I suspect you can infer a subscription when a thread goes to sleep. It's probably waiting on some kind of synchronization primitive (and I guess I/O would count

I suspect you can infer a subscription when a thread goes to sleep. It's probably waiting on some kind of synchronization primitive (and I guess I/O would count), or has explicitly slept for a period. You still need a timer, I think, but it doesn't have to be a fixed frequency. It gets set to the time slice period when each new thread is scheduled, and if it fires before the thread has given up its time slice then a new thread is selected. (Ideally this behaviour should be relatively rare.)