[electroly]

> Except it's not a context switch? You're jumping to another instruction in the program, one that should be very predictable. You might lose your cache, but it will depend on a ton of factors.

Java virtual threads are stackful; they have to save and restore the stack every time they mount a different virtual thread to the platform thread. They do this by naive[0] copying of the stack out to a heap allocation and then back again, every time. That's clearly a context switch that you're paying for; it's just not in the kernel. I believe this is what the person you're replying to is talking about.

[0] Not totally naive. They do take some effort to copy only subsets of the stack if they can get away with it. But it's still all done by copies. I don't know enough to understand why they need to copy and can't just swap stack pointers. I think it's related to the need to dynamically grow the stack when the thread is active vs. having a fixed size heap allocation to store the stack copy.