Look for cards and motherboards with this logo to ensure compatibility
Author: Terren Tong
Editor: Howard Ha
Publish Date: Monday, January 10th, 2005
Originally Published on Neoseeker (http://www.neoseeker.com)
Article Link: http://www.neoseeker.com/Articles/Hardware/Features/nvslitesting/
Copyright Neo Era Media, Inc. - please do not redistribute or use for commercial purposes.
The announcement of SLI and the nForce 4 in the middle of 2004 generated a lot of talk in the hardware community. There were the skeptics who thought that it would remain a limited release product and be relegated to a niche solution but when pretty much every single major manufacturer is producing a SLI board it is hard to substantiate that claim. It is apparent that a product is making an impact in the mainstream segment - this past weekend when I walked into a store and overheard guys inquiring about SLI boards and when they can get a pair of video cards to go along with it. For those that are not familiar with SLI (Scalable Link Interface), the one line summary is that with certain PCIe motherboards (with dual x16 slots), a pair of identical GeForce 6 based video cards can be used in tandem to nearly double video performance - a more extensive look at SLI can be found here. This has several advantages - for the uncompromising hardcore enthusiast, this means they can take a performance leap into next-generation levels today. For mild-mannered Clark Kent and those of us that are constrained by a budget, this means that 6 months down the road when we are craving more performance, we do not need to shell out the big bucks for the ultra expensive high end card but can drop in another identical card to double performance.
Look for cards and motherboards with this logo to ensure compatibility
Currently, the only two chipsets that are SLI certified is NVIDIA's nForce 4 and Intel's Tumwater chipset. In our testing today, we will be looking at Gigabyte's GA-K8NXP-SLI and a pair of reference 6800GTs from NVIDIA along with Gigabyte's single slot SLI 6600GT solution in the 3D1 (review here).
It is important to note that SLI requires two of the same cards from the same manufacturer. This means that a 6800GT cannot be paired with a 6600GT. We have a reference 6600GT and a Gigabyte 6600GT that refuses to go into SLI mode even though the Gigabyte is just a reference board. Until there is a tool that allows a work around for this, end users should take care in choosing a card / brand that they are comfortable with as this is the only case where compatibility is more or less guaranteed.
The twins...
There are apparently several implementations of the motherboard SLI setup and the one from Gigabyte is the NVIDIA-approved solution. There is a bridge piece that looks sort of like a small stick of memory. One side disables the secondary PCIe graphics slot leaving the first slot in x16 mode. The second side splits the x16 lanes into a dual x8 configuration for SLI. The two graphics cards are connected with a small bridge piece (provided by the mainboard manufacturer).
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Along with the mainboard and graphics card, a SLI rig should be paired up with a very fast processor - the almost doubling in performance only applies to situations where the game is graphics bound, not CPU bound. In a CPU bound situation there will be no gain in performance.
The final piece to the equation is a quality power supply. We've had a bunch of low end systems with cheap power supplies blow out and it decided to take no prisoners - everything inside the computer was dead. Power requirements for even a single 6800GT and 6800 Ultra are quite hefty already so do spend the money for a reputable PSU.
The installation process for SLI is not all too different from a single graphics card but as mentioned, there are a couple extra steps.
This is PCIe lane changing mechanism. The SLI side needs to be inserted into the slot to enable the second PCIe graphics slot. I do not know if this is consistent across all boards, but the side that is inserted into the connector is the side that is enabled.
Insert the two graphics into each slot. Hook up the extra power connectors as necessary. Be sure to keep the two cards on separate external power rails so that there is not a huge draw on a single rail.
This bridge piece goes over the MIO connector of each card. The reason why this piece is included with the motherboard instead of the graphics card is that the physical SLI specification allows for variance in the distance between the two cards so it makes a lot more sense for the mainboard manufacturer who decides on the spacing to include the piece themselves. Note that there is a label on the bridge piece for SLI_1 and SLI_2. Each end is NOT identical so it is probably a good idea to insert it the right way.
The traces run to different pins on each side of the MIO connector
The new 66.93 drivers from NVIDIA include support for SLI. We got a set of 71.24 from Gigabyte for their single board dual 6600GT solution, the 3D1, which we took a look at earlier this week.
The final step is actually enabling SLI in the NVIDIA driver control panel. It does require a system reboot so for anyone finishing off their Nobel Prize winning paper, save first.
For relatively complicated technology, it is good to see that the setup and installation process remains straightforward for anyone who is comfortable enough to assemble their own system.
