File Session Cleanup

I'm on the very latest version of CI, using file sessions. What I noticed was that my sessions folder is full of session files when it probably shouldn't be. It seems like old session files are not being deleted. What part of CI or my PHP config is responsible for this and how can I check if it's working properly?

Bonus round: I'm on a digital ocean ubuntu box, is there a part in CI docs that breaks down all folders / files and what should their permissions be set to?