After many fruitless attempts to get an SLI system running we found out the reason: our cards were too "old". Our models were very early NVIDIA reference samples that according to the manufacturer do not yet function properly with SLI. As these cards had not been in stores for long anyway at the time we received them, no particular importance should be attached to this matter. With the latest 6800 Ultra and GT samples SLI worked right off the bat. That of course raises the question of whether it is at all possible to later purchase a second graphics card for SLI operation, perhaps even from a different card maker. At present we are unable to answer that question.
We also had big problems with the SLI motherboards. With the ASUS A8N-SLI it completely blew the BIOS every time we restarted the computer - a problem we encountered not only with our test board....The source of the problems turned out later to be a series of defective Bios components. A quick change-out made possible by the socket mount took care of the problems. After that the A8N-SLI Deluxe ran with no problems and at a good speed.
The MSI K8N Diamond, on the other hand, had major memory module problems with SLI operation. Our Kingston HyperX, set up for 2/2/2/6 DDR 400 timings, caused constant crashing.....Only after setting the timing down to 2.5/3/3/7 did the MSI K8N Diamond run stably with SLI in operation. Our test sample also appears to have a problem concerning memory bandwidth. Even with 2/2/2/6 timings, Sandra displayed a memory bandwidth of only 4.9 GB/s, corresponding to only DDR 333!
We also had to lower the memory timings for SLI operation somewhat for the ASUS board as well, as crashes occurred here too with 2/2/2/6 timings. With 2/3/3/6 timings the board then ran stably. We anticipate and hope that these problems are a result of the motherboards' revision status and will no longer occur with models that reach the stores. ASUS and MSI jostled each other over the past few weeks to be the first to introduce a working SLI motherboard. Under such conditions, problems are pre-programmed. What is striking though is that most of the instability occurred only in SLI operation.
After several benchmark tests we noticed some relatively slow performance in two of our games. After turning on SLI HUD in the driver we saw that SLI was not active during these games. Even our efforts to force SLI operation through the expanded driver settings had no effect. After talking with NVIDIA we learned the cause: SLI is not available with some games. NVIDIA has so-called SLI Profiles for games that are defined in the driver. The driver recognizes the game via application detection and executes the SLI mode (split or balancing) designated for that profile. If no SLI profile exists for a game, there is no SLI rendering. It is not possible to force SLI mode or generate your own profile. According to NVIDIA however the driver already contains over 50 profiles for games running with SLI. For newer titles this therefore means that SLI system owners have to wait for a new driver. But even then there is no gaurantee
NVIDIA has said it intends to set up a dedicated SLI homepage listing all games that are SLI-compatible. There is also supposed to be a list to be found there of games not compatible with SLI. We'll be glad when we see that.
NVIDIA thought up a trick for the nForce 4 chipset. When operating a graphics card, all 16 lanes are available to the slot. In SLI operation however the 16 lanes are simply distributed among the two x16 slots. So the PEG slots are only operated with x8 PCIe....In practice that has no negative effect on performance, as the x8 PCIe bandwidth still corresponds to AGP 8x performance.
according to ASUS problems can arise when using an ATX power supply with 20-pin main power connector with 2 graphics cards. In the manual, ASUS asks for an ATX 2.0 power supply with 24-pin main power connector despite the additional power connector....ASUS recommends a 450Watts or even better PSU, to drive a SLI system with Athlon FX 55 and GeForce 6800 Ultra - which means at least 281 Watts just for these items!
The Northbridge cooler remains the only downer. Experience has shown that these components have only a limited lifespan. A purely passive cooling system would make more sense.
According to information provided by MSI, using a 20-pin ATX power cable is possible however as long as at least 18A are provided along with the 12V power supply. These reveals are in opposite of what we heard from ASUS, which claim that a SLI system needs much more power than that. We'll have a closer look at that and report our findings.
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.
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.....In shader-intensive games like Doom3 and Farcry, the SLI performance boost outbalances the drawbacks
And lastly, one final test we just couldn't say no to. Even back in the days of Voodoo2, users had a choice of whether they would rather play 3D games via graphics card or a separate Voodoo2 card. So why not deploy a Radeon X800 XT PE alongside a GeForce 6800 Ultra
It was to be expected: an SLI computer with GeForce 6800 Ultra or GT beats everything for performance! The performance reserves of an SLI system are tremendous. Even with an Athlon64 4000+ the cards almost always have to wait on the CPU.
the power is what really kinda has me second thinking...
