Power consumption of Asus Eee 901

Ondrej 'SanTiago' Zajicek, santiago@crfreenet.org

There are five important factors that affect the power consumption of the Asus Eee 901 notebook - backlight brightness, CPU activity, CPU frequency, cpufv setting and connected devices.

Backlight brightness - As usual, more intensive backlight leads to bigger power consumption. The brightness can be queried or set by file /sys/class/backlight/eeepc/brightness (when eeepc_laptop driver is active). Unless i am outdoor, i use brightness level 4.

CPU activity - As usual, high CPU load leads to bigger power consumption, but it is not very significant compared to other notebooks. I found that the number of wakeups is completely insignificant on Eee 901.

CPU frequency - As usual, can be set by cpufreq-set tool from cpufrequtils.

cpufv setting - This magic setting is not implemented in vanilla Linux 2.6.28, but it is available in kernel provided by Asus in Linux versions of Asus Eee 901 notebook. I don't know what exactly it is, but it has a significant effect on the power consumption. I guess that it might set FSB or RAM frequency. Similarly to the other ACPI settings of Eee 901, this setting persists over poweroff. Therefore, you can boot Asus provided kernel and configure cpufv to value you want and later use your standard system and kernel.

Cpufv setting in Asus provided kernel is accessible after modprobing asus_acpi as /proc/acpi/asus/cpufv . Values for write are 0, 1, 2. For read it returns, 768, 769, 770. Asus provided kernel and modules here for download. It also contains driver for integrated wifi card, which is not in vanilla kernel.

It would be nice to port this setting to vanilla kernel, but the source code from Asus i found seems not to work and they are probably for Eee 701 or 900 (with different CPU). If anyone know about official source code of Asus kernel modifications, please send me a e-mail.

connected / enabled devices - Obviously, external USB devices take some power. A connected ethernet cable increases power consumption. Enabled VGA output (regardless of whether a cable is connected) increases power consumption. There are four internal devices that can be connected or disconnected using write to files in /sys directory - wifi card, bluetooth controller, camera and card reader. When enabled, these devices increase power consumption even if not used. You can use eeepc_laptop driver to access these settings, they are files wlan, bluetooth, camera, cardr in /sys/devices/platform/eeepc/ directory. In Linux kernel 2.6.28 the files wlan, bluetooth are missing, and these settings are accessible using rfkill interface.

It is pity that although vanilla Linux kernel does not have driver for integrated wifi card, the wireless card is enabled after modprobing eeepc_laptop driver even when disabled in BIOS setting (in 2.6.28).There is also bug in Linux kernel 2.6.28 that after resume from STR the power consumption is slightly bigger.

I did some measurements of power consumption of my Asus Eee 901 notebook. All these measurements are done under Linux kernel 2.6.28 and Debian Lenny userspace. The notebook was completely disconnected (no USB device, no Ethernet cable and no power cable) and the four internal optional devices were disabled. Brightness was at level 4. CPU fan was disabled too (~ 0.2 W difference). Power consumption was measured by internal battery sensor that is accessible using ACPI and powertop tool. Frequency and cpufv columns show the configured state of the notebook, idle and burn columns show the power consumption in a idle mode and when running Nbench benchmark. Nbench column shows measured integer index result of Nbench benchmark (relative CPU speed in fixed point operations).

Freqency (MHz) Cpufv Idle (W) Burn (W) Nbench
800 2 5.6 5.9 2.429
1067 2 5.7 6.1 3.319
1330 2 5.8 6.3 4.141
1600 2 5.9 6.6 4.975
800 1 7.0 7.3 3.332
1067 1 7.1 7.5 4.443
1330 1 7.2 7.9 5.575
1600 1 7.3 8.3 6.708
800 0 7.1 7.4 3.455
1067 0 7.3 7.7 4.659
1330 0 7.4 8.1 5.869
1600 0 7.5 8.5 7.087