How to Choose SLI or CrossFire

Gamers demand performance from their gaming PCs. Over the last few years, games have become so graphically power hungry that video cards are an exploding area of development and a huge part of gaming PC cost.

No longer can 1 GPU with 256MB of memory deliver the visual fidelity “power gamers” expect. Try turning on full-screen anti-aliasing, full shadows and anisotropic filtering in the latest blockbuster and watch your frame rate crawl.

How does a PC gamer cope? Well, aside from buying prohibitively expensive CPU and RAM upgrades you should consider adding multiple video cards to your system to help.

We cut through the hype and explain what the multiple-GPU landscape of SLI and CrossFire really means to you and how to choose the best solution to match your hardware and budget.

Table of Contents

Official Disclaimer:
The technology used in graphics processing evolves in 6-10 month intervals and some of this information may become dated before the article receives an updated version. Be sure to consider the date this article was published.

Step 1: Assumptions
This “how to” is aimed at giving casual PC gamers a basic understanding of the multi-GPU technology and the options offered by the two leading performance graphics chipset makers Nvidia and AMD/ATI. While it will mention some specifications and describe the technical concepts it is not meant to be an exhaustive educational piece on specific chipsets.

Part 2: Why Use Multiple-GPUs to Play Games?
Farcry is a game that was well known among gamers a few years back as a title whose graphic engine pushed the limits of available hardware at the time. A few years later, not surprisingly Crysis (a game using an upgraded version of the same engine) made gamers rush out to buy video cards with the best specs, most power and largest memory to try and approach the fluid beauty the game promised in its deep rendered shadows and extremely high resolution textures.

By and large as a gamer, the multi-GPU question centers on how well do you want your cutting edge games to run. Theoretically and in the promises of both ATI and Nvidia running multiple GPUs can increase the performance and visual fidelity of games by a huge percentage. Games can be run at maximum resolution, with all video options enabled at over 60 frames per second. Theoretically.

The reality is a little less satisfying. Games that recognize multiple GPUs will utilize them but few gamers have the right combination of hardware and software to make all their games run fast and smoothly. It’s a good idea to research your favorite games and see if it supports multiple GPUs out of the box. The next step is to find out which one it works best with – Crossfire or SLI.


Step 3: How Does SLI work?
SLI or Scalable Link Interface is a technology developed originally by graphics company 3dfx. This tech allowed multiple video cards to be linked together to share the processing load of graphics calculations and rendering.

When 3dfx was acquired by Nvidia the technology was overhauled and refined and the current version works only with PCI-Express x16 based graphics cards and motherboards. The video cards work in a master-slave setup and share the work of rendering a 3D scene thanks to parallel processing. SLI also requires Nvidia-based motherboard chipsets to be compatible.

One video card will draw a portion of a scene while the other will complete the task on the remaining part. How this happens and which part is rendered by each card is determined by the SLI mode the system or program is set to use. There are four primary modes of SLI processing, three for rendering and one to make anti-aliasing faster:

  1. Split Frame Rendering (SFR): The graphics cards divide up the scene 50/50 and work on the top or bottom respectively.
  2. Alternate Frame Rendering (AFR): The graphics cards render a series of images working on either even or odd frames. This method scales better when dealing with geometry and intense workloads in rapidly changing scenes.
  3. Alternate Frame Rendering of Split Frame Renders (AFR on SFR): This is a rendering mode only available on Quad GPU configurations where the SLI system will split the frame and then divide the odd and even scenes to the second set of GPUs before rendering them all together and displaying them.
  4. SLI Antialiasing: The graphics card work in tandem to produce smoother images by rendering a slight offset to the pixels and combining them. This mode is not intended to make visual rendering faster. It is focused on making image fidelity and clarity higher, resulting in slower frame rates but better looking images.

