X Compositing for Free

Current Linux desktop environments incorporate window-system level compositing to present a rich user environment. The process of merging application windows together comes at a steep cost though -- each pixel on the screen will have been copied multiple times before landing in the scanout buffer, and each of these copies consumes memory bandwidth and power. Some desktop environments provide special case optimizations for full-screen windows, or offer quick ways to disable and re-enable compositing. Neither of these is particularly aappealing; what we want is a completely composited desktop without making any copies.

This presentation describes on-going work in the X window system to eliminate copies in the compositing process. A double buffered application is given enough information to construct its image in a way that the pages containing the frame can be mapped to the scanout buffer directly. The work involves changes to both the direct and indirect rendering paths so that Xrender and OpenGL based applications can both benefit. The presentation will include a demonstration of the system and discussion of how to integrate this into current Linux desktop environments.