ubuntu freeze

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.

This is the SysReq key:

SysReq key

NOTE: There exists less radical way than rebooting the whole system. If SysReq key works, you can kill processes one-by-one using Alt+SysReq+F. Kernel will kill the mostly «expensive» process each time. If you want to kill all processes for one console, you can issue Alt+SysReq+K.

NOTE: You should explicitly enable these key combinations. Ubuntu ships with sysrq default setting 176 (128+32+16), which allows to run only SUB part of REISUB combination. You can change it to 1 or, which is potentially less harmful, 244. To do this:

sudo nano /etc/sysctl.d/10-magic-sysrq.conf

and switch 176 to 244; then

echo 244 | sudo tee /proc/sys/kernel/sysrq

It will immediately work! You can test this by pressing Alt+SysReq+F. For me, it killed active browser tab, then all extensions. And if you will continue, you can reach X Server restart.

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More info on all the Alt+SysReq functions here.