io_uring
The Linux kernel's modern async I/O interface — two ring buffers (submission + completion) shared between kernel and userspace, with syscall batching and zero-copy as defaults.
Designed by Jens Axboe (2019, kernel 5.1). Addresses the limitations of epoll (readiness only, not completion) and POSIX AIO (which in practice only works on direct I/O). Now supports network I/O, file I/O, accept, splice, send/recv-zerocopy and most other syscalls.
Databases and proxies (ScyllaDB, RocksDB, Cloudflare Pingora) use it for latency. The security history is rocky — multiple CVEs in early versions, and some cloud providers disable it via seccomp.