Athlon 3000+ (Winchester Core) @ 2461 Mhz (273.5 Mhz FSB)
Gigabyte GA-K8NXP-SLI (nForce 4 SLI chipset)
OCZ Platinum Edition PC4200 2*512MB Dual Channel Memory Kit
OCZ Modstream 520W Powersupply
NVIDIA GeForce 6800GT SLI
NVIDIA GeForce 6800GT
ATI RADEON X850 XT Platinum Edition
Gigabyte 3D1 (6600GT SLI)
NVIDIA Reference 6600GT
Western Digital WD1200 SATA HD
Windows XP SP2
DirectX 9c
Because we are finding it hard to justify forking out over a grand in Canadian dollars for a Athlon FX chip, we decided the next best thing to do was to spend mere pennies and get a 90nm Winchester core based Athlon 3000+. Actually it was a bit more than that but seeing as how we can get six of these as opposed to a single Athlon FX it seemed like the wiser decision especially since these things can overclock fairly well. From the slightly pokey default 1800 Mhz we managed to squeeze 2461 out of the chip which is certainly in the clock speed range of the Athlon FX. Again we will stress that end users who are looking at a SLI solution should look to mating it with a fast processor as the CPU will become the bottleneck.
Drivers consisted of the official 4.12s from ATI which seem to be a more recent build than the RC2 drivers provided for the X850 XT PE Launch. Drivers on the NVIDIA side are the 71.24s provided by Gigabyte and the same set was used in testing across all the NVIDIA cards.
For both the X850 XT PE and the 6800GT SLI set up we ran an additional set of benchmarks labeled as custom - ATI went with a 6x AA setting while NVIDIA has gone with something they call 8xS which is a combination of multisampling and supersampling. Both ATI and NVIDIA currently use a multisample technique which has been in place since the introduction of the GeForce 3. Supersampling was the classic brute force method of antialiasing where a higher than displayed resolution was rendered then subsequently scaled down. Because of the performance hit involved, it is not a technique that is normally used. Because of the different supported antialiasing settings and techniques, the benchmark set labeled CUSTOM is not directly comparable between the ATI and NVIDIA solutions.
Benchmarks
The features test on the 3DMark05 test suite highlights what SLI is capable of in extremely video dependent situations. In most cases we're at about a 90% increase of a single 6800GT.
Despite being fairly shader heavy, at stock resolutions, Aquamark 3 is almost CPU bound on the 6800 SLI setup. Everyone else has a much sharper rate of decrease. In Aquamark we're seeing the 6800GT SLI rig being able to handle 8xS AA / 16AF at faster framerates than the single 6800GT with just 4xAA / 8xAF.
At 1024, it looks like we are running into CPU bounds with the HL2 Video Stress Test though the X850 does manage to distance itself slightly compared to the 6800GT and the SLI setup. SLI flexes its muscles at 1280 and beyond.
* Note that the custom setting is not a direct comparison between the NVIDIA and ATI card as the antialiasing setting is different - 8xS mode on the NVIDIA and 6xAA mode on the ATI.
We're seeing practically no difference between the 6800GT and the 6800GT-SLI in Call of Duty across all resolutions. NVIDIA includes a load balancing feature to show the GPU load on each card but it would not display on Call of Duty. We do have fairly high framerates at 1280x1024 with the 8xS / 16x AF setting so those stuck on that resolution on LCDs may be interested in this as an alternative.
Some funny numbers on Jedi Knight 2 also. Load balancing showed very little work being down by the GPU at any resolution AA/AF setting. Looks like our benchmark is very bound and the 6800-SLI setup actually takes a big hit. The fact that we're seeing strange results in both Quake 3 engine games and that this also occured during testing of the 3D1 (the 3D1 was also slower here) leads us to believe that this may be a driver problem. We'll keep an eye out on this. Again we're getting fairly good framerates at 1280x1024 with 8xS / 16x AF so that is an option for LCD users.
* Note that the custom setting is not a direct comparison between the NVIDIA and ATI card as the antialiasing setting is different - 8xS mode on the NVIDIA and 6xAA mode on the ATI.
Halo is very shader dependent and the 6800GT-SLI and the 3D1 simply tear it up here. We're seeing about a 67% jump over the single 6800GT. The CPU does play a role here in limiting performance so we are not seeing the 90%+ like in 3DMark05 but it is still impressive nonetheless.
Our UT2k4 demo is also fairly heavily CPU bound with the current high end cards. We once again see the overhead that is involved with the SLI setup as the single 6800GT is a tad faster on several occasions. Framerates on the 6800GT-SLI setup stay excellent up to the 1280x1024 8xS / 16x AF setting before it plummets, likely due to memory constraints. We see this problem with the 3D1 as each GPU only has access to 128MB of memory and it takes a sharp nosedive at 1280 with 4xAA/8xAF like other 128MB cards.
* Note that the custom setting is not a direct comparison between the NVIDIA and ATI card as the antialiasing setting is different - 8xS mode on the NVIDIA and 6xAA mode on the ATI.
We're not seeing much of an advantage with SLI in Splinter Cell as the 6800GT and the SLI setup are pretty much neck and neck throughout all resolutions. Splinter Cell has a very small hit with AF and does not work with AA due to the rendering style.
