How many cores for innodb?

I came across two articles on Planet MySQL with some benchmarks on Innodb scalability on machines with multiple cores. The first one being “MySQL, Innodb, DBT2 Core Scalability Graphs” and the second one being “Innodb Multi-core Performance“.

What is interesting is that both seem to suggest that Innodb peaks around 8 cores, though I would be curious how many CPUs there actually were and what the memory bandwidth was. I guess what I am really saying is that I am curious where the bottleneck actually is.