If all [else] fails, you may still be able to communicate with the Kernel.

예전에 본거였는데, 돌아다니다가 보여서 적어둬요☆

Alt+SysRq

If all else fails, you may still be able to communicate with the Kernel.

To use, just press the Alt and SysRq (PrintScreen) keys and one of the keys listed below, all together. You should try switch into a terminal window, if possible, first by pressing Ctrl+Alt+F2.

If your computer is not responding you can use the following steps to perform a reboot with less chance of fscking your files. This is the method suggested on Wikipedia's Magic SysRq key page.

1. Press Ctrl+Alt+F2 to switch to a terminal window. Sometimes, this is not possible.
2. Press Alt+SysRq+R to get the keyboard
3. If pressing Ctrl+Alt+F2 before failed, try it again now.
4. Press Alt+SysRq+S to sync your disks.
5. Wait for OK or Done message.
6. Press Alt+SysRq+E to term all processes.
7. Press Alt+SysRq+I to kill all processes.
8. Press Alt+SysRq+U to umount all disk drives.
9. Wait for OK or Done message.
10. Press Alt+SysRq+B to reboot.

The Full list of available commands is shown below. It's largely taken from Documentation/sysrq.txt in the Linux Kernel Sources.

Alt+SysRq+	Action	Uses
R	UnRaw	Turns off keyboard raw mode. This allows input from the keyboard even if X-Window has crashed.
K	SAK - Kill all on console	Secure Access Key - Kills all programs on the current virtual console. This is useful if you want to make sure there are no programs waiting on the console to grab your password, or if a process won't let you switch consoles.
S	Sync	Attempts to sync all filesystems. This lessens the chance of data loss and fscking. Syncing is complete when "done" or "OK" is printed.
U	Umount	Attempts to remount all filesystems as read-only. Umounting is complete when "done" or "OK" is printed.
B	Reboot	Will immediately reboot without syncing or unmounting any disks. Before using this use Alt+SysRq+S and Alt+SysRq+U to avoid data loss.
C	Crashdump	Will perform a kexec reboot, in order to take a crashdump. Before using this use Alt+SysRq+S and Alt+SysRq+U to avoid data loss.
O	Power Off	Turns off the computer without syncing or unmounting disks. Before using this use Alt+SysRq+S and Alt+SysRq+U to avoid data loss.
P	Show Pc	Attempts to dump all registers and pointers to console.
T	Show Tasks	Attempts to dump a list of all tasks to console.
M	Show Memory Info	Displays memory info to console
V	Voyager processor info	Dumps Voyager SMP processor info to your console.
0-8	Kernеl Error Verbosity	Set the console log level for kernel messages. Setting to 0 only shows messages like PANIC and OOPS
F	OOM Kill	Calls oom_kill to kill a memory hog process
E	Term	Sends SIGTERM signal to all processes.
I	Kill	Kills (sends SIGKILL signal to) all processes.
L	Kill + Kill Init	Kills (sends SIGKILL signal to) all processes, including init. You will not be able to do anything else after using this!
N	Nice	Make real-time processes nice-able.
H	Help	Prints some help

출처 : http://en.wikibooks.org/wiki/Linux_Guide/Freezes
관련 : http://en.wikipedia.org/wiki/Magic_SysRq_key
참고 : http://jeongsw.tistory.com/178