What's wrong with backing store?

Owen Taylor otaylor at redhat.com
Tue Oct 5 19:15:41 UTC 2004


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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