I have a 2008 Mac Mini with 4 GB of RAM. Recently (post installation of Lion), it started to behave extremely sluggishly, so I used Activity Monitor to see what was using up the system resources. Turns out that kernel_task is using 2.95 GB of RAM. Not a typo: it's using ~3 GB of RAM.
I use this Mac Mini as a home media server, connected to a TV and a Drobo. In other words, beyond running iTunes, I don't really use it for much. I don't have a ton of applications running, nor do I have any accessories other than the Drobo and the TV connected to it.
What on earth could kernel_task possibly need 3 GB of RAM for, and how can I tell it to stop eating all my RAM and dragging my system to a screeching halt?