In the case of Zen/Zen2, some of the dies didn't have direct access to main memory but went through an adjacent die's memory controller and channel. This had a

In the case of Zen/Zen2, some of the dies didn't have direct access to main memory but went through an adjacent die's memory controller and channel. This had a performance and latency effect, and required some extra work on the part of OS schedulers to make sure that tasks weren't being scheduled on related cores. It's (relatively) easy to have lots of cores on one die, it's not so easy to keep them fed.