Comment by pron

The efficiency and complexity of user mode threads heavily depend on constraints imposed by the particular language. E.g. if the language supports pointers into the stack, user mode threads would be less efficient; if the language is largely dependent on manual memory management -- user mode threads would be more expensive; if the language already has some other concurrency primitives (like async/await) -- user mode threads will be more expensive (although in this case in terms of complexity rather than runtime efficiency). Because Java exposes relatively little of its implementation details, we've been able to implement efficient user mode threads even without any FFI overhead.