Interesting, but probably a bit out of date, as it is based on Ruby 2.0, and Ruby 4 has just been released. Also, we now have more concurrency primitives like Fibers and Ractors, as well as the Threads discussed in the article.
True. Do you know of any vaguely equivalent resources for learning about those?
Pickaxe Book, Chapter 12 - but it is terser and aimed at someone with more experience. I think that the article is a good introduction to Threads, and once you understand how they work, then Fibers and Ractors start to become easier to understand.