What's wrong with backing store?

Owen Taylor otaylor at redhat.com
Wed Oct 6 18:47:27 UTC 2004


On Wed, 2004-10-06 at 16:38 +1000, Steffen Kluge wrote:
> 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.

This doesn't really help - you want to backing store the *visible* 
portions of subwindows, what you don't want to do is to backing
store portions of subwindows that are obscured by other subwindows.

> > 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...

Imagine a 10,000 item list that is 150,000 pixels by 1000 pixels - 
e.g., 450 megs of memory. 

Under some circumstances, the X backing store code can be fooled into
backing storing the *entire list*.

Regards,
						Owen

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