[duped]

> I would assume any modern processor would make a context switch a one instruction affair. Interrupt -> small scheduler code picks the thread to run -> LOAD THREAD instruction and the processor swaps in all the registers and the instruction pointer.

It can't be a single instruction, since the details of what a "context" contains depends on the OS and ABI. For example on Linux, the signal mask is a part of the OS thread context (but usually not user thread contexts) which requires a syscall to retrieve it from kernel memory before saving it in the context.

The reason why user threads are so much faster than OS threads is precisely because it can be reduced to a handful of instructions without caring about all the details that OS threads need to care about.

> Which only works if really every blocking IO is modified to include yielding behavior. If you call a blocking OS function, I assume something bad will happen.

That's exactly what Go does, they introduce yield points into function prologues and i/o ops. You don't have direct FFI calls in Go so it's not as big of an issue. It's roughly the same problem as GC safepoints in multithreaded interpreters that support FFI.