On Tue, 2004-10-05 at 11:34 +1000, Steffen Kluge wrote: > Hi, > I've just got to ask this, please forgive me if it is only marginally > on-topic for this list. > > Back when I started using X11 (late 80's, Ultrix, X11R3), turning on > save-unders and backing store was an essential performance tuning step, > that drastically improved the perceived "speed" of the desktop (e.g. > disappearing menus exposed the underlying window instantly, without it > having to be re-drawn). > > I've used save-unders and backing store ever since. Which also means > that ever since starting to use RH desktops I had to turn it on > manually, by editing the X command line in [gx]dm.conf. All RH and FC > desktops I've used so far disable backing store (and at the same time > save-unders - the two seem to be inextricably linked) by default. > > Why is that so? I have to admit that with modern hardware the perceived > speed-up is small and may even be entirely based on self-suggestion. > However, I'd like to understand the pros and cons of turning on backing > store with modern X servers and on modern hardware. Well, the biggest problem with backing store is that it is per-window not per *toplevel* window. In X, toplevel windows can have subwindows (try xwininfo -tree). In certain circumstances, X can end up storing huge amounts of entirely useless pixel data for subwindows because it mistakes them for obscured toplevels. Save unders are broken because they are just a quick hack that works by turning on backing store for the windows under the popup. The COMPOSITE extension is a much better way of doing backing stored windows. It's slightly more expensive than classic save unders because it saves the entire window, not just the obscured parts, but it's a lot more robust, and allows doing a lot more (previews in your pager, alpha transparency, etc.) We'll probably have it on by default by the FC5 timescale. Regards, Owen
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