NVIDIA's Scalable Link Interface: The New SLI
by Derek Wilson on June 28, 2004 2:00 PM EST- Posted in
- GPUs
Scalable Link Interface
As we first saw during Computex this year, the enigmatic NV45 had a rather odd looking slot-like connector on the top of the card. We assumed that this connector would be for internal NVIDIA purposes, as companies often add testing and diagnostic interfaces to very early hardware. As it turns out, this is NVIDIA's Scalable Link Interface connector.
Notice the gold connector at the top of the card.
In order to make use of NVIDIA's SLI technology, two NVIDIA cards are placed in a system (which requires 2 PCIe x16 slots - more on this later), and the cards are linked together using a special piece of hardware. Currently, this communications hardware is a small PCB with a slot connector at each end. No pass through cable is needed, and one video card acts as the master (connected to the monitor) and the other is the slave.
SLI PCB top view.
SLI PCB bottom view.
When asked whether it would be possible to connect the cards together with something along the lines of a cable, NVIDIA indicated that the PCB approach had afforded them superior signaling qualities, but that they continued to look into the viability of other media. As this is new technology, NVIDIA is slightly weary of sharing some of the lower level details with us. We asked whether their SLI uses a serial or parallel interface (usually fast parallel interfaces are more sensitive to signal routing), but we were told that they may or may not be able to get back to us with that information. Either way, this is going to have to be a very high bandwidth connection as it's over this path that the GPUs will communicate (this includes sending framebuffer data for display).
As previously mentioned, this setup requires having 2 PCIe x16 slots available on one's motherboard. Not only is this going to be difficult to come by in the first few months of PCIe motherboard availability, but currently, none of Intel's chipsets support more than 24 PCIe lanes. The current prototypes of motherboards with two PCI Express x16 slots are actually only using one PCI Express x16 interface and one x8 interface, simply with an x16 connector (so it's physically an x16 slot, but electrically, an x8 slot). This reduces the bandwidth available to the 2GB/s up and down (which is still more than AGP 8x can handle). That's not to say that PCIe bandwidth is necessary for gaming at the moment. The real problem is that there would be no other PCIe slots available for expansion cards. But x1 and x4 PCIe expansion cards haven't been making many waves, so until chipsets support more than 24 PCIe lanes and more PCIe expansion cards come out, it might be possible to get away with this.
NVIDIA Quadro connected in SLI configuration.
Until now, we've just mentioned NV45 as supporting this, but NVIDIA seems to be indicating that all their PCIe cards will have the capability to run in SLI configurations. This includes the Quadro line of workstation graphics cards. This is very interesting, as it shows NVIDIA's commitment to enhancing performance without degrading quality (CAD/CAM professionals can't put up with any graphical artifacts or rendering issues and can always use more graphics power).
But let's move on to the meat of the technology.
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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.