Today we have a very interesting review. Quite a long time ago, when platforms with PCI-Express support had only started to appear, NVIDIA announced that its High-End video cards, and then Middle-End as well (GeForce 6600GT) could work in pairs. And considering that all patents and trademarks of the former 3dfx are a property of NVIDIA, there was no need in inventing a name for the new technology, it was decided to use a well known abbreviation – SLI. But! This is not the Scanline Interleaving that we saw in a combo of two Voodoo2 cards. SLI means Scalable Link Interface.
SLI for Voodoo2 distributed operations between the two cards PER LINE. That is odd lines were processed by the first video card, even lines – by the second. This was hardcoded in the driver. The data (processed lines) to be assembled in the buffer of the primary video card was transferred via the VGA-LOOP cable synchronized by a special SLI adapter looking like a piece of a floppy cable.
NVIDIA SLI operates differently. Like in the latest multi-chip solutions from 3dfx, the pair is formed by a Primary (Master) card and a Secondary (Slave) card. A special buffer is allocated in a master card to assemble the image. But we have written much about operating methods of multi-chip solutions, so we shall not repeat ourselves.
Note that there is a special unit inside the corresponding chips (NV45, NV43), which is responsible for SLI, working with shared resources of the two video cards. The data is exchanged via a bus and synched via a special SLI connector plugged to both cards (see the photos below).
Theoretically, SLI can operate in two modes:
The second mode is not operable so far (to be more exact, it cannot be enabled), so let's consider SLI as dividing a single frame between video cards.
At present SLI is officially supported by NVIDIA GeForce 6800 GT/Ultra PCI-E (NV45) and GeForce 6600GT (NV43). Though, as I have already mentioned above, all video cards based on NV45/43 should theoretically support this mode. So the lack of a MIO port for a SLI connector on the 6600/6200 PCB is a purely marketing move, and thus we can safely assume that later on there may appear cheap cards up to $100 with SLI support.
We have a lot of questions concerning how SLI operates in present 3D environments from the point of view of rendering resources. In old days of Voodoo everything was more or less clear: interleaved operations, just divided the operations (trivial thing) in half and that's all. Geometry and illumination (in case of vertex one) calculations were up to CPU.
Now everything is different. Just take shader operations, which are used to process object surfaces (roughly speaking). What about objects located on the borderline between areas of responsibility of the two GPUs? Should both processors calculate the same shader and do the redundant work? Or should one GPU calculate the entire object and pass half of the data to the second one? - It's not physically possible. There will probably be some performance losses for overheads on the borderline objects between the areas of responsibility of the first and the second GPU.
From bad to worse: how will adaptive anisotropy work in cases when more data is required than available in one GPU? It's a murky secret so far, we are going to investigate further into the matter. One thing we know for sure: geometry calculations and texture loading are duplicated by both cards.
Today we are interested in practice. What's in all of it for us.
In practice one will have problems with availability of... no, not cards or SLI connectors (though the latter are obviously in deficit so far). But of the platforms with two PCI-Express x16/x4 (x8/x8) slots. You know that motherboard manufacturers have not yet released such motherboards based on i9xx for desktop and game systems, and now you can see this in action only in server solutions based on i75xx. That's why SLI will bring no joy to the Pentium4 camp, until the x16+x4 or x8+x8 layout will be designed for two sterling video slots. Either VIA or SIS will probably announce revisions of their solutions with SLI support for P4, but we know nothing about this so far. Having got the license for P4 bus, NVIDIA is obviously preparing a corresponding nForce with SLI support for this platform as well, as nForce4 is already officially released for AMD Athlon64/FX/Opteron.
Or nForce4 SLI + AMD processor of the latest generation or an almost server platform based on i75xx with one or two Intel Xeon CPUs. It's clear that the first solution will be much cheaper.
Will SLI be an additional AMD trump card for promoting its solutions among Hardcore Gamers? It's hard to say so far, because nForce4 is not yet properly available (only early samples). Besides, as I have already said, SLI connectors are not to be found on sale yet and they are not shipped with video cards so far. And we have information that they will not be shipped later either. Only vendors selling motherboards based on nForce4 SLI will include such adapters into the bundle.
Thus, we are approaching the chapter devoted to testing video cards. As it's clear from the title of the article, we shall test only Intel platforms today. Readers should understand that this solution is not designed for games, and even Xeon 3.4 GHz together with a powerful motherboard demonstrated that the performance dropped even lower than previously on Pentium4 3.2GHz (i875P) (registered DDR333 memory is much slower than well-known DDR400). Nevertheless we got very interesting results.
