In my previous post on Multi-thread scaling issues with Python and Ruby, I had described the threading implementation within Ruby as using green threads. It seems that this has changed in Ruby 1.9 …

Ruby 1.9 threads do map to native threads, unfortunately the 1.9 interpreter forces user created threads to acquire a global mutex lock before executing. The upshot is that 1.9 thread execution is serialized and unable to benefit from multiple cores.

See Ruby in a multicore world for further discussion.