In the section "Imagine a non-C processor"

"Caches are large, but their size isn't the only reason for their complexity. The cache coherency protocol is one of the hardest parts of a modern CPU to make both fast and correct. Most of the complexity involved comes from supporting a language in which data is expected to be both shared and mutable as a matter of course. Consider in contrast an Erlang-style abstract machine, where every object is either thread-local or immutable (Erlang has a simplification of even this, where there is only one mutable object per thread). A cache coherency protocol for such a system would have two cases: mutable or shared."