What's wrong with backing store?

[Steffen Kluge]

On Tue, 2004-10-05 at 15:15 -0400, Owen Taylor wrote:
> Well, the biggest problem with backing store is that it is per-window
> not per *toplevel* window. In X, toplevel windows can have subwindows

Hmm, interesting. It seems, though, that the X11 forefathers have
thought of that, to a degree. There used to be (still are?) the
"WhenMapped" and "WhenUseful" backing store attributes. There still
seems to be the -wm server switch, turning on "WhenMapped" for all
windows.

> In certain circumstances, X can end up storing huge amounts of entirely
> useless pixel data for subwindows because it mistakes them for
> obscured toplevels.

How much of an issue is this? I vaguely remember reading once that
backing store uses off-screen frame buffer memory (there should be
plenty of that on a 128MB graphics card running at 1280x1024). Does
backing store actually affect the X server memory footprint? Then again,
doesn't the reported memory footprint of the X server contain all the
mapped in frame buffer memory as well? The more I think about it the
more I realise my utter ignorance in all of this...

Cheers
Steffen.

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