I assume that you want to limit it because it is taking away from what you want to do, making your desktop sluggish, etc. If so, you can renice (or kill and start with the nice command) with a positive value. Every process in linux has a niceness value. You can see it yoursefl with top. I would expect that mysql starts with the default 0, which means it has the same priority as most everything else. If you give it a nice of say, 10, it will 'get out of the way' of most other processes with a higher priority and use less CPU time.