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 The theory and practice of using of technology NVIDIA SLI 

The theory and practice of using of technology NVIDIA SLI

There are two key concepts of development of progress of mankind.They are extensiveness and intensity. Extensive and intensive ways of development of a society, technologies, and all mankind in general replaced each other throughout many centuries. There are a lot of examples in all spheres of economic activities. But agriculture and cattle breeding arent interested us today,we are inerested in information technologies and computers. For example, we will examine the development of the central processors.

Let's not go deeply in the early 80s-90s, we will begin at once with Pentium 4. After the long intensive development of technology NetBurst, escalating of "steroid" megahertz, the industry has rested against a wall it has become very difficult to disperse one-nuclear processors further and to receive much more powerful processor, the great expenses are necessary. And two- nuclear processors  have appeared. It was the extensive decision of a problem. And soon they have ceased to provide the necessary productivity, and the new architecture Core appears which at smaller clock frequencies gives more productivity - the intensive decision of the problem. And it will go on further.

And what happens with the development of the second component, important for the fan of entertainment games on the personal computer - a videocard?It happens the same with the processor. At this stage of development,one chip  video cards cannot provide the nessesary productivity any more. Therefore both leading market companies of video cards (NVIDIA and AMD) develop and actively introduce their  own technologies increasing productivity of a videosystem by combining of two, three, and even four video cards in one personal computer. In this article we take the first company (because it has presented the working out the first) NVIDIA and its technology SLI.

What is technology SLI?

Technology NVIDIA SLI is a revolutionary method to scaling of graphic productivity by combining of several video cards NVIDIA in one system.


In 1998 the company 3dfx has presented graphic processor Voodoo2 among which had the innovating technology SLI (English- Scan Line Interleave changing of lines) which assumed teamwork of two chips Voodoo2 on image formation. With the  technology SLI cards of various manufacturers, and cards with a different memory size could even work The SLI-system allowed to work with the permission to 1024x768, that at that time it seemed improbable. The high price  of accelerators ($600) and the strong thermal emission had been lacks SLI from 3dfx. However video cards had been transferred from bridge PCI to faster allocated graphic AGP-port. As on motherboard this port was only one release of video cards with support SLI on a short time stopped.

In 2000 with release of new chip VSA-100 3dfx it has become possible to realise SLI on AGP, but this time two or four similar chips are placed in one board.

However boards on the basis of SLI-system had anormous consumption of energy and failed because of problems with power supplies. It has been sold about 200 pieces of boards Voodoo5 6000 in the world but only 100 pieces of them were really working. So unsuccessful advancement of the promising accelerator which had been invested much money in, has led actually to the  bankruptcy of the company. In 2001 NVIDIA buys 3dfx for 110 million dollars.

In 2004 with the publishing of the first decisions on the basis of new bridge PCI Express NVIDIA declares its support in the products of technology multichipped data processing SLI which has already been deciphered in another way - Scalable Link Interface (the scaled interface).

Firstly the advancement of technology SLI promoted not so smoothly, first of all because of constantly revealed defects in drivers, and also because of necessity to "sharpen" them under each concrete application or the player did not receive any advantage of purchase of the  pair of accelerators. But generations of accelerators were being changed, the drivers were being improved, the list of supported games was extended. Finally, at the end of 2007 the technology of Triple SLI has been placed in operation, allowing to unite in a sheaf of 3 video cards NVIDIA:

The manufacturer promises a gain of productivity to 250 % in comparison with the single chip. But it did not become a limit, Quad SLI is appearing now, allowing to force to work over visualisation of a game scene already 4 GPU, but in pair of  structure of two chip video cards.

Algorithms of construction of images

 Split Frame Rendering

The scheme of algorithm Split Frame Rendering

It often used a mode when the image breaks into some parts which quantity corresponds to quantity of video cards in a sheaf. Each part of the image is processed by one video card completely, including geometrical and pixel components. (Analogue in CrossFire - algorithm Scissor)

Alternate Frame Rendering

The scheme of algorithm Alternate Frame Rendering

The processing of shots occurs gradually: one video card processes only even shots, and the second - only odd. However this algorithm has a lack. The reason is that one shot can be simple, and another one is difficult for processing. Besides this algorithm, is patented by ATI during the release of two chip video cards. 


