Intel RC6 Support On The Sandy Bridge Desktop

While RC6 support remains off-by-default as Intel developers are faced by RC6-related bugs affecting a small minority of Sandy Bridge users, this power-savings feature is not limited to only Intel mobile graphics. As discovered at Phoronix, RC6 can manage to boost the graphics performance beyond just extending your battery life. The RC6 performance boost is also quite visible on Intel Sandy Bridge desktop hardware too.

Having RC6 power-savings support enabled by default for supported configurations has been a mess. Intel keeps flip-flopping as they end up enabling it by default for certain configurations in the upstream Intel DRM driver but only to later disable the feature after a new system hits an issue with the code. RC6, which allows the graphics hardware to rest in a deeper state, was on for the Linux 3.2 kernel but then disabled at the last minute. With the Linux 3.3 kernel, RC6 has yet to be attempted. This is in terms of the Sandy Bridge support as the Ivy Bridge RC6 support is still enabled (at least last time I checked), as it is more stable with the next-generation Intel hardware.

While RC6 isn't on by default for Sandy Bridge right now, on modern kernels the feature can be flipped on at boot-time by having the "i915.i915_enable_rc6=1" kernel parameter. Previous Phoronix tests have been focused on mobile Sandy Bridge hardware since that is where power consumption matters the most, but the feature also works for desktop systems to conserve power -- and as a positive side effect, to boost the performance too.

In this article are just some quick pre-FOSDEM benchmarks of an Intel Core i5 2400S with an ASRock H61M/U3S3 motherboard. The Core i5 2400S is a quad-core part (no Hyper Threading) with a 2.5GHz base frequency, 3.3GHz Turbo Boost frequency, 6MB of Intel Smart Cache, and Intel HD Graphics 2000 (GT1) integrated graphics with an 850MHz base frequency and 1100MHz maximum frequency.

With this testing a recent Ubuntu 12.04 x86_64 daily snapshot was pulled in with X.Org Server 1.11.3 and the Unity desktop. The Linux 3.3 kernel, Mesa 8.0-devel, and xf86-video-intel were built from Git to have the latest Intel Linux graphics stack. Several OpenGL benchmarks were run at a variety of resolutions and the only change between test runs was enabling the RC6 support via the "i915.i915_enable_rc6=1" parameter. This shows the RC6 performance difference for a desktop with HD 2000 Sandy Bridge graphics. All benchmarking was done in a fully automated manner via the Phoronix Test Suite.