Multi-GPU World Tour 2006, Part 3 - Choosing the right motherboard

INTRODUCTION

Earlier this year Bjorn3d and a bunch of other websites were approached by NVIDIA regarding the possibility to cooperate on a big article testing the best Multi-GPU systems from both NVIDIA and ATI. As it was NVIDIA asking us about this it was natural that we were a bit hesitant at first even though the whole idea sounded exciting and something we would love to do. However, after NVIDIA assured us that the controll over the content in the articles were entirely up to us we decided to join.

THE PAST AND FUTURE ARTICLES

Just so you don't miss any of the parts already released or that are getting released, here is a list of all the parts in this article series.

• Introduction - PenstarSys - Read the article
• GPUs - bit-tech - Read the article 
• Motherboards - Bjorn3D - Your reading it .... 
• Common Benchmarks - HardInfo - Read the article
• Xfire most played - NeoSeeker - Read the article
• Uncommon Benchmarks 1 - NVNews - Read the article
• Uncommon Benchmarks 2 - Legit Reviews - Read the article 
• Uncommon Benchmarks 3 - Rage3D - Read the article 
• The Verdict - Guru3D - Read the article 

MOTHERBOARDS - THE FOUNDATION OF A MULTI-GPU SYSTEM

When building a complete system the choice of the components of course is very important. Many concentrate on picking a good CPU and video card but forget that the road to a stable and fast system starts with the motherboard. If you plan on building a system for two video cards the choice is pretty simple; a motherboard using NVIDIA’s SLI-technology or a motherboard using ATI’s Crossfire technology.

Some of the motherboards on test today.
I'm actually fed up with them now ...

The goal with this article is to give you a few suggestions on what motherboards to get when building your Multi-GPU system as well which CPU's to go with them.

SLI VERSUS CROSSFIRE

Bit-Tech already has covered the ins and out of Crossfire and SLI in their second part of this article series so I suggest you go read it if you have not already. What follow here is a short recap on the respective technologies.

As long as you only plan to use a single video card it really doesn’t matter which chipset your motherboard has, whether it is VIA, AMD, ATI or NVIDIA. The moment you start thinking of adding another card it becomes much more complicated. Now you have to first decide whether you want to buy NVIDIA video cards or ATI video cards and then pick the appropriate motherboard.

The first commercial enthusiast solution with multiple cards was released by 3dfx many years ago. They allowed you to hook up two Voodoo2 video cards for double the performance. These cards were not hybrid 2D/3D-cards so you still had to have a 2D video card in your machine. While an expensive solution it still worked well and was pretty popular, at least among the hard-core gamers.

After 3dfx’s demise (bought out by NVIDIA) nothing happened on the Multi-GPU front for common users for many years. Then in 2004 NVIDIA revealed SLI (Scalable Link Interface). While it has the same acronym as 3dfx technology it works in a different way.

SLI

In short SLI works by letting each video card work in parallell on the frames. There are several methods that can be used. AFR (Alternate Frame rendering) gives each card a frame to render before outputting alternate frames from each card to the screen. SFR (Split Frame Rendering) on the other hand lets the cards work in parallel where each card renders half of the frame. The frame is divided dynamically so that the work-load is shared equally between the two cards. This means that sometimes one card will render 70% of a frame while the other card renders 30%. The parts are then synchronized and then sent to the screen. The dynamic load balancing is done by the Forceware drivers.

SLI works with basically every new NVIDIA PCI-E video card. You can mix and match cards from different brands but not different models. To enable SLI you need a SLI-bridge that connects the two cards. This is provided with the motherboard. Then you just hook up your monitor to one of the cards which will act as a master.

SLI bridges. I like ASUS approach with a flexible bridge.

Games need a profile in the drivers to benefit from SLI. If the game you play do not have a finished profile you can manually create one in the controlpanel for the ForceWare drivers.

Crossfire

Crossfire has a bit different requirements than SLI. First of all you’ll need a Crossfire Master card (or at least in some cases you do – more about that later). This card has an extra compositing engine chip which handles the task of putting the image together from the two video cards. The cards are not connected through a bridge, instead you use a cable that hooks up to both cards and then to the monitor. Very little traffic is supposed to be going over the PCI-E bus with this method.

ATI uses a cable for its high-end video cards when enabling Crossfire.

Just as with SLI there’s a bunch of different ways an image will be handled by the two video cards. Super Tiling splits the frame up in small tiles where every other tile is rendered by each card. Scissors splits the frame up in two parts where each card handles one parts of the frame and Alternate Frame Rendering renders alternative frames on the two cards.

One advantage with ATI’s Crossfire is that on some level it works with any game without any need for special profiles. However - ATI still are working on adding game profiles since the default picked method. Super Tiling, doesn't work well on all types of games. Unfortunately there isn't any easy way for the user to create own profiles for games (you can rename the game-exe but that hardly is considered 'easy') which means that gamers are stuck with the rendering method the drivers decide regardless if it works well or not.

I mentioned that you need a Crossfire Edition master card in the beginning of this section. This is only partly true now. ATI has released updated drivers that allow you to hook up some models without the need of either a Crossfire Edition master card or a special cable. The data is instead transfered over the PCI-E bus. So far it is only the X1600 Pro that supports cable-free Crossfire but ATI has promised to include the X1900GT in that list soon.

Crossfire allows you to hook up not only different brands but even different models of cards. Thus you can in theory hook up a X1800XT and a X1600XT if you wanted. Basically what happens is that both cards will run at the slower speed and specifications. In the end it still makes much more sense to hook up two cards of the same model.