In a 2000 interview (that is, pre-YARV), Matz said
Matz: I'd like to make it faster and more stable. I'm planning a full rewrite of the interpreter for Ruby 2.0, code-named "Rite". It will be smaller, easier to embed, thread-safe, and faster. It will use a bytecode engine. It will probably take me years to implement, since I'm pretty busy just maintaining the current version.
What was meant by "thread-safe" in this context? An interpreter that allowed you to use green threads? An interpreter that allowed you to use native threads? An interpreter that didn't have a global interpreter lock (GVL in YARV Ruby terminology)?