NVIDIA's Scalable Link Interface: The New SLI
by Derek Wilson on June 28, 2004 2:00 PM EST- Posted in
- GPUs
Final Words
In our conversation with NVIDIA, we noted that this technology should work with all software without any modification necessary, and NVIDIA was quoting numbers around 1.87x performance increase over a single card (under 3dmark '03). Of course, this may be an exception rather than the rule, and we do want to run our own tests (as impact will change over graphical complexity). Even more intriguing is the fact that NVIDIA talked about developers being able to take advantage of this technology in their games. It was indicated that, by doing so, games could see a linear (2x) performance increase. If realized, this would be unprecedented in real-world applications of this type of technology.In speaking about why multiple graphics card configurations haven't seen more light in the recent past, NVIDIA mentioned that the PCI bus was the limiting factor in being able to execute something like this on modern hardware. At this point, they still have an unbalanced bandwidth situation due to the x8 bandwidth limit on secondary x16 PCIe slots, but that's nothing like the difference between PCI and AGP (especially as full PCI bus bandwidth can't always be guaranteed to one device).
And then there's the problem of finding motherboards with multiple PCIe slots. The only one that we've seen so far is a multiprocessor board, which hasn't been released yet. To be fair, NVIDIA is targeting the system builders first, and won't be pushing a consumer SLI upgrade package until later (possibly this fall). The success or failure of this product will likely not rest on its technical merits, but rather on the availability of suitable motherboards, and the cost of the upgrade. We can see some hardcore gamers out there spending $500 on a card. We could see some even going so far as to upgrade their entire PC if it meant better performance. But it is hard for us to see very many people justifying spending $1000 on two NV45 based cards even for 2x the performance of one very fast GPU; perhaps if NVIDIA cripples some of its GPUs to work only as slaves and sells them for a (very) reduced price as an upgrade option. Even then, this isn't going to have as easy a time in the marketplace as the original 3dfx SLI solutions.
Now, what would really work in NVIDIA's favor is if they engineer their NV5x GPUs with backwards compatible SLI technology. Even the promise of such a thing might be enough to get some people to pick up an NV45 with the knowledge that their future upgrade would actually be an upgrade. For those who upgrade every generation, there would be a way to get more power for "free", and those who wait 2 or more generations to upgrade might have a new reason to take the plunge. This scenario is probably wishful thinking, but we can dream, can't we?
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SpeekinSfear - Tuesday, June 29, 2004 - link
I also prefer NVIDIA 6800s over ATI X800s (Especially the GT model) but I requiring two video cards to get the best peformance is an inconsiderate progression. They're even encouraging devs to design stuff specially for this. It almost makes it like they cant make better video cards anymore or else like they care enough to try hard. Almost like they wanna slow down the video card performance pace, get everyone to buy two cards and make money from quantity over quality. NVIDIA better easy up if they know what's good for them. They're already pushing us hard enough to get PCIe*16 mobos. If they get their heads to high up in the clouds, they may start to lose business because no one will be willing to pay for their stuff. Or maybe Im just reading too much into this. :)Jeff7181 - Tuesday, June 29, 2004 - link
I thought it was a really big deal when they started combining vga cards and 3d accelerator cards into an "all-in-one" package. Now to get peak performance you're going to have two cards again... sounds like a step back to me... not to mention a HUGE waste of hardware. If they want the power of two NV4x GPU's, make a GeForce 68,000 Super Duper Ultra Extreme that's a dual GPU configuration.NFactor - Tuesday, June 29, 2004 - link
NVIDIA's new series of chips in my opinion are more impressive than ATI's. ATI may be faster but Nvidia is adding new technology like an onchip video encoder/decoder or this SLI technology. I look forward to seeing it in action.SpeekinSfear - Tuesday, June 29, 2004 - link
DerekWilsonI get what you're sayin'. I just think it's crazy! I try to stay somewhat up to pace but this is just too much.
DerekWilson - Tuesday, June 29, 2004 - link
SpeekinSfear --If you've got the money to grab a workstation board and 2x 6800 Ultras, I think you can spring for a couple hundred dollar workstation power supply. :-)
SpeekinSfear - Tuesday, June 29, 2004 - link
Im sorry but I thought lots of people were having a hard enough time powering up one 6800 Ultra. Either is absurd or I dont know something. What kind of PSU are gonna need to pull this off?TrogdorJW - Monday, June 28, 2004 - link
The CPU is already doing a lot of work on the triangles. Doing a quick analysis that determines where to send a triangle shouldn't be too hard. The only difficulty is the overlapping triangles that need to be sent to both cards, and even that isn't very difficult. The load balancing is going to be of much greater benefit than the added computation, I think. Otherwise, you risk instances where 75% of the complexity is in the bottom or top half of the screen, so the actual performance boost of a solution like Alienware's would only be 33% instead of 100%.At one point, the article mentioned the bandwidth necessary to transfer half of a 2048x1536 frame from one card to the other. At 32-bit color, it would be 6,291,456 bytes, or 6 MB. If you were shooting for 100 FPS rates, then the bandwidth would need to be 600 MB/s - more than X2 PCIe but less than X4 PCIe if it were run at the same clockspeed as PCIe.
If the connection is something like 16 bits wide (looking at the images, that seems like a good candidate - there are 13 pins on each side, I think, so 26 pins with 10 being for grounding or other data seems like a good estimate), then the connection would need to run at 300 MHz to manage transferring 600 MB/s. It might simply run at the core clockspeed, then, so it would handle 650 MB/s on the 6800, 700 MB/s on the GT, and 750+ MB/s on the Ultra and Ultra Extreme. Of course, how many of us even have monitors that can run at 2048x1536 resolution? At 1600x1200, you would need to be running at roughly 177 FPS or higher to max out a 650 MB/s connection.
With that in mind, I imagine benchmarks with older games like Quake 3 (games that run at higher frame rates due to lower complexity) aren't going to benefit nearly as much. I believe we're seeing well over 200 FPS at 1600x1200 with 4xAA in Q3 with high-end systems, and somehow I doubt that the SLI connection is going to be able to push enough information to enable rates of 400+ FPS. (1600x1200x32 at 400 FPS would need 1400 MB/s of bandwidth between the cards just for the frames, not to mention any other communications.) Not that it really matters, though, except for bragging rights. :) More complex, GPU-bound games like Far Cry (and presumably Doom 3 and Half-life 2) will probably be happy to reach even 100 FPS.
glennpratt - Monday, June 28, 2004 - link
Uhh, there's still the same number of triangles. If this is to be transparent to the game's then the card's themselves will likely split up the information.You come to some pretty serious conclusions based on exactly zero fact or logic.
hifisoftware - Monday, June 28, 2004 - link
How much CPU load does it add? As I understand every triangle is analyzed as to where it will end up (top or bottom). Then this triangle is sent to the appropriate video card. This will add a huge load on CPU. Is this thing is going to be faster at all?ZobarStyl - Monday, June 28, 2004 - link
I completely agree with the final thought that if someone can purchase a dual-PCI-E board and a single SLI enabled card with the thought of grabbing an identical card later on, then this will definitely work out well. Plus once a system gets old and is relegated to other purposes (secondary rigs) you could still seperate the two and have 2 perfectly good GPU's. I seriously hope this is what nV has in mind.