In MRI (aka "CRuby", standard ruby), you don't get parallelism with threads either, due to the GIL. (Similar to CPython). A lot of people are used to thinking

In MRI (aka "CRuby", standard ruby), you don't get parallelism with threads either, due to the GIL. (Similar to CPython). A lot of people are used to thinking mostly of that scenario, where ruby doesn't currently give you parallism for threads either. JRuby does give you parellism for threads -- but I believe currently doesn't actually implement true coroutines (Fibers are just implemented on top of threads), so there are now two reasons this wouldn't be as useful on JRuby, true.