SLI

From Overclocking Wiki

SLI, an abbreviation for Scalable Link Interface, is the use of two separate, identical GPUs together for added performance. SLI requires two SLI-supporting graphics cards, a motherboard with a SLI chipset and the proprietary SLI bridge, and high-wattage power supply in addition to the usual required computer components. Note that SLI is for specific NVIDIA cards only; for multiple-GPU solutions with ATI cards, see Crossfire).

SLI logo

1 What are the benefits to SLI?
2 Disadvantages to SLI / Alternatives
3 Using SLI
4 SLI Profiles / SLI modes
5 SLI-Supporting graphics cards
6 SLI FAQs
7 External links

What are the benefits to SLI?

SLI allows two GPUs on two separate cards (in most cases, the 7950gx2 is an exception) to work together to give a theoretical 2x performance boost. While gains in most real-world scenarios are usually not as significant as the theoretical gains, they are still significant enough for most hardcore gamers to want SLI. Unfortunately, to run SLI you will need a motherboard with SLI support, two SLI-supporting graphics cards, and enough power and cooling for SLI. To ensure that any component you buy is fully approved for SLI use, you can look for the NVIDIA SLI logo.

Disadvantages to SLI / Alternatives

For some reason, many people believe that low-end SLI can outperform high-end cards with ease. This is not true. Most of the time, SLI is not even worth looking into unless you have the most current high-end card. For example, the 7900GT is about equivalent to 7600GT SLI, and a 7900GTX easily outperforms both. SLI is generally excessive, especially as-of today when you can buy an 8800GTX which will outperform any 7xxx series SLI configuration. For ATI users, Crossfire is the SLI configuration. Unfortunately, Crossfire is not as easy to set up as SLI because you have to have a hardware-specific master and slave card, whereas SLI has an identical master and slave that have identical or similar hardware. SLI is usually overkill for gaming systems also, especially at low resolutions. Because the human eye can distinguish only about 60 fps (not to mention vsync restricts fps to 60 anyways), SLI really only shows huge visible differences in high resolutions or in games with intense detail. Another negative of SLI is cost; most of the time you can buy an outperforming single card for less than two lower cards in SLI. Therefore, it is only very useful to those who wish to upgrade over time or to those who want to top the charts in all benchmarks with the highest-end cards in SLI.

Using SLI

Once you have purchased a SLI supporting motherboard, an adequate power supply, two SLI-Supporting graphics cards, and the rest of the basic components you will need to create your computer, begin assembly. Construct the computer normally, plugging every component in as usual. When you install the graphics cards, put one in each of the PCI-E slots and connect the SLI Bridge to the SLI connectors on each video card.

SLI bridge

Next, you will need to enable SLI on the motherboard. The steps to enable SLI vary with each motherboard, but you may have to change SLI jumpers or enable SLI in the BIOS. Since enabling SLI varies with the motherboard, look in the manual for specific instructions. Once you have loaded and installed Windows, enable multi-GPU mode in the video card drivers advanced properties section. You are now ready to game.

SLI Profiles / SLI modes

NVIDIA's drivers have pre-made profiles for many mainstream games in SLI. Each profile has specific settings to work best with the game. For some games, you may need to create your own profile. Usually you can find a profile by searching online, as someone else will probably have made one for the game you want to play. In order to create or modify profiles, you must download Coolbits.

There are four methods of SLI rendering, three of which combine GPU for higher FPS and one for higher image quality:

Split Frame Rendering. This method allocates half of the processing of each frame to each GPU.

Alternate frame rendering 1 and Alternate frame rendering 2 have each GPU do every other frame. This mode produces a bit less than double the performance of an individual card because data from the slave card has to pass through the master in order to be outputted. Some games do not support this mode.

SLI Antialiasing has each frame is processed on both cards through separate antialias filters for a better image.

Each profile runs one of the three first modes, then can run a mode of SLI Antialiasing or not. That way, you can adjust your settings for optimal framerates and highest quality based on the game you are playing and the hardware you are running.

SLI connector

SLI-Supporting graphics cards

The following cards support SLI (list courtesy of NVIDIA.com):

NVIDIA GeForce 8800 GTX
NVIDIA GeForce 8800 GTS
NVIDIA GeForce 7950 GX2
NVIDIA GeForce 7950 GT
NVIDIA GeForce 7900 GTX
NVIDIA GeForce 7900 GT
NVIDIA GeForce 7900 GS
NVIDIA GeForce 7800 GTX 512
NVIDIA GeForce 7800 GTX
NVIDIA GeForce 7800 GT
NVIDIA GeForce 7600 GT
NVIDIA GeForce 7600 GS
NVIDIA GeForce 7300 GT*
NVIDIA GeForce 7300 GS*
NVIDIA GeForce 7300 LE*
NVIDIA GeForce 7100 GS*
NVIDIA GeForce 6800 Ultra
NVIDIA GeForce 6800 GS
NVIDIA GeForce 6800 GT
NVIDIA GeForce 6800
NVIDIA GeForce 6800 XT
NVIDIA GeForce 6800 LE
NVIDIA GeForce 6600 GT
NVIDIA GeForce 6600*
NVIDIA GeForce 6600 LE*
NVIDIA SLI-Ready Quadro GPUs

* indicates that the card does not use the SLI bridge. Also note that SLI is only supported on the PCI Express interface. AGP does not support SLI.

SLI FAQs

Q: Can I use dual-monitors with SLI?
A: Simply put, no. The hardware is fully capable, and if you disable SLI, you can even use quad monitors with your dual video cards! But, the drivers that NVIDIA has released simply don't support dual monitors.

Q: Can I use my 7600GT with my 7900GT for SLI?
A: No. You have to have two cards with the same GPU.

Q: I have an XFX 7900GT and an eVGA 7900GT, can I use SLI with different brands of card?
A: Yes. As long as they have the same GPU.

Q: What if the cards have different clock speeds, RAMs speed, or quantities of of RAM?
A: Both cards default to the lowest speed or quantity and will still run SLI. You do need to use Coolbits to do so though.

Q: Is there such thing as quad-SLI?
A: Of course, there is 7950GX2 SLI, and also the Gigabyte Quad Royal for four independent cards. Most users will never need quad-SLI performance though.

Q: What about oct-SLI?
A: In theory, you could put four 7950GX2's in a Quad Royal, but there is no driver support. Therefore, it is not possible.

Q: Why am I seeing little or no performance gain on my SLI setup?
A: Make sure you have SLI enabled properly. You also should be sure there are no bottlenecks in other places in your system. Almost every game will see a significant boost in fps because of SLI.

Q: My system is unstable, what can I do to solve this?
A: Make sure your cards are not overheating and that your cards are getting enough power. Test each independently and make sure both are fully functional. Be sure you have the most recent drivers too.

Q: My 8xxx series card has two SLI connectors on it, what do I do?
A: The second is for future use (possible triple / quad SLI full support). Simply place the provided SLI bridge on the same connector on both cards and you will be fine.

Q: I see a green line while gaming, how do I get rid of it?
A: Uncheck the GPU-load balancing box in your graphics properties.

Q: Some cards claim to have SLI support, while others say they are SLI ready. What is the difference?
A: There is really no difference. A card either supports SLI or is incapable of running SLI. A good way of telling whether or not a card can run SLI is by seeing whether or not it has a SLI connector at the top of the card.