X Compositing For Free

Current Linux desktop environments incorporate window-system levelcompositing to present a rich user environment. The process of mergingapplication windows together comes at a steep cost though -- eachpixel on the screen will have been copied multiple times beforelanding in the scanout buffer, and each of these copies consumesmemory bandwidth and power. Some desktop environments provide specialcase optimizations for full-screen windows, or offer quick ways todisable and re-enable compositing. Neither of these is particularlyaappealing; what we want is a completely composited desktop withoutmaking any copies.This presentation describes on-going work in the X window system toeliminate copies in the compositing process. A double bufferedapplication is given enough information to construct its image in away that the pages containing the frame can be mapped to the scanoutbuffer directly. The work involves changes to both the direct andindirect rendering paths so that Xrender and OpenGL based applicationscan both benefit. The presentation will include a demonstration of thesystem and discussion of how to integrate this into current Linuxdesktop environments.