth*", RUN+="/usr/bin/ethtool -s %k wol d"

To enable powersaving on all wireless interfaces:

ACTION=="add", SUBSYSTEM=="net", KERNEL=="wlan*", RUN+="/usr/bin/iw dev %k set power_save on"

In these examples, %k is a specifier for the kernel name of the matched device. For example, if it finds that the rule is applicable to wlan0, the %k specifier will be replaced with wlan0. To apply the rules to only a particular interface, just replace the pattern eth* and specifier %k with the desired interface name. For more information, see Writing udev rules.

In this case, the name of the configuration file is important. Due to the introduction of persistent device names via 80-net-name-slot.rules in systemd v197, it is important that the network powersave rules are named lexicographically before 80-net-name-slot.rules, so that they are applied before the devices are named e.g. enp2s0.

Bus power management

Active State Power Management

If the computer is believed not to support ASPM it and will be disabled on boot:

$ lspci -vv | grep ASPM.*abled\;

ASPM is handled by the BIOS, if ASPM is disabled it will be because [ref]:

  1. The BIOS disabled it for some reason (for conflicts?).
  2. PCIE requires ASPM but L0s are optional (so L0s might be disabled and only L1 enabled).
  3. The BIOS might not have been programmed for it.
  4. The BIOS is buggy.

If believing the computer has support for ASPM it can be forced on for the kernel to handle with the pcie_aspm=force kernel parameter.

  • Forcing on ASPM can cause a freeze/panic, so make sure you have a way to undo the option if it does not work.
  • On systems that do not support it forcing on ASPM can even increase power consumption.

To adjust to powersave do (the following command will not work unless enabled):

echo powersave | tee /sys/module/pcie_aspm/parameters/policy

By default it looks like this:

$ cat /sys/module/pcie_aspm/parameters/policy
[default] performance powersave

PCI Runtime Power Management

ACTION=="add", SUBSYSTEM=="pci", ATTR{power/control}="auto"

Device power management

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: Mass enabling for nearly all devices has negative side effects (unwanted devices such as USB are included, worse performance of the udev rule since matching devices that enable power management by default is useless, harder troubleshooting, etc). Enabling per bus/class for the devices that need it should be recommended instead. (Discuss in Talk:Power saving#)

Enable power management for (almost) all devices, including USB:

# Various subsystems runtime power management (by bus or class)
ACTION=="add", SUBSYSTEMS=="*", TEST=="power/control", ATTR{power/control}="auto"

# Various subsystems power saving (by module)
ACTION=="add", SUBSYSTEMS=="*", TEST=="parameters/power_save", ATTR{parameters/power_save}="1" 

To list the various devices affected:

ls /sys/bus/*/devices/*/power/control
ls /sys/class/*/*/power/control
ls /sys/module/*/parameters/power_save

To enable power management for all systems except USB, you could replace the first rule with individual rules for each subsystem instead (/sys/bus/some_subsystem, /sys/class/some_subsystem):

ACTION=="add", SUBSYSTEM=="some_subsystem", TEST=="power/control", ATTR{power/control}="auto"

USB autosuspend

The linux kernel can automatically suspend USB devices when they are not in use. This can sometimes save quite a bit of power, however some USB devices are not compatible with USB power saving and start to misbehave (common for USB mice/keyboards). udev rules based on whitelist or blacklist filtering can help to mitigate the problem.

The most simple and likely useless example is enabling autosuspend for all USB devices:

ACTION=="add", SUBSYSTEM=="usb", TEST=="power/control", ATTR{power/control}="auto"

To allow autosuspend only for devices that are known to work, use simple matching against vendor and product IDs (use lsusb to get these values):

# whitelist for usb autosuspend
ACTION=="add", SUBSYSTEM=="usb", TEST=="power/control", ATTR{idVendor}=="05c6", ATTR{idProduct}=="9205", ATTR{power/control}="auto"

Alternatively, to blacklist devices that are not working with USB autosuspend and enable it for all other devices:

# blacklist for usb autosuspend
ACTION=="add", SUBSYSTEM=="usb", ATTR{idVendor}=="05c6", ATTR{idProduct}=="9205", GOTO="power_usb_rules_end"

ACTION=="add", SUBSYSTEM=="usb", TEST=="power/control", ATTR{power/control}="auto"

The default autosuspend idle delay time is controlled by the autosuspend parameter of the usbcore kernel module. To set the delay to 5 seconds instead of the default 2 seconds:

options usbcore autosuspend=5

Similarly to power/control, the delay time can be fine-tuned per device by setting the power/autosuspend attribute.

See the Linux kernel documentation for more information on USB power management.