The present algorithm is aimed at image improvement of quality. The same picture is generated on all video cards with different templates of smoothing. The video card makes smoothing of a shot with some step concerning the image of other video card.Then the received images are  mixed up and deduced. This way the maximum clearness and detailed elaboration of image is reached. The following modes of smoothing are accessible: 8x, 10x, 12x, 14x, 16x and 32x. (Analogue in CrossFire - SuperAA)

Construction principles

To construct the computer on the basis of SLI it is necessary to have:

  • a motherboard with two and more sockets PCI Express, supporting technology SLI (as a rule, at the name of a motherboard there is a word SLI).
  • powerful enough power unit (usually it is recommended from 550 Watt ),
  • video cards GeForce 6/7/8/9/GTX or Quadro FX with tyre PCI Express;
  • the bridge connecting video cards.

It should be mentioned, that support of chipsets for working with SLI is carried out software instead of hardware. But video cards should belong to one class, thus version BIOS of boards and their manufacturer dont have the principal meaning.

At present,the technology SLI is supported by following operational systems:

  • Windows XP 32-bit
  • Windows XP 64-bit
  • Windows Vista 32-bit
  • Windows Vista 64-bit
  • Linux 32-bit
  • Linux 64-bit (AMD-64/EM64T)

The SLI-system can be organised in two ways:

  • By means of the special bridge;
  • By program

In the last case the loading on tyre PCIe increases, that influences badly on productivity, thats why this way has not received a wide circulation. Such mode can be used only with rather weak accelerators.

If the bridge SLI is not established, the driver gives out the prevention, that mode SLI will not work in full force.

Here is a picture of the special bridge for the connectin of videocards when they are made on textolite. Besides the flaxible bridgers are also widespread, because they are cheaper in their manufacture. But for technology activation 3-Way SLI the rigid special bridge is only used, which (in fact) has itself three ordinary bridgers in ring mode.

As it has been told before, system Quad SLI is also popularised It assumes the association in a single system of two two-chip boards. . Thus, it turns out, that 4 chips participate in image construction, however the destiny of inveterate enthusiasts and for ordinary users doesnt have the practical value. Another thing is very important for the ordinary user, thanks to it the present technology has been thought.. Buying a motherboard with support SLI, you have a good reserve for personal computer upgrade in the future as it gives the possibility of addition of one more videocard. No other way of "upgrade", except full change of system, even close gives that gain of game productivity which is provided with the second video card.

It looks like this. You buy the good personal computer with a good modern video card.You play modern games easily, but in a year-one and a half your system cannot satisfy you with quality of the image any more and speed in last games. Then you instead of replacement of an old video card (or even all the personal computer) simply add one more and receive almost doubled capacity of a videosystem.

However it sounds simply in the theory. And practically the user should face set of many difficulties.

The first one is games and applications. From a terrible word "optimisation" anybody has not yet thought up a medicine - yes, not only drivers, but also games should be optimised for technology SLI to work with it correctly. NVIDIA declares, that technology SLI supports the longest list of games. If a game interesting you is not present in the list, the company suggests to create own profile of options for it.

Unfortunately, there are still  many games which are not compatible with SLI, and profile creation does not correct it in any way. Ther is only one decision- to wait for patches from developers of games and new drivers from the company NVIDIA. However these games are practically old and the single modern video card can solve the problem with any problems or a little popular which are not interesting to the overwhelming majority of players owing to bad "gameability". And the new powerful games have already provided of using  of two video cards. For example, in the game Call of Duty 4 in the section of options of a drawing there is a quite concrete switch.

About inclusion of function SLI you can learn from the special indicators directly in the game, corresponding "test" adjustment of the driver.

If on the screen of the monitor you see horizontal or vertical green strips, it means, that a mode to Show visual indicators SLI is switched on in control panels NVIDIA Display. Switching on  this option it is possible to see, how the graphic loading on GPU is distributed, and names of SLI components in your system will be changed: SLI for two GPU will be named SLI, the name of 3-way NVIDIA SLI will be replaced on SLI x3, and Quad SLI will be named Quad SLI. For games which visualisation technology Alternate Frame Rendering (AFR) are used, the vertical green strip will grow or decrease depending on expansion scale. For the games which  use  the visualisation technology Split-Frame Rendering (SFR) is used, the horizontal green strip will rise and fall, showing as loading between all GPU is distributed. If the level of detailed elaboration of the top and bottom half of screen is identical, the horizontal strip will be close to the middle. To connect or disconnect this possibility, simply open control panel NVIDIA and choose Options 3D.

