I'm surprised no one mentioned the Rainbows app server, a multi-threaded version of unicorn. Unless you're running jRuby, you're limited by Ruby's green thread

I'm surprised no one mentioned the Rainbows app server, a multi-threaded version of unicorn. Unless you're running jRuby, you're limited by Ruby's green threads. If you want to saturate all of the cores on your server, you need 1-2 processes per core and then a thread pool within each to keep it well utilized. I've been running a 4x10 rainbows process/thread setup in production with good results on AWS dual core systems.