> The internet is full of posts declaring io_uring wins over mmap.

Internet is also full of idiots not having done their homework.

Blindly throwing io_uring for mmap and hoping for better perfomance in a highly concurrent environment is a recipe for thread contention and latency spike.

Add to that the kernel lock contention on mmap. Later kernels have tried to increasingly mitigate this. But significant latency on kernel lock contention still exists.

Try mmap + numa like pinning (from software atleast via hashing the work id) and always have the same mmap be used from the same node. This usually yields better latency compared to throwing wonder weapons.

Our database Dip uses mmap burst for opening massive amounts of mmaped kv engine files along with numa pinning of the same mmap kv engine file to the same core so as to reduce cross cache contamination and page cache relocation.

io_uring (available only in more modern kernels) has so far only increased Dip's latency. Hence we have no reason yet for using it. It is a net negative for our usecase.

Use io_uring where appropriate and don't expect every mmap replaced by io_uring to do wonders.