LessWatts.org - Saving Power on Intel systems with Linux

Tips & Tricks

AC97 audio power saving mode

The previous generation onboard audio chips (called AC97) support power saving, where the analog parts (codec) are powered down when no program is using the audio device. This power saving is disabled by default in current kernels and distributions (there is the potential of hearing a little click when the power savings kick in). However this power savings can be enabled manually.

To see if you have an AC97 generation sound chip, you can perform this command:

# ls /sys/module/snd_ac97_codec/

If that command doesn't return any files, you don't have an AC97 sound card in use.

To check if you have the power-saving mode enabled, use this command:

# cat /sys/module/snd_ac97_codec/parameters/power_save
N

In the example above, the power savings are disabled.

To enable the power savings, two steps are needed:

enable the power save value
play a little sound, so the driver can enter power save mode after that.

The following two steps perform these functions described in the steps above:

echo 1 > /sys/module/snd_ac97_codec/parameters/power_save
echo 1 > /dev/dsp

HDA-Intel power saving mode

The current generation onboard audio (often called "High Definition" or HDA) has power savings capabilities similar to AC97. The Linux ALSA drivers support these capabilities, as of version 1.0.15 of the ALSA subsystem. These are scheduled to appear in the 2.6.24 kernel.

Unlike the AC97 power saving mode, the HDA power saving mode is enabled by default.

If you are running an older kernel, you can download the ALSA sound drivers from the ALSA website.

To see if power save is enabled for the HDA controllers, you can use this command:

# cat /sys/module/snd_hda_intel/parameters/power_save
10

Power saving is enabled if the value that this shows is greater than zero.

BIOS settings

Sometimes, BIOSes come with very conservative settings. For example, avoiding showing a yellow bang symbol in the hardware manager of certain operating systems.

There are a few settings that will help Linux save power. These settings aren't present in all BIOSs, but when they are present in your BIOS, you can make sure they're set to the optimal value:

Processor C1E support: This enables maximum power saving of the processor when idle.
Enhanced Speedstep (EIST): This allows Linux to optimally reduce the frequency and voltage of the processor when not using the maximum capacity.
Fan control: Set to "auto speed"; this allows the fans to slow down (and use less power) when the temperatures in the machine allow this.
Enable the HPET (often called "Multimedia timer") option.: This allows Linux with tickless idle to maximally save power by being idle longer.

Don't use "fake" cpu hot unplugging

Linux has the ability to, at runtime, take cores offline. Some people use this feature in an attempt to save power. However, measurements show that an offline core actually uses more power than a core that's in an idle sleep state.