Rails 3.2/4 are thread-safe -- or at least, they are architected and intended to be. So few people actually use Rails in a multi-threaded deploy environment -- and concurrency bugs can be so tricky -- that it would not surprise me if there are still some bugs hiding out.
There really is such a huge performance advantage to a multi-threaded deploy environment -- EVEN with MRI, for the typical I/O-bound webapp, although you'd definitely want both multi-process and multi-threaded under MRI GIL. (Both puma and passenger enterprise can give you this, although not passenger free).
I'm hoping people start to catch on to this, and multi-threaded app servers get more popular, making them more mature and feature-rich, and in turn flushing out remaining bugs in Rails, causing yet more interest in multi-threaded deploy environment, virtuous circle blah blah
There really is such a huge performance advantage to a multi-threaded deploy environment -- EVEN with MRI, for the typical I/O-bound webapp, although you'd definitely want both multi-process and multi-threaded under MRI GIL. (Both puma and passenger enterprise can give you this, although not passenger free).
I'm hoping people start to catch on to this, and multi-threaded app servers get more popular, making them more mature and feature-rich, and in turn flushing out remaining bugs in Rails, causing yet more interest in multi-threaded deploy environment, virtuous circle blah blah