[xakahnx]

Keeping the directory coherent is the difficult part when translating directory-based cache coherence protocols to other distributed systems problems. The directory is like an oracle that sees every transaction in order. This is hard in most network distributed systems problems where you have to worry about availability, network partitioning, or durability of this node.

[yvdriess]

Indeed, the evolution will probably in the other direction, with the on chip network adopting algorithms from wider networks to deal with scaling problems. DRAM interfaces use to be pretty simple, now they are being trained almost like a DSL line.