IMO, adding feedback mechanisms (e.g. LB polling app or server agent) are among the best ways to create smart, fast LBs without keeping a lot of expensive state

IMO, adding feedback mechanisms (e.g. LB polling app or server agent) are among the best ways to create smart, fast LBs without keeping a lot of expensive state. Calculating busyness is another question entirely but it's also a much easier problem to solve than stateful routers.