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As far as I know, all JavaScript interpreters and JIT VMs are also single-threaded (web workers aren't real threads because they can't directly share memory wit

• remexre 5y ago
  except via SharedArrayBuffer?
• vlovich123 5y ago
  Web workers are real threads in the sense that they map directly to low-level threads (+ shared memory via SharedArrayBuffer which was only temporarily disabled due to Spectre). The bigger thing is that the Python runtime has a GIL because that's kind of how you solved the problem when multiple cores started becoming mainstream in the 90s. The Linux kernel had a similar GIL. It's an easy way to technically support running in a multi-threaded environment while reducing maintenance issues at the cost that you can never use more than 1 core at a time. JS runtimes don't have this problem because the runtime support layer is almost non-existent & there's no multi-threading support at all (indeed, V8 by itself comes with almost no JS APIs - there's a clear delineation between language runtime & "app runtime"). Service workers is extremely thin & works on channels instead of shared memory, further simplifying the design. The JS GC is typically written to support concurrent multi-threaded operation to minimize the "stop the world" phase runtime whereas Python's cycle detector needs to stop the world due to the GIL. These are all small decisions that add up.