Ractors vs Fibers: Ruby Concurrency Without the Hand-Waving

Ruby has had a concurrency story for years. For most of that time, the story was "threads exist but the GIL means they don't give you parallelism for CPU-bound work, and Fibers are for cooperative scheduling if you want to manage it yourself." Ruby 3 changed the first half of that sentence. Ractors (introduced in Ruby 3.0, still experimental as of 3.3) give you genuine parallelism — multiple Ractors run on multiple OS threads without the GIL constraining them. Fibers (overhauled in Ruby 3.1 with