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Thursday
Feb232012

Laptop Power Saving

Kernel Options

Back in August phoronix came out with an article about some kernel options for power savings on sandy bridge processors. In order to use these you can add the following to your kernel line:

pcie_aspm=force i915.i915_enable_rc6=1 i915.i915_enable_fbc=1 i915.lvds_downclock=1

pcie_aspm=force enables aspm power saving. There was a bug that was introduced in an earlier kernel which prevented aspm from working correctly. The fix for this was rolled into 3.2.5. There's also supposed to be another fix going into 3.3. I'm unsure if this option will be needed once 3.3 is released.

i915.i915_enable_rc6=1 enables an extra power savings mode for the GPU. On some machines this is said to cause artifacts. I have not experienced any issues with this, but I also don't have many tasks that require 3d support.

i915.i915_enable_fbc=1 enables frame buffer compression. This saves some video memory. I'm unsure how much this would actually save power wise.

i915.lvds_downclock=1 enables the kernel to lower the clockrate of the LCD panel. This should save a little power, but might cause flickering. Again, I have had no issues using this.

laptop-mode-tools

laptop-mode-tools is a set of scripts that are designed to save power when a laptop isn't plugged in. I suggest installing acpid and ethtool in order for these scripts to work correctly. You can add acpid and laptop-mode-tools to your DAEMONS=(..) section in the /etc/rc.conf file if they aren't already there.

laptop-mode-tools is mainly configured using the file /etc/laptop-mode/laptop-mode.conf. It's well documented, so I suggest going through and seeing if there's anything you feel you need to change. I mostly changed the options regarding low battery charge, increasing them a few %. This is more a personal preference, as it bothers me when my laptop drops below 10%.

There are other configuration files in /etc/laptop-mode/conf.d. This includes features such as auto hibernate, usb autosuspend, ethernet power savings, and other commonly used features. I'd suggest taking a look in that directory if you want to get a better idea of other areas you can tweak with laptop-mode-tools.

I did have an issue with my u24e shutting down on battery power with laptop-mode-tools enabled. Disabling ethernet power saving by editing /etc/laptop-mode/conf.d/ethernet.conf and changing CONTROL_ETHERNET="auto" to CONTROL_ETHERNET="0" fixed this issue.

module blacklisting

Some hardware will take a small amount of power if the kernel module for it is loaded. You can disable these by creating (or editing) the file /etc/modprobe.d/blacklist.conf and putting a list of modules you don't want loaded prefixed by the word blacklist. For instance, I don't use the webcam so I blacklist the driver:

# Disable webcam
blacklist uvcvideo

Powertop

There's also a good power diagnosis tool called powertop, which can be used to determine what's waking a computer and what other tweaks can be made. Be aware that powertop 1 hasn't been updated in awhile and gives a lot of false tips, such as disabling usb autosuspend when it's already disabled. Powertop 2 was being worked on and has some interesting features in it. I believe the best way to install this is to use powertop-git in the AUR.

Reader Comments (1)

When you are not going to be using your laptop for a even longer time, put it into hibernate(Shift+Standby Button) mode. It will have the same effect as turning the computer off, but it will allow you to go back to working on your laptop just like in standby and uses little to no power from the battery.

April 2, 2012 | Unregistered CommenterAbram Adam
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