[trunkszi]

Summary

Add a Criterion 0.8.2 hot-path benchmark suite with zero-allocation pre-flight gates across liburing-rs, refactor OperationSlab and CompletionSocketTable slot state into compact #[repr(u8)] Hierarchical State Machines (OperationPhase and SocketSlotPhase), and pack OperationSlab slot metadata to improve submission and completion hot-path latency.

Cargo.toml                                     |   1 +
Cargo.lock                                     | 392 +++++++++++++++++++++++++
crates/liburing/
├── Cargo.toml                                 |   5 +
├── benches/uring_hotpath_bench.rs             | 366 +++++++++++++++++++++++ (new)
└── src/
    ├── fixed_buffers.rs                       |  11 +-
    ├── identity_map.rs                        |  42 ++-
    ├── operation_slab.rs                      | 311 ++++++++++++++------
    └── socket_io.rs                           |  42 ++-

• Criterion 0.8.2 Benchmark Suite & Zero-Alloc Pre-Flight:
  • Register criterion = { version = "0.8.2", default-features = false, features = ["cargo_bench_support"] } in root [workspace.dependencies] and inherit via criterion = { workspace = true } in crates/liburing/Cargo.toml, with bench = false under [lib].
  • Add crates/liburing/benches/uring_hotpath_bench.rs covering single-shot, multishot, zero-copy, and cancellation OperationSlab lifecycles, OperationId payload bit-packing, FixedBufferRegistry lease/release, FixedFileSet lease/release, CompletionSocketTable close transitions, and real kernel io_uring NOP submit/wait dispatch, with a CountingAllocator zero-allocation pre-flight check outside the timed b.iter() loop.
• Hierarchical State Machines (#[repr(u8)]) & Slot Packing:
  • Replace the 4-state OperationState + 3 sidecar booleans (multishot, zero_copy, zero_copy_notified) in crates/liburing/src/operation_slab.rs with a single 1-byte #[repr(u8)] OperationPhase whose low 2 bits encode OperationState directly (state() is a single bitmask + projection, size_of::<OperationPhase>() == 1).
  • Pack the free-list index into a u32 alongside generation: u32 in #[repr(C)] struct Slot<Meta> and flatten steady-state completion, cancellation, and zero-copy notification transitions.
  • Introduce 1-byte #[repr(u8)] SocketSlotPhase (Closed, Active, ClosingPending, size_of::<SocketSlotPhase>() == 1) in crates/liburing/src/socket_io.rs.
  • Add an IdentityMap linear-scan fast path for small tables (<= 8 entries) in crates/liburing/src/identity_map.rs while keeping IdentityMap crate-private (pub(crate)).

Evidence

Before vs. After Criterion Benchmarks (origin/main @ 8155225 vs. Branch, Pinned Interleaved A/B on Core 29)

Benchmark	origin/main Mean	Branch Mean	Speedup / Delta	Steady-State Heap Allocs
operation_slab/zero_copy	2.71 ns	2.15 ns	1.26x faster (-20.7%)	0 B / 0 allocs
operation_slab/single	1.84 ns	1.57 ns	1.17x faster (-14.3%)	0 B / 0 allocs
operation_slab/cancel	2.17 ns	2.01 ns	1.08x faster (-7.4%)	0 B / 0 allocs
operation_slab/multishot	2.15 ns	2.11 ns	1.02x faster (-1.9%)	0 B / 0 allocs
operation_id/payload_pack	605.77 ps	517.50 ps	1.17x faster (-14.6%)	0 B / 0 allocs
proactor/submit_dispatch_nop	165.36 ns	163.44 ns	1.01x faster (-1.2%)	0 B / 0 allocs
fixed_file/lease_release	260.05 ns	252.42 ns	1.03x faster (-2.9%)	0 B / 0 allocs
fixed_buffer/lease_release	19.86 ns	19.96 ns	~1.00x	0 B / 0 allocs

Verification Gates

• cargo fmt --all -- --check — PASS
• cargo clippy --workspace --all-targets -- -D warnings — PASS
• cargo test --workspace — PASS (129 passed, 0 failed)
• cargo bench --workspace --no-run — PASS
• Max .rs file length across modified/added files: 786 lines (crates/liburing/src/operation_slab.rs, <= 800 limit).

Merge Danger

• Door: Two-Way Door. Public OperationState, OperationSlab, CompletionSocketTable, FixedBufferRegistry, and FixedFileSet APIs preserve full backward compatibility; IdentityMap and OperationPhase remain private to the crate.
• Blast Radius: crates/liburing/src/{fixed_buffers.rs, identity_map.rs, operation_slab.rs, socket_io.rs} and the new Criterion benchmark suite in crates/liburing/benches/uring_hotpath_bench.rs.