Improving the performance of VNC for high-resolution display walls

A tiled display wall comprises several computers and projectors for a total of twenty to one hundred megapixel displayed over a whole wall. It is well suited for collaboration because multiple users can easily fit in front of it, simultaneously viewing much more information and interacting with many more applications than what is possible using standard-sized displays. We use a virtual network computing (VNC) server with a desktop at 22 megapixels for our display wall. A VNC viewer runs on each of the display wall computers displaying a tile each of the total desktop. However, the frame rate at the display wall becomes very low because of performance bottlenecks in the computer running the VNC server, and because of not enough network band width. The applications also suffer because they must run at the same computer as the VNC server. We report on an effort to identify the performance bottlenecks, and how we reduced them. First we profiled, measured and compared the performance of two existing implementations of the VNC model, TightVNC and RealVNC, when playing back a 3 megapixel and a 7 megapixel video. Then we selected the best performing implementation, TightVNC, and modified it by using the Intel SSE2 instruction set to speed up data movement, and by using assembler language to speed up the encoding of the pixels. These techniques improve the frame rate from 8.14 to 10 for the low resolution video, and from 4.3 to 5.9 for the high resolution video. While this is indeed a noticeable improvement in practice, and contributes to making VNC more effective in some of the collaborative settings we discuss, we conclude that more fundamental changes are needed to approach 25 FPS and make a display wall using VNC even better suited for collaboration.