Constant CHUNK_BITS 

pub const CHUNK_BITS: u32 = 14;

Entries per chunk of the table, as a power of two. A chunk is 512 KiB: above glibc’s mmap threshold (which the engine fixes at 256 KiB), so each chunk is mapped on its own instead of sitting in a thread’s heap among loaded runs’ short-lived blocks, where it would keep the pages around it resident after those are freed.