The arena serves two purposes. It is a structure that is

the top-level state of the MPS, and as such contains a lot of fields

which are considered "global". And it provides raw memory to pools.

 

An arena belongs to a particular arena class. The class is selected

when the arena is created. Classes encapsulate both policy (such as

how pool placement preferences map into actual placement) and

mechanism (such as where the memory originates: operating system

virtual memory, client provided, or via malloc). Some behaviour

(mostly serving the "top-level datastructure" purpose) is implemented

by generic arena code, and some by arena class code.