It is going on two years since support for Scalable Link Interface (SLI) was introduced into NVIDIA's Linux binary display driver. This support had come a year after it was officially launched and supported by the Windows ForceWare display driver. As we had seen at the end of 2005 with two GeForce 6 graphics cards in SLI, its performance was very sluggish, and there were a number of problems to be found with Linux SLI. While we have routinely tested new NVIDIA graphics cards under Linux SLI internally, there hasn't been much to report on as the experience has been very foul. However, things have changed recently and with the recent NVIDIA 100.14.19 display driver release using GeForce 8 hardware -- we finally have some modest numbers to report on in a Linux SLI configuration. Linux SLI is still far from perfect, but in this article we've used two GeForce 8600GT graphics cards in an SLI configuration under both Linux and Windows to compare the single and dual GPU performance under both operating systems.
Support for Scalable Link Interface was one of the major highlights that came with the Linux 1.0-8XXX display driver series. SLI can be enabled from the nvidia-xconfig utility, but to this point there remains to be seen any dedicated control panel for SLI profiles or options under Linux (aside from enabling the SLI HUD in nvidia-settings). Under Linux we had run SLI in AFR and SFR modes. AFR is for Alternate Frame Rendering mode where the two GPUs switch between rendering each frame. On the other hand, SFR is Split Frame Rendering where the screen is split horizontally and each GPU takes a side. There is also an AA mode for SLI that distributes the anti-aliasing workload, but that was not tested independently since it's designed for CPU bound environments.
Due to issues with SLI, the only benchmark we are using in this article is Quake 4. However, once we are permitted to distribute Enemy Territory: Quake Wars benchmarks, we will be delivering those results as well. With Linux SLI only benefiting when running at the native X resolution in full-screen mode, we had run all of our benchmarks at 1680 x 1050. We had run each of the graphics tests in a single card configuration as well as running in AFR (Alternate Frame Rendering) and SFR (Single Frame Rendering) modes. On Windows we had used the automatic mode for SLI. On Linux we had used the NVIDIA 100.14.19 display driver while when running Microsoft Windows we had used the NVIDIA 162.18 WHQL release.
The hardware had consisted of two NVIDIA GeForce 8600GT graphics cards with an ASUS P5N-E SLI (nForce 650i SLI) motherboard, 2 x 1GB OCZ DDR2 Reaper HPC memory, SilverStone 750W PSU, and Seagate 7200.10 SATA 2.0 hard drive. Software-wise we had used Fedora 7 32-bit with the Linux 18.104.22.168 kernel and for our Windows tests was Microsoft Windows XP Professional SP2 32-bit.