It depends what you're doing. If you're running a ruby web app server and talking to the database, all the io you're doing is most likely synchronous. In the one-server-per-cpu model, anyway.
It's been a while, but I thought that MRI basically slept during IO and released the GIL so that other threads could do work. In that case, you'd still be handling more requests while the IO was performed. I could be wrong. This kind of thing: http://ablogaboutcode.com/2012/02/06/the-ruby-global-interpr...
You're right that it can do async io. But you compared a framework to a language. Rust has Tokio for async - Iron is just not using it.
It's similar with Ruby/RoR - yeah, they can do async. But not on their own. With unicorn server, you still get no threading and just a bunch of processes. With puma you can do threading (async cooperative really) - as long as you keep the configuration/code within the limits of what's allowed.
And due to the extra care needed whenever you do caching/storage things, I expect unicorn is still the king in RoR deployments. (GH uses that for example)
It's definitely preemptive rather than cooperative. Ruby/Puma is actually using one OS thread per Ruby thread, so when one hits a DB call and blocks on sync IO, it releases the GVL and another Ruby thread can proceed. There's a timer thread Ruby runs and pre-empts each thread to schedule them.
