Nouveau Reclocking: Buggy, But Can Boost Performance
Over the weekend I shared that the Nouveau driver project, which seeks to provide an open-source NVIDIA graphics driver for Linux and other platforms via reverse-engineering, hit a major milestone. The Nouveau driver now supports re-clocking for several generations of NVIDIA GeForce hardware, which allows the open-source driver to put the graphics cards at their properly designed operating frequencies for maximum performance. This can result in the Nouveau driver performing much better against the official closed-source NVIDIA graphics driver, but the support is still very experimental. Initial testing over the weekend found this support to perform well when it works, but that overall it is still very buggy.
As said in the earlier re-clocking article, this work hit the Nouveau DRM Git tree last week and will be queued up for the Linux 3.4 kernel in a few months. This re-clocking support is for "NV50" class hardware, which is from the GeForce 8 series through the GeForce 300 series, inclusive. The re-clocking support for the modern NVIDIA GeForce 400/500 "Fermi" series is still a work-in-progress and not currently available. What this new code allows for the NV50 hardware is to properly change the GPU core, graphics memory, and shader clock speeds in real-time along with the memory timings, voltage, and fan speed (though the power management itself is still premature and if manipulating the clocks it will just force the fan(s) to run at 100% speed).
Even if pulling down this latest Nouveau kernel code right now, re-clocking will not be activated by default. Until the support is considered safe and stable, you must first boot your system with the "nouveau.perflvl_wr=7777" kernel module parameter. When the system is then booted, by default it is still running at the frequencies that the graphics card was set at when booting. To change the performance level, you must then write the performance level number to "/sys/class/drm/card0/device/performance_level." The performance levels and their given performance attributes can be found by examining the "/sys/class/drm/card0/device/performance_level_*" files or by looking at the Nouveau output within the kernel dmesg. If all went well, after re-clocking the graphics card, "/sys/class/drm/card0/device/performance_level" should reflect the new performance level information.
That is at least how the support is designed and should work in theory when taking advantage of this new feature. Unlike the Radeon driver and its "dynpm" power management support, there is not an option within the Nouveau driver right now for dynamically switching between performance levels / frequencies based upon the current workload of the GPU. However, the Nouveau driver does at least support separate performance levels to be set for when running on AC and battery power, for those with mobile NVIDIA GeForce/Quadro graphics hardware. Another limitation of this first-cut support is that if using multiple monitors you are forced to run in the highest power state or if your desktop resolution exceeds 1920 x 1200, this is to workaround complexities in re-clocking where scanning out from the video memory is more difficult to do in an effective manner.