Change niceness (priority) of a running process

Terminal

If you're at a terminal you can use renice

renice [-n] priority [[-p] pid ...] [[-g] pgrp ...] [[-u] user ...]

A simple example would be

renice 8 31043
31043: old priority 5, new priority 8

You can also pass it hard flags, but it follows that order (you have to pass priority first and then the pid - if you change the order it will show the usage messagge)

renice -n 5 -p 31043
31043: old priority 8, new priority 5

Priorities work on a scale of -20 to 19 - The lower the number, the higher it's priority on the system.

If you own the process then you won't need root - however, if the process is owned by another user or if you plan on changing the group/user of the process root (via sudo) will be required.

GUI

• Alt+F2 and type sudo gnome-system-monitor

Prior to 11.04: System > Administration > System Monitor. Choose Processes.
In 11.04: Alt+A and search for system monitor.

The id's are shown in the image here:

And nice does not require sudo for increasing niceness if you own the task. It does if you need to decrease niceness.

Start a command with nice:

nice -n 19 your_command

Renice process:

renice 19 $(pidof your_command_name_like_for_ex_ffmpeg)

Priority could be between -20 and 19. 19 is the lowest priority.