Thank you very much for sharing this, very insightful.
https://github.com/openzfs/zfs/pull/16896
I doubt that this will make us as fast as ext4 at unlinking files in a single thread, but it should narrow the gap somewhat. It also should make many other common operations slightly faster.
I had looked into range lock overhead years ago, but when I saw the majority of time entering range locks was spent in an “unavoidable” memory allocation, I did not feel that making the operations outside the memory allocation faster would make much difference, so I put this down. I imagine many others profiling the code came to the same conclusion. Now that the memory allocation overhead will soon be gone, additional profiling might yield further improvements. :)