The second widespread problem of calculations by several graphic chips is synchronisation of the last. As it has been told before, the method of rendering AFR has one feature: one shot can be simple, and another one can be difficult for processing.Having the big difference in a shot complex processing one video card  can process much faster it than the second one. It leads to "micrologs" - small delays of the image, like jerks, but the general number FPS looks comfortable. Such effect appears seldom enough, but how to struggle with it even developers do not know. There is a simple decission for the users - in games where "micrologs" with method AFR are being controlled, change method AFR with method SFR, having specified it in a game profile.

In mode SFR the display is divided in two parts. The first card gives out the top part of a picture, and the second - bottom. Thanks to dynamic balancing of loading the driver regullary distributes loading between two cards.

The third problem is a processor. The reason is when using of several video adapters, loading on the processor increases not only from using of heavy graphic modes, but also from  synchronisation of video chips. Therefore, to open all potential of a sheaf from several video cards you need the powerful processor. Though nowadays it is not so thorny question, as it was a couple years ago.

Additional functions of new technology

The new generation of motherboards and video cards with support SLI give users of much more functions, than usual acceleration of a three-dimensional drawing.

Hybrid SLI provides more rational use of a mode of sharing of the graphic kernel which has been built in a chipset, and a discrete video card. 

The technology consists of two parts: GeForce Boost and HybridPower.

The first one will be involved in the applications working intensively with a 3D-drawing. The accelerator will take itself the part of calculation of a three-dimentional scene with GeForce, which is integrated into a set of a system logic that will give an appreciable increase in productivity to those systems where not so fast discrete adapter is established, for example GeForce 8500 GT or GeForce 8400 GS.

HybridPower, particulary, will allow to use the built in drawing, disconnecting the external accelerator when the user works in the Internet, office applications or watches video.The greatest benefit, according to NVIDIA, from the technology the owners of laptops with the allocated video card will receive, time of their independent job considerably will increase.

Now Hybrid SLI it is supported by desktop GPU GeForce 8500 GT and GeForce 8400 GS for organisation GeForce Boost; productive GeForce GTX 280, GeForce 9800 GX2, GeForce GTX 260, GeForce 9800 GTX +, GeForce 9800 GTX and GeForce 9800 GT in mode HybridPower; and also system boards for processors AMD on nForce 780a, nForce 750a, nForce 730a (only function GeForce Boost) with built in graphic accelerator GeForce 8200.

Obviously, the economy of the electric power can reach impressing values.

There is a very useful  function for the overlocks because it allows the basic video map to "have a rest" at the moment of idle time and to prolong its service life, especially if extreme dispersal is used (exept economy of the electric power).

One more useful additional feature of a SLI-sheaf is the possibility of using to 4 monitors simultaneously.

Practical application

Weve already tested various video cards in mode SLI many times. Let's try to sum up the received information a little.

In a case with video map XpertVision GeForce 8800 GS 768MB Sonic we have got the effective job of mode SLI almost in all games that you will not tell about synthetic tests. The speed gain has been swept up everywhere and its average makes from 20 to 60 %. In heavy games of type Crysis or Company of Heroes with high options of a drawing the speed gain has made up to 90-100 %.  

More powerful video map GIGABYTE GeForce 8800 GT 512Mb also has shown excellent results in productivity scaling in mode SLI. The technology has proved itself from the best sides almost in all our tests. The synthetic tests, which were limited by our processor, showed their insensibility, respects by the used processor, but in any way it doesnt worry us, because the user should play comfortably games, instead of looking at"parrots" in 3DMark. The productivity gain in this case has averaged 20-60 %, however, unlike the previous videocard, it remained at a mark of 60 % - there were no sharp jumps of productivity transitting in very heavy graphic modes.

We move further. We will consider the test of much more powerful video card ASUS EN8800GTS/HTDP/512M/A.

The situation is similar to the previous. The average gain makes 20-60% and splashes are absent.

But videocard NVIDIA GeForce9800 GX2 which  has the SLI-system inside, has shown a productivity gain of all to 30 % in compare with the one chip analogue.

And this card uses 1 socket PCI affects and has twice smaller throughput for data to exchange with the system. Besides, for such accelerators the optimisation of drivers are still proceeding.

3-Way SLI

Our review of video map GIGABYTE on NVIDIA GeForce 9800GTX 512 MB can be an example of using of the present mode.

We will remind once more, according to the recommendations of the manufacturer, correct connection of adapters of feeding will be the important condition of reliable job of video cards. For connection Molex-connectors it is necessary to use various lines of a food 12V,other wise the power unit can not sustain loading that can lead to constant failures in job or even its exit out of operation.

