I've been meaning to look into it, so these are mostly notes, that others might find useful,

arenas are x86_64 only, not sure how involved are ports, but there's at least a VOP, that as of right now exists only on x86_64. (the build obviously fails on other systems)

I'm trying it on m1, so I can build with

    arch -arch x86_64 sh make.sh --with-system-tlabs

and then run with

    arch -arch x86_64 sh run-sbcl.sh

it's pretty raw, so for example allocating object larger than arena results in ldb. there's a lot of sample code in tests/arena.impure.lisp

and the simplest possible test code just to see what's up,

    (gc :full t)
    (room)
    ;;;    7,084,208 bytes for  36,307 simple-vector objects
    (progn (make-array 10000000) (values))
    (room)
    ;;    86,991,136 bytes for  36,080 simple-vector objects
    (gc :full t)  
    ;;     6,847,824 bytes for  35,865 simple-vector objects
    (use-package :sb-vm)
    (defvar a (new-arena 100000000))
    (with-arena (a) (make-array 10000000) (values))
    (room)
    ;;     7,050,080 bytes for  36,233 simple-vector objects

well, so at least we know the array is put somewhere. finally one calls (destroy-arena a), but you can still access the data, so presumably there's no checking involved, and being undisciplined about retaining arena pointers will create all kinds of interesting bugs.

neat!