Has anyone looked at http://ringojs.org? Seems like a full featured, multi-threaded javascript impl on top of rhino on the jvm?

I wonder it’s possible to hack threading support into jsc?

or just add planck apis that can instantiate a new jsc runtime from clojurescript/js

JavaScriptCore does have support for multiple threads, so Planck could make use of that capability for interesting things

The JavaScriptCore API is thread safe—for example, you can create JSValue objects or evaluate scripts from any thread—however, all other threads attempting to use the same virtual machine will wait. To run JavaScript concurrently on multiple threads, use a separate JSVirtualMachine instance for each thread.

it does suggest that you have to make a second instance of the vm if you want true parallelism, though?

Yes