Monthly Archives: April 2013

Story of a bug in Ubuntu

I just came across this lovely writeup of a particularly annoying bug I helped squash in Ubuntu 12.04. The bug caused applications like evince and eog to load incredibly slowly, taking almost a full minute to open a PDF. It turned out to have a relatively banal cause — an initialization procedure didn’t immediately signal its failure and instead the library would wait a full 5 seconds for the initialization to finish before giving up.

Thanks for the writeup, Emanuele Rocca. I think this shows the power of open source software. An obscure bug affecting only a handful of people can still get fixed, even without much help from the original developers.

Changing screen resolution in RealVNC

These days I do most of my work in a RealVNC session on a remote Ubuntu workstation. This provides me with a consistent environment, regardless of whether I’m at work, a coffee shop, or at home. One major annoyance with the default configuration is that the screen resolution cannot be changed after the virtual desktop is started.

The key to getting around this problem is to provide RealVNC with a complete list of screen resolutions you would like to have available when starting RealVNC. For example, my .vnc/config file contains

# Additional Resolutions
-randr 800x600,1024x768,1280x800,1280x960,1280x1024,1344x756,1680x1050,1920x1080,1920x1200,3360x1050,1024x700,1200x740,1600x1000,3200x1000,1680x1020,768x1024

After restarting RealVNC, you can easily change the screen resolution using the xrandr command. You can list screen resolutions by running xrandr:

$ xrandr
 SZ:    Pixels          Physical       Refresh
 0    800 x 600    ( 203mm x 152mm )   0
 1   1024 x 768    ( 260mm x 195mm )   0
 2   1280 x 800    ( 325mm x 203mm )   0
 3   1280 x 960    ( 325mm x 244mm )   0
 4   1280 x 1024   ( 325mm x 260mm )   0
 5   1344 x 756    ( 341mm x 192mm )   0
 6   1680 x 1050   ( 427mm x 267mm )   0
*7   1920 x 1080   ( 488mm x 274mm )  *0
 8   1920 x 1200   ( 488mm x 305mm )   0
 9   3360 x 1050   ( 853mm x 267mm )   0
 10  1024 x 700    ( 260mm x 178mm )   0
 11  1200 x 740    ( 305mm x 188mm )   0
 12  1600 x 1000   ( 406mm x 254mm )   0
 13  3200 x 1000   ( 813mm x 254mm )   0
 14  1680 x 1020   ( 427mm x 259mm )   0
 15   768 x 1024   ( 195mm x 260mm )   0
Current rotation - normal
Current reflection - none
Rotations possible - normal
Reflections possible - none

To switch to a different resolution just run xrandr -s <resolution>, where the resolution is either the item number, like 12, or resolution, like 1600x1000.

Afterwards, especially in recent versions of Ubuntu (12.04 and later), you may find that the background Nautilus desktop didn’t get the memo that the screen resolution was changed. To work around this, it’s often easiest to quit and restart Nautilus:

$ nautilus -q; sleep 1; nautilus -n > /dev/null 2>&1 & disown %

If you find yourself doing this a lot, as I did, you may want to consider writing a little script to automate the task. I named my script xres and have posted the source as a gist.

RealVNC on Ubuntu 13.04 Raring Ringtail

I just spent a few hours getting the free version RealVNC working in Ubuntu 13.04 Raring Ringtail (Beta 2). Here are the steps that I needed to take:

  1. Disable the X Server Render extension. Copy /etc/vnc/config to /etc/vnc/config.custom and add
    -extension RENDER
  2. Create a custom xstartup file. Copy /etc/vnc/xstartup to /etc/vnc/xstartup.custom and make the following changes:
    • Remove the line
      [ -x /etc/vnc/xstartup.custom ] && exec /etc/vnc/xstartup.custom
    • Add "gnome-fallback" to the list of valid sessions by replacing
      for SESSION in "ubuntu-2d" "2d-gnome"; do


      for SESSION in "ubuntu-2d" "2d-gnome" "gnome-fallback"; do
    • Add to the top of the file:
      unset XDG_RUNTIME_DIR

I found it necessary to unset XDG_RUNTIME_DIR because I could not figure out a way to guarantee the directory it points to actually exists. When the directory did not exist I experienced 100% CPU utilization bugs in indicator-datetime-service and gnome-settings-daemon.

Briefly, the XDG_RUNTIME_DIR variable and directory that it points to are managed by a PAM module in libpam-xdg-support. When jsmith logs in over SSH, the PAM module creates a directory /run/users/jsmith and increments a session count stored in /run/users/.jsmith.lock. When jsmith later logs out, the session count is decremented. When the session count reaches zero the directory is deleted. Since RealVNC does not appear to use the PAM session machinery, there is no way to guarantee that the session count remains positive if only a VNC session is active.