How can green threads have zero impact on FFI in the presence of callbacks across the boundary? I mean, as soon as the other side uses TLS in any way, for example, you have to figure out how to handle that, and it's not free.
In every user-mode thread or stackless coroutine (async/await) implementation I know, the construct is pinned (i.e. can't be preempted) while there's a foreign frame on the stack. There is no advantage to either design on that front.
But 1. there's no speed penalty in Java for doing that, and 2. In Java, calling native functions that block (which is the only time this limitation matters) is rare, especially in high concurrency situations where you'd use virtual threads in the first place.
But 1. there's no speed penalty in Java for doing that, and 2. In Java, calling native functions that block (which is the only time this limitation matters) is rare, especially in high concurrency situations where you'd use virtual threads in the first place.