New Linux kernel fixes power-saving issues
Greg Kroah-Hartman has released long-term kernel 3.0.20 and stable kernel 3.2.5. Both contain just a single bug fix that allows PCIe power-saving technology ASPM (Active State Power Management) to be used on systems with a BIOS that activates ASPM on some components, but states in the FADT (Fixed ACPI Description Table) consulted by Linux that ASPM is not supported.
According to Matthew Garrett, who developed the patch, the change can reduce the power consumption of a Thinkpad X220 by 5 watts. The H's associates at c't magazine in Germany tested systems in their laboratory and experienced savings in the range of 1 to 3 watts which resulted in noticeably longer battery life. The patch only has this effect on systems with the firmware problem detailed above, which can be identified by using
dmesg to display kernel messages and looking for the message "ACPI FADT declares the system doesn't support PCIe ASPM, so disable it".
The change fixes one of two problems which garnered serious attention last year as a result of reports on the Phoronix web site. Several weeks ago, the patch was merged into the main Linux development tree, which will form the basis of March's Linux 3.3 release. Patches like this, which could potentially cause problems on isolated systems, are not usually merged into long-term and stable kernels. The Fedora Project has, however, been testing the patch as part of its current distribution for some time, and Kroah-Hartman is therefore confident that the change will not cause any major problems. In view of the margin for error in this prediction, however, this is the only change in Linux 3.0.20 and 3.2.5.
Another problem which probably affects many more systems remains, however. By default, current Linux kernels do not activate the RC6 graphics power-saving feature on Intel's popular Sandy Bridge processors (which include a graphics core), as it has caused crashes and graphics errors on isolated systems. According to measurements made by c't, RC6 typically saves 3 to 5 watts when idling, which for laptops means reduced fan noise and longer battery life. It can be enabled for testing purposes using the kernel argument
For more on the latter problem, see:
- Kernel Comment: Untapped power saving potential, a comment feature from The H.