Besides the Nouveau driver performance being faster thanks to experimental re-clocking when using the Linux 3.16 kernel, there are also performance improvements to note with some generations of AMD graphics processors.
The changes found within Linux 3.16 to benefit the Radeon DRM graphics performance are the GPU VM optimizations and large PTE support. Separate from this performance-related work for this kernel-side open-source AMD update is also HDMI deep color support, HDMI audio clean-ups, and other bug-fixes.
The performance improvements with Radeon hardware for Linux 3.16 is mostly focused around GCN hardware (SI/CIK). For this article I ran some Linux 3.15 benchmarks against the Radeon drm-next code using the Radeon HD 7850, HD 7950, and R9 270X graphics cards. For reference purposes I also ran a Radeon HD 6870 from the Northern Islands line-up.
Testing was done from an Intel Core i7 4790K Devil's Canyon system running Ubuntu 14.04 plus the Oibaf PPA for tapping Mesa 10.3-devel and other user-space driver updates. Besides changing out the graphics cards, the only other changes during testing was swapping the 3.15 mainline stable kernel build for my build of the drm-next tree after the merge window to pull in the Radeon updates.
All benchmarking was handled via the Phoronix Test Suite.