Kino: a Ractor web server for Ruby 4.0 that runs one process on all cores
Kino: A high-performance Ractor web server for Ruby 4.0
Kino is a high-performance Ractor web server for Ruby 4.0+ that runs your app on every core in one small process, using a Rust tokio/hyper front-end and parallel Ractors. On an 8-core server, it beats a Puma fork cluster by 1.5-2x on I/O-light endpoints and uses about 7x less memory on a tiny Ractor app. It includes production features like graceful drain, crash supervision, and a --check tool to identify Ractor blockers. Ractors are experimental in Ruby 4.0, so Kino also offers a threaded fallback mode for Rails.
The GVL allows only one Ruby thread to run at a time. To use all cores, Ruby servers fork processes, and every fork costs a full copy of the app. Ractors do not have this limit: each one has its own lock, so one process can run Ruby in parallel.