[cpr]

I don't understand the "smurfing" solution he references, but CPython's runtime uses reference counts in each referenced value to detect garbage (when a value can be freed), which means even read-only values can be modified in memory by the runtime as object references come and go.

Those modifications force pages which were created on forking a child process as copy-on-write (meaning they share the same physical page until the page is modified by the child) to be copied and thus blow out any memory savings that would normally happen with copy-on-write.