this makes sharing objects and synchronising access while still getting good performance more difficult. I can't back up my claims yet; we'll see how Hydra VM works out.
In the long term, a VM that can run its green threads (aka Process) on multiple OS threads (aka pthreads) should be the long-term goal.
This is debatable. Why are you convinced that fine-grained concurrency will not involve a large performance hit due to CPU cache invalidations? I haven't heard a compelling argument that this won't be a problem (and increasingly so, as the number of cores grows). We can't pretend that it takes zero time to make an object available for processing on a different core. As I've said before, I'm willing to be convinced otherwise.