What is a reasonable amount of inotify watches with Linux?

Question

I am working on a daemon that monitors file events via inotify to trigger various types of events when files are accessed. I have read that watches are a little expensive, because the Kernel is storing the full path name of every file being watched.

How many watches would be too many?

Edit: Mostly, I'm wondering .. have you ever seen a noticeable performance hit, if so, at how many watches did it happen? Yes, I have to monitor / recursively (however its a minimal bootstrapped system).

Answer 1

AFAIK the kernel isn't storing the pathname, but the inode. Nevertheless, there are 540 bytes per Watch on a 32bit system. Double as much on 64bit.

I know from Lsyncd (maybe you want to check that out?) people who have a million watches. It just eats a Gigabyte of memory.

Answer 2

You can find the system limits by reading /proc/sys/fs/inotify/max_user_instances (maximum number of inotify "objects") and /proc/sys/fs/inotify/max_user_watches (maximum number of files watched), so if you exceed those numbers, it's too many ;-) The maximum number of watches is usually several tens of thousands or higher - on my system, 262143 - which is probably more than you'd ever need unless you're trying to watch every file in a file system, but you shouldn't be doing that. I would say, just try not to use more inotify watches than you need to, and don't worry about it unless you notice a significant decrease in performance.