Thanks for the explanation! Much clearer and I get the concept. In the case of load balancing, we'd need a ton of servers (1000s?) for this to pay off vs just comparing all, right? Cache updating aside, most of the overhead would be in reading the load numbers in. Comparing a thousand numbers has to be quick in comparison, no?
The problem with load balancing is herd behavior. Stats for load are usually at least a little stale, because it's a distributed system where you can't afford to wait for consistency. When there are traffic spikes a whole herd of new connections will go to the least loaded server for a window of time where the cached "load" number is out of date. Picking two at random helps keep from a bunch of connections racing to one server, even when you're only running 3-4 of them.
