Multiple issues with wallpaper handling

[Chris Dekter <cdekter gmail com>]

Hi list,

I am experiencing a very frustrating performance gremlin on my 
workstation with a dual-monitor setup. I have been trying for weeks to 
nail down the source of the problems, and have finally figured out that 
it is a combination of issues centering on wallpaper handling. Firstly 
my system:

Ubuntu 11.10 x86_64 with Gnome 3.2.1
Various kernel versions tested in the 3.0 and 3.1 series (all show the 
same symptoms)
Radeon HD 5750 with 1GB RAM running the open drivers
Two 24" monitors at 1920x1200@60hz
(base system is Core i7 with 8GB of RAM)

When I initially load shell, everything is smooth as butter. However, 
when I perform any of the following tasks, the overview animation slows 
to a crawl (although all other animations are still fine):
- Lock the screen using Ctrl+Alt+L and then unlock
- Change the wallpaper (image, svg or colour, doesn't matter)

If I disable Nautilus from displaying the desktop (using 
gnome-tweak-tool), matters improves somewhat, but there is still a 
drastic drop in overview performance as soon as the wallpaper is 
changed. If I restart shell using Alt+F2 and r, performance becomes 
normal again. Also if I disable and then re-enable the second monitor, 
performance is recovered.

I believe that this is a bug in the wallpaper handling in Gnome Shell. 
My theory is that an excessive number of copies of giant wallpaper 
pixmaps are being loaded into graphics memory. I also believe that 
possibly under some conditions the wallpaper pixmap is being loaded into 
memory not at the individual monitor's resolution, but at the X screen's 
resolution (in my case 3840 x 1020). I also think that possibly 
wallpaper pixmaps are not being removed from memory when a new wallpaper 
is set. Combine these issues, and you get a set of pixmaps that will 
choke even a 1GB graphics card.

Thanks,
Chris Dekter