As for results of test applications, they are ambiguous. In some cases the productivity gain really makes the promised 150-170 %, however such applications are rare (in case of our testing just one). The average gain makes 60-120 % that is no doubt higher than 20-60 % using two video cards.

Let's sum up results

First of all, it is necessary to notice, that the greatest gain of productivity is observed at average level video cards, in a price range 150-250$. And, the more hard the graphic mode and permission, the more advantage is brought by the second video card in system. And to tell the truth it is the real victory! NVIDIA has achieved the main purpose- correctness of "upgrade" of the computer by buying of the second video card.In this case the user just also receives +100 % of productivity promised in advertising. Do you agree to play Crysis at 41 FPS instead of 22 is much more pleasantly? The general peculiarity for all examined video cards was that the peak efficiency of sheaf SLI was reached in those games where pixel-shaders are plentifully used.

For the advanced users there is an occasion to be glad too. On videocards so -called Performance class (in a price category 250-350$) the productivity gain to 60 % allows to use the heaviest graphic modes on monitors from above 24 without any losses in speed. Technology SLI will allow to transform the computer into the present game centre. It is also necessary to note the economic feasibility of application SLI in this segment - usually the buying of such 2 video cards is cheaper than the buying of the most exellent decission. As for High-End class application SLI is justified only in the presence of very powerful processor - but for the provided users for whom speed at any cost is important, it is not a hindrance, the productivity gain on 30% will not be nessesery ever.

In any case - the technology works, but it doesnt work  as it was a couple years ago  and it really provides the acceleration almost in all popular games. It is necessary to give due NVIDIA which could debug the working out to the status which can be recommend the ordinary users. If you search for a motherboard for the new personal computer we recommend you to pay attention to the boards with the support of several video cards. And whether to establish in your system two videocards, you can dicide later. This choice is stronly influenced by a current situation in the market of videocards. The price is a principal battle-field of 2 video cards of middle class and TOP PRODUCTS. It also is a final argument.

As for technology 3-way SLI. As it has been noted in our review, a bottleneck in a sheaf of three powerful video cards is the volume of the memory established in them . Technology is arranged in such a way that their memory sizes are not summarised and all three graphic chips use a memory size of a single video card (in case of our test there were 512 MB). But it is possible to consider this lack partially solved because today there are the new video cards GTX280 on sale which memory size is 1GB. Dont forget and about video cards where the manufacturer establishes a bigger memory size than the previous model has. However it is not all yet. Except these three video cards we  require also a very powerful processor.

Taking into consideration the prospective monetary investments for acquisition of three video maps and the most powerful processor, it becomes clear that everyone can not permit to estimate the possibilities of 3-Way SLI. Therefore the technology hardly becomes popular, and, perhaps, it reminds an ostentatious marketing course of company NVIDIA which purpose is the desire to draw attention of buyers and to keep a superiority palm tree.

About the additional functions - Hybrid SLI. They, certainly, are not superfluous, but we have not seen the real advantage of technology GeForce Boost.Think a litlle, if the user buys to himself the inexpensive computer for office job then the modern integrated video card will be enouph for all needs as they have even hardware support of decoding of HDTV-video. And if suddenly there will be a desire to play games then the buying of a video card of class GeForce 8500 GT or GeForce 8400 GS will look, at least,funny , as their productivity is not enough for modern games, even taken together with the integrated adapter.

But the function HybridPower has much more potential. As it has happened, it successfully works with one, two and even with three videocards. In view of the fact as each generation of video cards consumes more and more electricity, and also in view of constant increase of tariffs for this, saving energy technologies become very actual. Well and users of laptops should rejoice twice - with technology HybridPower in the laptop it becomes possible the association of incompatible concepts: a powerful video subsystem and a long time of independent job.

Advantages of technology SLI:

  • lightness of an upgrade without need to sell an old video map;
  • possibility of activation of the improved modes of smoothing,
  • possibility of using to 4 monitors simultaneously.


  • the motherboard supporting SLI is necessary
  • the power unit of corresponding capacity is necessary
  • 2-3 video cards take more places in the system case
  • occurrence of effect of "micrologs" in an AFR-mode

Authors: Peter Nosik, Alexander Chernoivan

By article preparation the information from official site NVIDIA and free encyclopaedia Wikipedia was used.

: 30.09.2008
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