Less autonomy under Linux? The fault of the BIOS and Windows

For several months, under GNU/Linux computers consumed more energy than under Windows. The reason is known for a time: the management of the NCDT has been modified in the 2.6.38 nucleus and - quite oddly - the operation of the technology was altered.

NCDT (Active State Power Management) is a technology related to the PCI-Express bus to reduce the consumption of the various PCI Express links when they are unused. The main problem is simple: the Linux kernel checks to the BIOS if the technology is supported and if the BIOS indicates, the NCDT is enabled. Otherwise, it is disabled. The kernel therefore uses a simple method, but it has a large defect: some BIOS do not indicate if the technology is supported, as it is. And the kernel, in this case specific (and a priori current) disables the technology, which has a quite visible impact on consumption and therefore - directly - autonomy.

In inquiring, a developer has understood the problem: Windows - by far the most used OS - does not test the rules on how to manage the NCDT. Specifically, the NCDT is regarded as being supported automatically unless the BIOS indicates explicitly that it is not. A patch for the kernel Linux - pending its integration in the future - has therefore developed to modify the behavior of the latter. With the patch, instead of manually disable the NCDT when the BIOS does not information, the kernel will do nothing. The only time where technology will be disabled manually is when the BIOS indicate it explicitly. The source is therefore linked to Windows - which does not perform the tests correctly - and, in fact, developers of the BIOS that indicate not correctly support, given that the most common operating system works without this information.

With this patch of 60 lines, consumption is much reduced, the developer indicates win 5 W on his Thinkpad X 220 in idle, which is important on a device of this type. Note that it is also possible to manually force the use of technology: it is supported on the vast majority of the recent laptops.