Ubuntu – what to do when everything freezes?

(this post is just for a personal reference)
If it locks up completely, you can REISUB it, which is a safer alternative to just cold rebooting the computer.

REISUB by:

While holding Alt and the SysReq (Print Screen) keys, type REISUB.

R: Switch to XLATE mode
E: Send Terminate signal to all processes except for init
I: Send Kill signal to all processes except for init
S: Sync all mounted file-systems
U: Remount file-systems as read-only
B: Reboot
REISUB is BUSIER backwards, as in “The System is busier than it should be”, if you need to remember it. Or mnemonically – R eboot; E ven; I f; S ystem; U tterly; B roken.

In case you like mnemonics: Raising Elephants Is So Utterly Boring, or Reboot Event If System Utterly Broken. I’ve also seen it as RSEIUB (Raising Skinny Elephants is Utterly Boring)

Reference: http://askubuntu.com/questions/4408/what-should-i-do-when-ubuntu-freezes?page=1&tab=votes#tab-top