Laptop battery life on FreeBSD

Bruce Cran bruce at
Tue Jan 27 22:10:05 PST 2009

On Tue, 27 Jan 2009 16:19:31 -0700
Shawn Badger <shawnbadger at> wrote:

> Hi, I recently installed FreeBSD 7.1 on my laptop, replacing Arch
> Linux and I noticed a significant drop in my battery life (from ~3
> hours to ~1.5 hours).  I realize that Linux has their "tickless"
> kernel, which I am sure explains the difference, but my question
> is...  is there anything I can do in FreeBSD right now to improve
> this?  And is there going to be any work done the kernel to allow the
> hardware to enter into a low-power state for an extended period of
> time? 
> My laptop is an Acer Travelmate 4100 which has the following hardware:
>    - Pentium M 1.6 GHz processer
>   - 2 GB RAM
>   - integrated video/sound
>   - 802.11bg / bluetooth
> I'm somewhat of a minimalist and don't use X11.  I've disabled the 
> bluetooth adapter, but frequently use my wireless adapter (intel 
> pro/2200bg).  Does anyone have any suggestions that could potentially 
> extend my battery life?

powerd isn't run by default, so the laptop will always be running at
maximum frequency.  By putting powerd_enable="YES" in /etc/rc.conf the
CPU will throttle back whenever the system is idle.

You can also tell FreeBSD to enter lower Cx states when idle by
setting dev.cpu.x.cx_lowest, if it's supported.  For example my VIA
EPIA system has the following dev.cpu tree:

> sysctl dev.cpu
dev.cpu.0.%desc: ACPI CPU
dev.cpu.0.%driver: cpu
dev.cpu.0.%location: handle=\_PR_.CPU0
dev.cpu.0.%pnpinfo: _HID=none _UID=0
dev.cpu.0.%parent: acpi0
dev.cpu.0.freq: 533
dev.cpu.0.freq_levels: 533/-1 266/-1
dev.cpu.0.cx_supported: C1/0 C2/90 C3/900
dev.cpu.0.cx_lowest: C1
dev.cpu.0.cx_usage: 100.00% 0.00% 0.00%

Bruce Cran

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