Struct Footprint 

pub struct Footprint {
    pub bytes: u64,
    pub shared_bytes: u64,
    pub entries: u64,
}

A deterministic byte estimate for the memory a subscription’s engine retains.

docs/05-tier-lowering.md §5.3 names per-session memory as one of three metrics to export, and Phase 0’s kill gate is written in kilobytes per idle session (docs/18-phase-0-report.md §18.3). An engine per subscription is a memory-for-time trade, so the number has to exist.

It is computed rather than sampled. A resident-set reading moves with the allocator’s arena and swung by 2× between runs of the same measurement; a counting allocator would be exact and needs unsafe, which this workspace forbids. So this walks what is actually retained and adds up size_of plus the bytes behind each allocation, counting shared structure once: a Todo an arrangement holds is the same Arc the accumulator holds, and charging a subscription for it would be the difference between “a handle per row” and “a row per row”.

What it excludes, and therefore under-reports: allocator overhead per allocation, which for many small allocations is substantial. It is a floor on the true cost, not a ceiling.

Fields

bytes: u64

Bytes retained by this engine’s cells, arrangements and keys.

shared_bytes: u64

Of those, the ones in arrangements that do not read the session — what §5.3 says a thousand subscribers should hold once between them, and this engine holds once each.

entries: u64

Trait Implementations

impl Clone for Footprint

fn clone(&self) -> Footprint

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Footprint

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for Footprint

fn default() -> Footprint

Returns the “default value” for a type.

impl Copy for Footprint