Power saving

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.

Audio

By default, audio power saving is turned off by most drivers. It can be enabled by setting the power_save parameter to a time (in seconds) to go in idle.

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

Intel

/etc/modprobe.d/audio_power_save.conf

options snd_hda_intel power_save=1

ac97

/etc/modprobe.d/audio_power_save.conf

options snd_ac97_codec power_save=1

Active State Power Management

To verify that ASPM is enabled:

$ cat /sys/module/pcie_aspm/parameters/policy

[default] performance powersave

Either [default] or [powersave] means you do not need to force it on.

Otherwise, it's either unsupported/broken on your hardware, or has to be forced on with pcie_aspm=force on the kernel line.

Warning: Forcing on ASPM can cause a freeze/panic, so make sure you have a way to undo the option if it doesn't work.

Warning: In some cases forcing on ASPM can even increase power consumption.

Bluetooth

Blacklist the hci_usb module if the driver is loaded automatically.

Alternatively, blacklist the btusb and bluetooth modules.

Another variant is to rfkill it:

# rfkill block bluetooth

Web-Camera

If you won't use integrated web camera then blacklist the uvcvideo module.

Disabling NMI watchdog

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

/etc/sysctl.d/disable_watchdog.conf

kernel.nmi_watchdog = 0

or add nmi_watchdog=0 as a kernel parameter to disable it completely from early boot.

Disabling Wake-on-LAN

Wake-on-LAN can be a useful feature, but if you're not making use of it then it's simply draining extra power waiting for a magic packet while in suspend.

Disabling for all Ethernet interfaces:

/etc/udev/rules.d/disable_wol.rules

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

You can use multiple names in the matches; for example, KERNEL=="lan0|eth*"

Note: This should be combined with static naming of devices, the eth* names are not static.

PCI Runtime Power Management

/etc/udev/rules.d/pci_pm.rules

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

Wireless power saving

Enabling for a specific interface:

Note: This should be combined with static naming of devices, the eth* names are not static.

/etc/udev/rules.d/wlan0_power_save.rules

ACTION=="add", SUBSYSTEM=="net", KERNEL=="wifi0" RUN+="/usr/sbin/iw dev wifi0 set power_save on"

Enabling for all interfaces:

/etc/udev/rules.d/wifi_power_save.rules

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

Writeback Time

Increasing the VM dirty writeback time can help to aggregate I/O together - reducing disk writes, and decreasing power usage:

/etc/sysctl.d/dirty_writeback.conf

vm.dirty_writeback_centisecs = 1500

To do the same for journal commits with ext4 and some other filesystems, use commit=15 as a parameter in fstab or with the rootflags kernel parameter.

Laptop Mode

/etc/sysctl.d/laptop_mode.conf

vm.laptop_mode = 5

SATA Active Link Powermanagement

Note: This adds latency when accessing a drive that has been idle, so it's one of the few settings that may be worth toggling based on whether you're on AC power.

/etc/udev/rules.d/hd_power_save.rules

SUBSYSTEM=="scsi_host", KERNEL=="host*", ATTR{link_power_management_policy}="min_power"

USB Autosuspend

To enable USB autosuspend after 2 seconds of inactivity:

/etc/udev/rules.d/usb_power_save.rules

ACTION=="add", SUBSYSTEM=="usb", TEST=="power/control" ATTR{power/control}="auto"
ACTION=="add", SUBSYSTEM=="usb", TEST=="power/autosuspend" ATTR{power/autosuspend}="2"

Device Power Management

echo auto | tee /sys/bus/i2c/devices/*/power/control > /dev/null
echo auto | tee /sys/bus/spi/devices/*/power/control > /dev/null

View Power Setings

This function shows various power settings. Note you either must be root or you must have sudo.

function aa_power_settings ()
{ 
  sudo bash -c '
    for i in `find /sys/devices -name "bMaxPower"`;
    do
        for ii in `find $i -type f`;
        do
            bd=`dirname $ii`;
            busnum=`cat $bd/busnum`;
            devnum=`cat $bd/devnum`;
            title=`lsusb -s $busnum:$devnum`;
            echo -e "\n\n+++ $title\n  -$bd\n  -$ii";
            for ff in `find $bd/power -type f ! -empty 2>/dev/null`;
            do
                v=`cat $ff 2>/dev/null|tr -d "\n"`;
                [[ ${#v} -gt 0 ]] && echo -e " `basename $ff`=$v";
                v=;
            done | sort -g;
        done;
    done;
    echo -e "\n\n\n+++ Kernel Modules\n";
    for m in `command lspci -k|sed -n "/in use:/s,^.*: ,,p"|sort -u`;
    do
        echo "+ $m";
        systool -v -m $m 2> /dev/null | sed -n "/Parameters:/,/^$/p";
    done
  ';
}

See also

• CPU Frequency Scaling