What is SLI Technology ? A Brief Introduction

By   /   Apr 25, 2009

SLI technology by nVidia was the most anticipated technological change on PC front. Scalable Link Interface or SLI is a technology that lets user use two graphic cards to render a same image. It basically divides the work between two graphic cards by having each render different parts of the same image. SLI technology gives users the freedom to use two different GPU’s for the same output.Lets say you are playing GTA 4 at 1200 resolution, one card will render 600 lines of it and other would render the remaining 600, the division of labor can differ.

You can use  a special PCI express SLI ready motherboard to use the processing power of two different graphic cards on the same system.The implementation of this technology use PCI bus to share date between the cards, and uses varying rendering modes depending on which games we are playing.

Rendering Via SLI Configuration
nVidia has two main methods of rendering under SLI configuration, AFR or Alternate Frame Rendering and SFR or Split Frame Rendering.
AFR is an alternate frame rendering for example card 1 renders frame 24 card 2 will render frame 25, card one renders frame 26  card 2 will render fame 27 and so on. This is the preferred method of rendering under SLI configuration is it allows max performance benefits going from single card to dual cards.

The other rendering mode SFR  works by splitting the workload of each individual frame between two graphic cards. Each card works its part then sends the result from secondary card back to primary card and then combining it for the output on the screen.

This mode of rendering complicates the work a bit as it requires more channels to show the final output as compared to AFR so performance benefits may not be the same. To minimize this nVidia drivers do the dirty work of balancing the workload between the GPU’s and smoothing the timeframe in which each card renders its part by bringing them as close together as possible.

SFR might seem like not so wanted rendering mode but its important to know that some games work better in Split frame rendering mode and others work better in Alternate frame rendering mode. if you run  a game in not so wanted mode for the game it may cause crashes of the game or visual artifacts.

So nVidia drivers choose appropriate modes for different game to function them correctly.And there are times when SLI integration instead of increasing the performance rather decreases it.

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