X2: Rolling Demo once again shows the power of SLI. Even at 640x480 it is fairly graphics intensive and the SLI setup manages to put some distance between it and its competitors. The SLI setup holds a comfortable lead throughout all resolutions and graphics settings and manages to stay ahead of the X850 while running at a higher AA setting in the last benchmark series.
* Note that the custom setting is not a direct comparison between the NVIDIA and ATI card as the antialiasing setting is different - 8xS mode on the NVIDIA and 6xAA mode on the ATI.
DOOM 3 is arguably the most hardware strenuous of all games to come out this year especially on the video side. Even though DOOM is largely CPU bound at 1024, the SLI setup manages to pull up ahead. As the resolution increases the gap continues to widen. For the ultimate DOOM experience, a SLI set up is a necessity.
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We did three runs each with Fraps on a single 6800GT and the 6800GT-SLI setup on the CPU Core level. Basically all the enemies in this one area have to be cleared up before a door turns green and the player can continue on. We played this sequence through until the door turned green and stopped the benchmark. The pie charts above show how the framerates were distributed - red being under 30fps, orange being 30-40 fps, yellow 41-50 fps, blue 51-55 fps and green 55+ FPS. I think the results pretty much speak for themselves as to how much smoother gameplay is with the SLI setup compared to a single 6800GT.
Half-Life 2 is heavily CPU dependent for much of the game also. Again we do not see much of a difference between the 6800GT and the SLI rig until we start hitting the high res, high AA settings. The X850 XT PE deserves a special mention here as it manages to stay ahead of the SLI rig until 1600 with 4x AA / 8x AF. The SLI rig manages to keep up very high framerates throughout practically all resolutions with the exception of 1600x1200 with 8xS / 16x AF.
* Note that the custom setting is not a direct comparison between the NVIDIA and ATI card as the antialiasing setting is different - 8xS mode on the NVIDIA and 6xAA mode on the ATI.
Our 6800GTs managed to overclock to 385 / 1.08 from the stock 350/1.0 so overclocking is not necessarily hampered by SLI. We only managed to squeeze a few megahertz out of the 3D1 so we are fairly certain that it was a card specific limitation.
SLI is fast and in graphics intensive tasks, it certainly does not seem that NVIDIA is exaggerating about the nearly doubling in performance. In the 3DMark05 features test especially, the theoretical max for fill rate, pixel and vertex shading were very close to a 100% increase on the 6800GT pair. For the most part, games had pretty good framerates even when enabling 8xS antialiasing - this is important for LCD users that are limited with resolution choices. For games that are heavily graphics dependent such as DOOM 3 and Halo, the effectiveness of the second graphics card is very apparent. For older games, the differences are not immediately seen until higher resolutions paired with high AA/AF settings are used. This leads into a design philosophy question - two blockbusters this year, DOOM 3 and Half-Life 2 seem to have taken tangents. id has gone with pushing graphical boundaries while Valve has put more of the emphasis on processor power. If developers continue down the id route, the benefits of SLI at lower resolutions and settings will be more apparent as this is where the performance jump with SLI will be seen. On the other hand, if a game engine is more dependent on the processor side, the power of SLI will be seen at the higher resolution coupled with high AA/AF settings.
On the upgrade side, SLI provides some interesting options for the end user - they will have a choice to simply add a second card to increase performance at some time in the future or the traditional route of upgrading to a more powerful video card. The former is pretty compelling as prices tend to drop and adding a second card should be less cost prohibitive than paying for something that is outright more powerful. Something to note however is that the same dataset is loaded onto both cards which means the total amount of memory available is not a simple addition of the onboard memory size. This is something that may be more relevant as we have seen in DOOM 3, the amount of memory available directly affects the texture quality setting that can be used comfortably.
The entry fee to the SLI party will be what the end user puts into it. For those that have hit their blood and/or other body fluid selling quota for the month, a 6600GT and a SLI board is not a huge additional investment over a nForce 4 Ultra board. With several manufacturers set to introduce nForce 4 Ultra based boards capable of SLI including DFI, the price gap should continue to shrink between SLI and non-SLI capable boards. This does not imply that SLI for hardcore users will be cheap - a pair of 6800GTs or 6800 Ultras will go for roughly 800-1000$ though that is not too different from shelling out for the latest Athlon FX. We also suggest that end users who are interested in a pair of 6800GTs or higher to get an appropriately fast processor as the bottleneck will not be the video card in the majority of cases.
Like with the rest of the GeForce 6 family, NVIDIA has done an impressive job with SLI and is just riding more than just hype - they have the goods to back it up and literally bring next generation performance to the desktop today. In the span of roughly 8 months NVIDIA has managed to more than quadruple processing power in their GPU lineup. It almost puts the damper on next generation parts, but hey, they can be SLIed too. It is going to be a good year on the hardware side.
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