Re: What's wrong with backing store?

[Kyrre Ness Sjobak <kyrre solution-forge net>]

tir, 05.10.2004 kl. 21.15 skrev Owen Taylor:
> 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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That's a looong wait. About a year or so... But, yes, i'm really looking
foreward to it :P