Question: what to do when centos freezes?
Answer: Press Ctrl+Alt+F2 to switch to a terminal window. ...  Press Alt+SysRq+R to get the keyboard. If pressing Ctrl+Alt+F2 before failed, try it again now. Press Alt+SysRq+E to term all processes. Press Alt+SysRq+I to kill all processes. Press Alt+SysRq+S to sync your disks. Wait for OK or Done message.