You call `GC.compact` in the parent right before forking off your child processes and because the memory in the children are copy-on-write (COW), it lets them share memory with their parent far longer than they normally would be able to.

Any change in a page (i.e. an object allocated or deallocated) causes it to be copied to a child process, and because previously pages were a mix of objects of all kinds of longevity and slots which may be empty or used, children tended to copy their parent's entire memory space very quickly. Running a GC and compact before forks improves the likelihood that shared pages are mostly full of still-in-use, longer-lived objects, and gives the COW strategy a fighting chance.