Yes, on every function call, also some internal functions implemented in native code are instrumented with those checks. On average thread is switched out after

Yes, on every function call, also some internal functions implemented in native code are instrumented with those checks. On average thread is switched out after about 2000 "reductions" as they are called. Also there's an optimization, where if you send something to another thread and you are waiting for reply, VM switches instantly to the other thread, which makes some message sending equivalent to a simple function call.