I should note that the main SLI requirement for today is the complete IDENTITY of video cards forming the SLI pair. I have tested several GeForce 6600GT cards from various manufacturers and I can say that only video cards from Leadtek and Palit managed to work together, they were correctly detected by the driver and organized into SLI. In other cases the driver reported that one of the two video cards is not SLI compatible. Besides the manufacturers mentioned above, the tests included Gainward and Gigabyte.
To make the experiment pure, we took pairs of reference cards, which are certainly identical within the pair.
We used the following PAIRS of video cards:
Installation and Drivers
VSync is disabled.
A pair of GeForce 6800GT PCI-E video cards
A pair of GeForce 6600GT video cards
Note: do not be surprised to see different CPU fans on the photos. As the standard fan was so noisy that it caused headaches (heavy whistling at high frequencies), we had to remove it and temporarily install a regular fan from a power supply unit, which successfully blew over the copper heatsink without any noise, the latter being almost cool (why tower this 7000-rpm noisy fan on this heatsink?).
And at the end of this chapter I can add that when SLI is detected, a new tabbed page appears in driver settings offering to enable SLI and an option to show load of each GPU (you can see it clear on the screenshot of driver settings). The latter (show GPU load balancing) is a horizontal stripe, which is approximately 1/3 of the screen below, overlaid above all applications with GPU load indicators – the left one is for the first GPU and the right one is for the second GPU. So you can see how the load is distributed between them. However, deviations from 50/50 were rare and did not exceed 40/60 or 60/40 in all cases.
Test results: performance comparison
We used the following test applications:
If you want to get the demo-benchmarks, which we use, contact me at my e-mail.
Serious Sam : The Second Encounter
Unreal Tournament 2004
Unreal II: The Awakening
The results are similar to the previous ones.
TR:AoD, Paris5_4 DEMO
I can't believe it... Wow! I repeat that geometry calculations and texture loading are carried out by both cards (duplication). But anyway, perfect sharing of shader calculations among the cards yields an impressive effect.
The same picture! Only in 6800 Ultra the SLI gains are a tad lower because of system resource limits. But anyway, 90% of performance gain in Far Cry – that's a worthy result! However the i7525 platform costs dear.. :)
Call of Duty, ixbt04
HALO: Combat Evolved
The same picture. The weaker the card is, the higher the SLI effect is.
Half-Life2 (beta): ixbt07 demo
The same picture. I guess, no need in comments.
Quake III Arena
No need in comments.
Return to Castle Wolfenstein (Multiplayer)
The same picture.
Excellent results in all the cards! It goes without saying that we mean the AA+AF mode. Though in 6600GT the SLI effect is impressive even without this mode.
The same to previous.
The same to previous.
Pay close attention to the FPS in time graph. Note the strong oscillations of FPS due to SLI in this test, especially in the first and middle parts. A lot of objects are located here falling into the jurisdiction of both GPUs. And the driver switches between processor operations very fast resulting in instant FPS drops. This may well be because of the untuned drivers and one GPU is periodically idle.
Do you need more comments? :) I think you don't.
So, I guess it's perfectly clear that the SLI technology is effective. It even provides excellent performance gains where system resources do not lay severe limits. All this happens even despite that one PCI-Express slot operates as x16, and the second – only x4 (there are almost no other options, only x8+x8 is theoretically possible)).
But there is also a fly in the ointment so far. During our tests we sometimes witnessed artifacts (ripples), which, by the way, obviously demonstrate half-frame operations (between GPUs). There occurred color distortions sometimes. That is NVIDIA still has some issues to fix in the drivers.
I have also tested SLI with GeForce 6600GT + GeForce 6800GT. It won't work. To be more exact, it works with glitches and problems.
On the whole one can say that SLI made its debut. In the second part of this material we'll analyze test results for nForce4 SLI and only then we'll draw our conclusions – how it fares on more gaming-like platform. Though untuned drivers for nForce may have their effect there, because drivers from Intel are well known for their stability and fine-tuned characteristics, especially system ones.
I think that if the market offers more motherboards with two PCi-E slots, SLI will definitely be successful, even stunningly successful.
In our 3Digest you can find more detailed
comparisons of various video cards.
Theoretical materials and reviews of video cards, which concern functional properties of the GPU ATI RADEON X800 (R420)/X850 (R480)/X800XL (R430)/X700 (RV410) and NVIDIA GeForce 6800 (NV40/45)/6600 (NV43)
Andrey Vorobiev (firstname.lastname@example.org)
December 21, 2004
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