Current threading model of Rails?

Can anyone tell me the current state of Rails and threading? Do you need
bunch of mongrel processes running on a single machine or is it possible
get away with just one that handles concurrent requests? I did a bunch
Google searches and couldn’t find a definitive answer on the current
request model used by Rails.

Rails can do either (config.thread_safe! turns on multithreading, and
be the default in rails 4), but the deciding factor is what ruby
implementation you use: MRI doesn’t allow true concurrency in general
(specific cases like blocking on IO, waiting for a mysql query do permit
some concurrency). jruby doesn’t have this problem and rubinius has also
done away with the global vm lock.