renice - - set nice values of running processes
Lets say you have started a process as root and you are updating something. You notice a HUGE slowdown in your system. You open a shell, and type "top" and notice that a process is using 94% of your CPU and hogging all the cpu time slowing everything else to a crawl!!! now what!!!!
Enter the command renice. Just as the manpages say, renice "set nice values of running processes". So, while running top, you notice the PID (process ID number) of the offending program/command is XXXXX (will actually be a number not x's).
As a root from shell:
root:~# renice 19 XXXXX
This will reset the nice value to a much lower (probably) nice value than it's currently running (top shows that as well).
root:~ # renice 19 22537
22537: old priority 0, new priority 19
The man pages give a pretty good description of all of this, plus they give several other specific examples on how renice can also be used.