SATA Active Link Power Management

Note: This adds latency when accessing a drive that has been idle, so it is one of the few settings that may be worth toggling based on whether you are on AC power.
ACTION=="add", SUBSYSTEM=="scsi_host", KERNEL=="host*", ATTR{link_power_management_policy}="min_power"
Warning: SATA Active Link Power Management can lead to data loss on some devices (e.g. Lenovo T440s is known to suffer this problem)

Hard disk drive

See hdparm#Power management configuration for drive parameters that can be set.

Limiting disk writes can help when the hard disk is in power save mode; generally though this thought is considered unrealistic: tracking all programs, and how and when they write to disk can be laborious. Laptop-mode-tools has a program that can display when and what programs write to disk if willing to try. Also little things like setting relatime or noatime (see Fstab) can help (though relatime is the default kernel option now). If enough RAM is available consider disabling (or limiting swapiness) as it has the possibility to limit a good number of disk writes.

CD/DVD spin down

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: something similar without using udisks? (Discuss in Talk:Power saving#)

To allow the CD/DVD rom to spin down after a while using udisks:

# udisks --inhibit-polling /dev/sr0

Tools and scripts


There are many scripts and tools which make use of the various settings described in the previous sections; of notability:

  • acpid — A daemon for delivering ACPI power management events with netlink support.
http://sourceforge.net/projects/acpid2/ || acpid
  • ftw — Script to configure udev rules to save power.
https://github.com/supplantr/ftw || ftw-gitAUR
  • Laptop Mode Tools — An utility to configure laptop power saving settings, considered by many to be the de facto utility for power saving.
https://github.com/rickysarraf/laptop-mode-tools || laptop-mode-tools
  • pm-utils — Suspend and powerstate setting framework.
http://pm-utils.freedesktop.org/ || pm-utils
  • Powerdown — Scripts that combine different settings to make the computer consume less energy.
https://github.com/taylorchu/powerdown || powerdown-gitAUR
  • powertop — A tool to diagnose issues with power consumption and power management.
https://01.org/powertop/ || powertop
  • TLP — Linux advanced power management.
http://linrunner.de/en/tlp/docs/tlp-linux-advanced-power-management.html || tlp

If you do not want to take care of the settings by yourself it is recommended to use these tools. But be aware of running only one of these tools to avoid possible conflicts as they all work more or less similar.

Tip: Have a look at the power management category to get an overview on what power management options exist in Archlinux.

Using a script and an udev rule

Since systemd users can suspend and hibernate through systemctl suspend or systemctl hibernate and handle acpi events with /etc/systemd/logind.conf, it might be interesting to remove pm-utils and acpid. There is just one thing systemd cannot do (as of systemd-204): power management depending on whether the system is running on AC or battery. To fill this gap, you can create a single udev rule that runs a script when the AC adapter is plugged and unplugged:

SUBSYSTEM=="power_supply", ATTR{online}=="0", RUN+="/path/to/your/script true"
SUBSYSTEM=="power_supply", ATTR{online}=="1", RUN+="/path/to/your/script false"
Note: You can use the same script that pm-powersave uses. You just have to make it executable and place it somewhere else (for example /usr/local/bin/).

Examples of powersave scripts can be found here: powerdown, powersave.

The above udev rule should work as expected, but if your power settings are not updated after a suspend or hibernate cycle, you should add a script in /usr/lib/systemd/system-sleep/ with the following contents:


case $1 in
    pre) /path/to/your/script false ;;
	if cat /sys/class/power_supply/AC0/online | grep 0 > /dev/null 2>&1
    		/path/to/your/script true	
    		/path/to/your/script false
exit 0

Do not forget to make it executable!

Note: Be aware that AC0 may be different for your laptop, change it if that is the case.

Now you do not need pm-utils anymore. Depending on your configuration, it may be a dependency of some other package. If you wish to remove it anyway, run pacman -Rdd pm-utils.

Print power settings

This script prints power settings and a variety of other properties for USB and PCI devices. Note that root permissions are needed to see all settings.


for i in $(find /sys/devices -name "bMaxPower")
	title=$(lsusb -s $busnum:$devnum)

	printf "\n\n+++ %s\n  -%s\n" "$title" "$busdir"

	for ff in $(find $busdir/power -type f ! -empty 2>/dev/null)
		v=$(cat $ff 2>/dev/null|tr -d "\n")
		[[ ${#v} -gt 0 ]] && echo -e " ${ff##*/}=$v";
	done | sort -g;

printf "\n\n\n+++ %s\n" "Kernel Modules"
for mod in $(lspci -k | sed -n '/in use:/s,^.*: ,,p' | sort -u)
	echo "+ $mod";
	systool -v -m $mod 2> /dev/null | sed -n "/Parameters:/,/^$/p";

See also