[clug] Multi-seating

David (Price) Lattimore david at dmprice.com
Wed Oct 13 22:45:22 MDT 2010

  Hi Francis,

I set up my laptop (Ubuntu 10.04) to be dual seat earlier this year.  I 
only used it like that for a couple of weeks with my wife and I both 
using it concurrently.  There were a few different approaches described 
in various places on the internet.  I ended up using Xephyr with GDM as 
the login manager.  It was a bit fiddly to get it going.  For some 
reason I couldn't get it to use the laptop's trackpad, so instead had to 
use two external mice.

The set up had GDM launch a regular X server with a large desktop that 
spanned both monitors and didn't bind any input devices.  GDM then also 
launched two Xephyr instances running at the sizes of the respective 
monitors and each bound to different input devices.  GDM then provided a 
login on each Xephyr instance.  At the time I was doing this GDM had 
recently been rewritten and didn't support this kind of configuration, 
so I had to use the older gdm-2.20.x.  Not sure if this is still the 
case.  If you don't need a login manager then you can avoid all this, 
which should make it substantially simpler.

Regarding video, you can test it out easily by installing Xephyr 
(xserver-xephyr on Debian and relatives) then running:

# Xephyr :2 &
# DISPLAY=:2 mplayer -fs something.avi &
# Xephyr :3 &
# DISPLAY=:3 mplayer -fs something.avi &

Concurrent video seems to work OK.  From memory though, I don't think 3D 
acceleration works at all.

In terms of stability, I don't think I used it for long enough to say.  
It may have crashed when I tried 3D, but other than that I don't recall 
any problems.

One slight annoyance is that if you unplug one of the input devices then 
plug it back in it wont work again until you restart Xephyr (log out and 
back in again).  This makes it less than ideal for a laptop, but would 
presumably matter less with a kiosk setup since you presumably wouldn't 
want things to be unplugged while in use anyway.

I think I still have it all configured on my laptop if you wanted copies 
of any of the files used in the setup.


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