There are multiple SLI configurations in use today from two GPUs/Video Cards to Tri(3) and Quad(4) card setups. The main limitations are that video cards used in an SLI configuration must use the same type of video chipset. They can be from different manufacturers or even individually overclocked but the chipset must be the same. SLI video cards do not have to have the same amount of video RAM, but the system will never use more RAM than the smaller cards have available on any SLI video card.
Step 4: How Does CrossFireX/CrossFire Work?
CrossFire is ATI’s multi-GPU solution. Created as a reaction to Nvidia’s SLI it takes a slightly different approach to how it achieved the parallel processing of 3D video frames to achieve faster rendering. Unlike the SLI architecture Crossfire is a bit more liberal with the hardware requirements. GPUs used in Crossfire do not have to be the same, just in the same chipset line.

CrossFire requires PCI-Express x16 slots on the motherboard and will work with AMD/ATI or Intel based chipsets.

CrossFire splits the load of rendering 3D scenes like SLI but since it supports “unmatched” cards in a configuration it handles the workload a little differently. There are four rendering methods in Crossfire configurations:

  1. Scissor: Similar to the SLI Split Frame Rendering but since Crossfire cards do not have to share the same GPU chip or speed this method uses dynamic load balancing to determine which video card has enough free power to render the scenes fastest and sends the work to that card. Effectively the faster card will render more of the split than the slower one.
  2. SuperTiling: This mode sub-divides each part or tile of the screen into smaller parts measuring 32×32 pixels each. The work is then balanced to favor the faster card which will do more work to render scenes quickly.
  3. Alternate Frame Rendering(AFR): This mode functions the same as the SLI cards, each card renders and alternating series of screens. Load balancing is not a factor so slower cards will slow down AFR rendering in Crossfire configurations.
  4. Super AA: Effectively this mode works the same way as the SLI version – and allows up to 14x antialiasing.

Crossfire also supports two and four GPU configurations (under CrossFireX).
Step 5: What are the Differences Between SLI and Crossfire?
SLI and CrossFire solutions often achieve the same results however the primary difference between the two is flexibility. Nvidia’s SLI relies upon similarity to achieve performance. SLI needs a SLI certified motherboard and video cards must be the exact same GPU to be used together.

ATI’s CrossFire allows similar GPUs within the same generations (i.e. HD 5750 and HD 5770) and works using AMD/ATI and Intel motherboard chipsets. ATI relies on its Catalyst A.I to render CrossFire profiles, when no profile is available it defaults to Scissor and cannot be changed. Nvidia’s SLI controls allow users to customize the method used to render applications as needed.

Step 6: When is One Solution Better than the Other?
Which solution is best for you will depend on a few things.

  1. Do you already have a preference for video cards?: A lot of gamers get religious over their video card experiences and pick sides. If you have a favorite then you probably should stick with it.
  2. Are you buying a new system or using existing parts?: Money is always a factor but if you’re working from a budget and are not building from scratch then consider the capabilities of your existing choices. SLI and CrossFire are comprable price wise for the top of the line, however ATI’s ability to support different GPUs can save you some money.
  3. How comfortable are you with crashes and driver issues?:Multi-GPU setups are not for the weak. While ATI and Nvidia have pretty solid communities of users to help you there are often configuration problems with new games and new video drivers from both manufacturers. Traditionally ATI has a better track record of late.
  4. Do you use multiple monitors?:Both solutions offer support for multiple monitors but ATI’s Eyefinity technology is truly impressive. One caveat though – as of the time of this writing – the ATI Catalyst 10.5 drivers are not playing well with Eyefinity on some Radeon HD Crossfire configurations.
  5. Which configuration do most of your favorite games support?:You are making this jump to multi-GPU configurations to improve your gaming performance.. it makes no sense to spend the money and do the work if your favorite game is not compatible with one of the two technologies. Most modern games will support either but you should research by checking out the game forums or forums at AMD/ATI or Nvidia and asking. Use the Internet, it’s here to help sometimes.

Regardless of your ultimate choice, playing games with multi-GPU solutions like SLI and Crossfire can totally alter your expectations. Having the freedom and power to crank up the visuals and maintain a reliable frame rate really helps make it seem like you’ve gotten your moneys worth and having bragging rights never hurts either.

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