The X Window System, Version 11, is the standard window system on Linux and UNIX systems. X11, designed in 1987, was “state of the art ” at that time. From its inception, X has been a network transparent window system in which X client applications can run on any machine in a network using an X server running on any display. While there have been some significant extensions to X over its history (e.g. OpenGL support), X’s design lay fallow over much of the 1990’s. With the increasing interest in open source systems, it was no longer sufficient for modern applications and a significant overhaul is now well underway. This paper describes revisions to the architecture of the window system used in a growing fraction of desktops and embedded systems 1

Performance was an important issue in the development of X from the initial protocol design and continues to be important in modern application and extension development. That X is network transparent allows us to analyze the behavior of X from a perspective seldom possible in most systems. We passively monitor network packet flow to measure X application and server performance. The network simulation environment, the data capture tool and data analysis tools will be presented. Data from this analysis are used to show the performance impact of the Render extension, the limitations of the LBX extension and help identify specific application and toolkit performance problems. We believe this analysis technique can be usefully applied to other network protocols. 1