In short, there’s no secret sauce. There’s an efficiency spread in (what I consider) edge-cases.
Every potential gain over other naive watchers implemented with kqueue is likely algorithmic. I store events in a historical map, compare differences to the current state of the file tree, prune them, and send events when they change. That’s the whole implementation: scan paths, record their attributes, check for differences in the map, and send events when they happen. I haven’t given much thought to exactly why it beats kqueue, nor are there any good tests showing by how much. (Again, this is worth doing.)
Makes sense. I have only used kqueue on macos to monitor a small number of files and I find it quite painful to use and the semantics were confusing, not sure if it is different on say freebsd.
Just as a heads up, one of the strange fsevents issues is that it fails if you register two directories where one directory is a prefix of the other. So say that you want to monitor directories $ROOT/foo and $ROOT/fo and you register an event stream first with $ROOT/foo and then $ROOT/fo, you will only receive events for paths in $ROOT/fo and no events for paths in $ROOT/foo (I just double checked that this is still the case in Monterey at least). I never bothered to report this to apple but worked around it by just registering a stream with $ROOT if I detected that one path name was a substring of another.
I moved to a minimal std::filesystem-based watcher and optimized it from there.
There hasn’t been a formal head-to-head test between the two. That should be about halfway down my todo list. It’s worth revisiting more formally.
My response to this question should help here: https://news.ycombinator.com/item?id=33247155#33251437
In short, there’s no secret sauce. There’s an efficiency spread in (what I consider) edge-cases.
Every potential gain over other naive watchers implemented with kqueue is likely algorithmic. I store events in a historical map, compare differences to the current state of the file tree, prune them, and send events when they change. That’s the whole implementation: scan paths, record their attributes, check for differences in the map, and send events when they happen. I haven’t given much thought to exactly why it beats kqueue, nor are there any good tests showing by how much. (Again, this is worth doing.)