Testing The Different Ubuntu 10.04 Kernels
The Ubuntu kernel benchmarking adventure began with the OpenGL gaming tests with the first being the ioquake3-powered Tremulous. Even though Tremulous is not too demanding of a game compared to modern commercial titles, with the current open-source Mesa stack for the ATI R600/700 hardware the frame-rates are not the best and barely playable even for a high-end ATI Radeon HD 4870 graphics card. Running with a resolution of 1280 x 1024, the six tested kernels were within three frames of each other. When just looking at the numbers, the best Tremulous performance was found to be with the Linux 2.6.32.9 mainline kernel while Ubuntu's default/generic and preempt kernels were right behind at about the same speed. The Ubuntu Server kernel ran ever so slightly slower than the other Ubuntu kernels. While the results are close, the lowest frame-rate in Tremulous was with the very latest Linux 2.6.34-rc1 kernel.
There is a larger spread with the OpenArena test results between the kernels, but because of a lone kernel. The frame-rates in OpenArena that run off the similar ioquake3-engine are much higher than with Tremulous where all of the tested kernels topped 60 FPS at 1280 x 1024 with the RV770 GPU. By a wide margin, the best OpenArena performance was found with the mainline Linux 2.6.32.9 kernel. The mainline Linux 2.6.33 (and continuing into 2.6.34 so far) the performance has regressed dramatically going from 98 FPS to 62 FPS. The default, server, and preempt kernels that are based off the Linux 2.6.32 kernel also came in at 62 FPS.
The Ubuntu kernels having this drop is not surprising, as the Ubuntu 2.6.32 kernels have back-ported the 2.6.33 DRM. It all started when the Direct Rendering Manager (DRM) code for the Nouveau driver entered the mainline Linux 2.6.33 kernel for providing better open-source NVIDIA GPU support, it was decided to pull in that DRM to facilitate the 2D and 3D support (the 3D support is provided by Gallium3D and is only available via a PPA for the Lucid Lynx release) as Ubuntu 10.04 would be shipping with Nouveau support rather than the crippled xf86-video-nv driver that does not use any DRM. Since then the core DRM and other DRM drivers from the Linux 2.6.33 kernel have been pulled into Lucid. The newer DRM provides better hardware support, new features, and many other changes, but it seems one of the regressions brought in adversely affects the OpenArena performance at least with our hardware configuration. Fortunately, we have the capabilities to autonomously bisect performance regressions in the Linux kernel using the Phoronix Test Suite, so we may end up tracking down the commit in the 2.6.33 DRM causing this severe performance drop.
Similar to our OpenArena results, the highest frame-rate was found with the mainline Linux 2.6.32.9 kernel while the newer kernels and the Ubuntu kernels that have the back-ported DRM were all noticeably lower. The Urban Terror regression was not as bad with the frame-rate going from 61 FPS to 51 FPS for Urban Terror, but still we are using a high-end graphics card and with those using slower ATI Radeon graphics cards may be more impacted with the ill-optimized Mesa 3D stack and this DRM regression.