da56ef239b12786e3a177afda14352dda4a70bc6 clusterlin: minimize chunks (feature) (Pieter Wuille)
Pull request description:
Part of #30289.
This was split off from #34023, because it's not really an optimization but a feature. The feature existed pre-SFL, so this brings SFL to parity in terms of functionality with the old code.
The idea is that while optimality - as achieved by SFL before this PR - guarantees a linearization whose feerate diagram is optimal, it may be possible to split chunks into smaller equal-feerate parts. This is desirable because even though it doesn't change the diagram, it provides more flexibility for optimization (binpacking is easier when the pieces are smaller).
Thus, this PR introduces the stronger notion of "minimality": optimal chunks, which are also split into their smallest possible pieces. To accomplish that, an additional step in the SFL algorithm is added which aims to split chunks into minimal equal-feerate parts where possible, without introducing circular dependencies between them. It works based on the observation that if an (already otherwise optimal) chunk has a way of being split into two equal-feerate parts, and T is a given transaction in the chunk, then we can find the split in two steps:
* One time, pretend T has $\epsilon$ higher feerate than it really has. If a split exists with T in the top part, this will find it.
* The other time, pretend T has $\epsilon$ lower feerate than it really has. If a split exists with T in the bottom part, this will find it.
So we try both on each found optimal chunk. If neither works, the chunk is minimal. If one works, recurse into the split chunks to split them further.
ACKs for top commit:
instagibbs:
reACK da56ef239b
marcofleon:
crACK da56ef239b12786e3a177afda14352dda4a70bc6
Tree-SHA512: 2e94d6b78725f5f9470a939dedef46450b85c4e5e6f30cba0b038622ec2b417380747e8df923d1f303706602ab6d834350716df9678de144f857e3a8d163f6c2
baa554f708