How does this compare to Jython? The blog post says they looked at alternative runtimes but didn't like that they had tradeoffs (implying that Grumpy has no tradeoffs??). But Jython has been around for quite a while and also has no GIL:
Every time I've tried to use Jython, I've found it won't work with pre-existing code all that well.
In part it has exposed CPython "implementation quirks" that people were wittingly or otherwise taking advantage of. In other cases there doesn't seem to be obvious reasons for the differences and has required special-casing the python code to handle it.
It has been great with code written from scratch, specifically for it.
That sounds like the kind of issue you'd have with any reimplementation of CPython though. It sounds like Grumpy doesn't support quite a few things, so I still wonder how they compare and why developing a new runtime was considered easier than reusing Jython.
http://www.jython.org/jythonbook/en/1.0/Concurrency.html
It can also handle Python's dynamic aspects.