Enum OidLayout 

pub enum OidLayout {
    Compact,
    Compact64Alloc,
    Aligned64,
}

Where the key array starts, and therefore how the stree keyspace sits against the 64-byte cache line.

What this actually controls, and what it does not

stree’s internal B-tree nodes are 8 × i64 = one cache line each, but they do not live in this section: STree64Mmap builds them into its own Vec<[i64; 8]>. Nothing in this header can move them. What the section owns is the leaf layer — the sorted key array that find_exact and lookup_batch_pipeline linear-scan (up to B + 1 = 9 keys) once the tree has routed them to a block. So this enum moves exactly one memory access per lookup: the leaf touch, which is also the coldest one.

A leaf block is 8 * 8 = 64 bytes. Its phase against the line is keyspace_base mod 64, and keyspace_base is allocation_base + header_len — which is why one arm is not enough to separate the two terms:

variant	header	allocation	keyspace phase	leaf lines touched
Compact	24	Vec<u8>, align-1 by type	unknown, not 0	2, sometimes 3
Compact64Alloc	24	64-aligned	24	2, sometimes 3
Aligned64	64	64-aligned	0	1, plus the overflow key

Compact64Alloc is the control that makes the experiment readable: it holds the allocator constant and moves only the offset, so a difference between it and Compact is the allocator and a difference between it and Aligned64 is the phase.

Variants

Compact

24-byte header, plain Vec<u8>. What every section written before this enum existed looks like, and the arm every other arm is measured against.

Vec<u8> is align_of::<u8>() == 1 by type. Large allocations happen to come back 16-byte aligned from glibc, but that is allocator behaviour and not a guarantee — which is exactly why the aligned arms below cannot be built by hoping.

Compact64Alloc

24-byte header, 64-aligned allocation. Keyspace at phase 24.

Aligned64

64-byte header, 64-aligned allocation. Keyspace at phase 0 — every stree leaf block is exactly one cache line.

Implementations

impl OidLayout

pub const ALL: [OidLayout; 3]

pub const fn header_len(self) -> usize

pub const fn name(self) -> &'static str

Trait Implementations

impl Clone for OidLayout

fn clone(&self) -> OidLayout

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Copy for OidLayout

impl Debug for OidLayout

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

Formats the value using the given formatter.

impl Default for OidLayout

fn default() -> OidLayout

Returns the “default value” for a type.

impl Eq for OidLayout

impl Hash for OidLayout

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl PartialEq for OidLayout

fn eq(&self, other: &OidLayout) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for OidLayout