Intel Developer Finds 50 Watt Power Regression In Linux
Written by Michael Larabel in Intel on 8 December 2013 at 03:45 PM EST. 21 Comments
An Intel Linux kernel developer has discovered that since the Linux 3.10 kernel one of his powerful Xeon-based systems is going through 50 Watts more energy while idling than on pre-3.10 kernels. This power regression could affect smaller systems too, but fortunately the issue has been bisected and an investigation is in process.

Len Brown of Intel OTC wrote yesterday to the Linux kernel mailing list about a 50 Watt idle power regression bisected to Linux-3.10. On a system with 40 Intel Xeon CPU cores, Len Brown discovered the system was running through 50 more Watts of power on all kernel version since Linux 3.10 -- up through and including the latest Linux 3.13-rc3 kernel.

The issue was tracked down to an x86 change for using a generic idle loop. The change made the system go from idling in its package C6 state nearly the entire time to never having a chance to idle in this low-power C6 state. A commit to the Linux kernel from September attempted to improve the generic idle loop code, but it failed to properly do so and the power regression is still outstanding.

A proper fix for the solution is still being investigated but it looks like setting the "idle=mwait" kernel command-line parameter may help the situation.

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Michael Larabel is the principal author of and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 10,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and automated benchmarking software. He can be followed via Twitter or contacted via

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