Yeah I'm not sure why it's not in the manual, as it's had arena allocation support since at least 2.4.x, but basically:
reply- use SB-VM:NEW-ARENA to make a new arena
- use SB-VM:WITH-ARENA to redirect ordinary allocation into an existing arena like you would use WITH-OPEN-FILE or similar macros
The only real doc is this internals note, and it doesn't even cover NEW-ARENA which I guess is left as an exercise to the reader: https://github.com/sbcl/sbcl/blob/master/doc/internals-notes...