[Rails] Current threading model of Rails?

Rails can do either (config.thread_safe! turns on multithreading, and will
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.

Fred