Constant ZERO_COPY_THRESHOLD 

pub const ZERO_COPY_THRESHOLD: usize = 4096;

The data size threshold at which we start using shared memory.

Shared memory works by sharing memory pages. This means that the smallest memory region that can be shared is one memory page, which is typically 4KiB.

Using shared memory for messages smaller than the page size still requires sharing a full page, so we have some memory overhead. We also have some performance overhead because we need to issue multiple syscalls. For small messages it is faster to send them over a traditional TCP stream (or similar).

This hardcoded threshold value specifies which messages are sent through shared memory. Messages that are smaller than this threshold are sent through TCP.