NVIDIA's SLI - An Introduction

[Steve]

The launch of NVIDIA's SLI is almost upon us, and today we bring you an introduction. We take a look at SLI and just what it does, including split-frame and alternate-frame rendering. We get up close with the PCIe interface used for SLI and the the Inter-GPU connector. What happens if you suddenly lose the Inter-GPU link between the SLI'd graphics card? We have screenshots and a video!

Of course, all that juicy information in itself is exciting enough, but we have even more for you! There's benchmarks galore, including 3DMark05, Doom III, Half-Life 2 and Far Cry.

Currently, SLI allows you to accelerate rendering using two effective modes, which map to around four general cases of multi-GPU rendering. The first, SFR or split-frame rendering, has each graphics card render a portion of the screen, split horizontally. One card, the primary device on the PEG host, is responsible for the top section of the screen, the secondary device therefore responsible for the bottom. That situation is then load balanced. More on that shortly.

The second mode is AFR, or alternate-frame rendering. For each graphics device in the system, the driver round-robins the graphics commands and data to each device, forcing each device to render a frame in turn. For a dual-GPU SLI setup, each device renders a frame, waits for a frame, then renders a frame, ad infinitum. Interleaved with the second device doing the same, that's AFR in a nutshell.

Check it out here.

We have more coverage coming over the next few days, so keep checking back as we bring you more.

[directhex]

doesn't seem worth £800 of graphics cards just at the moment

[Lee H]

I agree DirectX but the 6800GT do look good value when linked together as do the newer 6600GT cards

I expect that many systems will have 2 x 6600GT cards as they are cheaper than all the rest and 2 of these perform on par with the 6800GT and are cheaper to boot.

[Steve]

My plan is to build an SLI capable system next month/jan with a 6800GT. Then pop another in sometime next year.

[Lee H]

good plan Kez

Thats what a lot of people are doing and what I did back in the voodoo 2 days of gaming.

[DR]

My plan is to knock one together as soon as I have a spare 939 cpu

[Butuz]

My plan is to build an SLI capable system next month/jan with a 6800GT. Then pop another in sometime next year.

A good idea, but will it work? If the card needs to be identical, what are the chanced of things not moving on in 6 months time? I'm not sure whether trying to second guess the whole industry is a good idea

Butuz

[Steve]

I'm sure I'll find a solution to that Butuz. I mean, I can probably do a bit of BIOS flashing if I really need to.

[imashination]

Is there any chance of running a cinebench test on it? only takes 5 minutes and id love to see how sli affects professional 3d software.

www.cinebench.com

My database of results is up at www.3dfluff.com.mash/cbogl.php

Thanks in advance.

[Dazz]

I am thinking of getting 2x 6600GT cards to replace my 6800GE AGP card, just a cheap.....isg SLI rig for now and wait for the next gen video cards

[Rys]

Is there any chance of running a cinebench test on it? only takes 5 minutes and id love to see how sli affects professional 3d software.

www.cinebench.com

My database of results is up at www.3dfluff.com.mash/cbogl.php

Thanks in advance.

Certainly I'll run it in a short while.

Rys

[imashination]

Ah, sorry, my database is:

http://www.3dfluff.com/mash/cbogl.php

I had a slash in the wrong place, i didnt get arrested though ;-)

Oh, when you run the test, make sure you use the clipboard feature to get the CB scores, the framerates are useless information because it dynamically makes the test harder or easier depending how well it does.

[Rys]

I can only currently get SLI, using 4 different SLI modes, to give me about 4% more speed on the OpenGL render tests in Cinebench 2003. I've got a score set saved using overclocked 6800 GTs that'll go 3rd in your database.

Let me know if you want to submit still, even though SLI buys you very little with the current drivers.

Rys

[imashination]

yes please, this data is useful whether its at the top or not. If you could mention the mode and driver verion ill add that to the notes as it seems it will have a large impact as they continue to update the drivers.

Perhaps its also possible that completely dissabling the SLI mode would speed it up? I was anticipating a HW opengl score or around 5000 with such a setup

[Widget]

I have just built an SLI system myself and I notice in the review it mentions SFR and AFR modes. I have gone through all the settings boxes I can find on the graphics cards and all I can find is the profiles for the games. The only option I can find is for single or multi GPU rendering. How do you switch between AFR and SFR mode?