How to identify and kill hanging process, when system hangs and reacts slowly on user input?
There are two primary ways to identify hung processes. One is via the System Monitor GUI and the other is via top in command-line.
System Monitor
This can be found in System > Administration
You can also create a keyboard shortcut for this in this article.
Once the GUI launches you can select the Processes tab which will list all the running processes. Sort by the CPU column to find the most CPU intensive task
Lastly you can right click that task and choose to end it, stop it, or kill it. Killing it will immediately stop and remove that process from the system.
Command-Line
If you have a terminal open you can simply type top this will list all the running processes similar to that of the Processes tab in the GUI
Within top it is CPU sorted by default - so the top most CPU intensive tasks are at the top. At anytime you can press the letter k to kill a process
Simply type the PID of the process you wish to kill and press enter. It will ask for a Kill signal to send. To kill the process nicely use the default 15 - to kill it right away "Do not pass go, do not collect $200" use 9.
The process will then be terminated.
If you are experiencing sluggish interface you can try to SSH in remotely if that is enabled - or switch to a virtual console via Ctrl+Alt+F# Where F# is a Function Key (F3, F4, F5, etc). To return to the Desktop environment switch to either F7 or F8 depending on your version of Ubuntu.
Add the System Monitor applet to your panel (right click on the panel, select Add to Pannel and search for System Monitor) the and right click on it to open the System Monitor application.
You can launch the System Monitor application by pressing Alt+F2 and start the program gnome-system-monitor.
Go to the "Processes" tab and right click the appliction you intent to kill.
Most already said (I like Marco Ceppi's answer) but there also is a "Force Quit" applet you can add to your GNOME Panel. Right click on the panel and choose "Add to panel...":
This lets you quit a hanging application by simply clicking on it's window. Very fast and efficient.
When running Unity in 11.04 of course this application will no longer be available. There is a replacement project Indicator-Forceclose but it is not included in the repositories.
Alternatively we could run xkill from a terminal.