Well, there's a fine line here. Correct me if I'm wrong, but if it's hardware based IRP completion (like DMA done, network card received a packet or whatever),

Well, there's a fine line here. Correct me if I'm wrong, but if it's hardware based IRP completion (like DMA done, network card received a packet or whatever), your thread could wake up very soon after I/O completes, long before the next timer tick. But this is not something you can use for timing, unless of course your hardware is built to do so.