dnsdist/rec: Ponder re-hashing when a bounded-load bin is full

[rgacogne]

• Program: Recursor, dnsdist
• Issue type: Feature request

In both the recursor and dnsdist, we implement bounded-load load-balancing where we dispatch a query to the next bin if the selected one is full (and so on). This suffers from a "cascading overflow" effect that decreases the performance when some of the bins are getting full.
This paper 1 suggests that instead of going to the next bin, we could re-hash the already computed hash with a counter appended to it, and iterate as long as we select a full bin. This is a nice idea because that way we don't always overflow to the same bin, since different qnames will go to a second different bin when re-hashing with the counter. Using a counter also means we don't have to re-hash the whole qname, which is a bit slow.

It seems to make sense on paper, it would be nice to see if it also makes sense in our use case.