Even the initial performance comparisons make clear that SLI requires a very powerful CPU. In SLI operation the GeForce 6800 GT and Ultra cards, fast in their own right, really show their stuff and even speedy CPUs are not able to supply the cards with data fast enough. The true strength of SLI can only be seen in high resolutions of 1280x1024, or even better 1600x1200 with FSAA and anisotropic filters, i.e. in scenarios that place today's graphics cards under maximum load. In lower resolutions without FSAA and AF, SLI offers few advantages. With some games, SLI operation even has a slowing effect (UT 2004), possibly caused by the increased administrative burden that SLI operation places upon the CPU. We did, however, detect a decline in performance with SLI in Flight Simulator 2004 as well versus a single card, despite the fact that FS2004 cannot run at all in SLI mode, or put another way: even if only one card of an SLI system is operating, performance falls below that of a computer with just a single graphics card installed. Not by much, but by a measurable amount.
Here is an example using the game Unreal Tournament 2004. We tested a GeForce 6800 GT in 1024x768, without FSAA and AF. Both with and without SLI, the CPU is not able to exhaust the graphics cards' performance - the cards are therefore CPU-limited. The more powerful the CPU, the higher the performance:
- SLI enabled (2 cards): 157.2 FPS
- SLI disabled (2 cards): 163.2 FPS
- Single card (1 card): 163.4 FPS
While the phenomenon is still explainable with UT2004 and other games (possibly higher CPU load due to increased SLI administrative burden), we do not understand why it also occurs in games that are not at all SLI-capable and those in which only one card actively renders even with SLI activated (Flight Simulator 2004). In OpenGL games like Call of Duty though, the problem is not as pronounced.
In shader-intensive games like Doom3 and Farcry, the SLI performance boost outbalances the drawbacks in these quality settings and resolutions.
The game itself also has a big impact upon the effectiveness of SLI. Games with older 3D engines such as Call of Duty benefit from SLI only at the highest resolutions and quality settings. The performance is already so high even without SLI that it is not worth the trouble and expense. Modern games that make extensive use of shaders like Doom3 or Farcry, on the other hand, benefit from SLI even in lower resolutions.
To do away with speculation on the observed performance decline in lower resolutions with SLI, we analyzed the performance of a single card with both x8 PCIe and x16 PCI. The results were identical.
And one last measurement in UT2004 without the SLI connector hooked up, 1600x1200 4xAA/8xAF:
- With SLI connector (2 cards): 153.3 FPS
- Without SLI connector disabled (2 cards): 133.0 FPS
- Single card (1 card): 101.1 FPS