A thread-local Cell<Option<Pid>> per scheduler, checked before the shared
run queue. Runtime-selected via Config { wake_slot: bool }, default OFF
until the slot shootout accepts it (one binary benches both arms).
Push policy: slot-eligible iff the wake originates from actor context
(current_pid().is_some()) — the slot push replaces run_queue.push at the
tail of the unpark protocol's Parked → Queued CAS, so at-most-once-enqueued
holds verbatim as (slot ⊕ shared queue). Scheduler-context wakes (timer/IO
drain) and spawns always go shared (spawns never reach unpark_inner at all).
Displacement is Go semantics: newest wake takes the slot, occupant pushed
shared — moved, never copied.
Pop order: slot, then shared. Slot-popped actors skip reset_timeslice() and
inherit the waker's remaining slice; a handoff chain is bounded by one
slice, after which the preempt-yield re-enqueue goes shared (a yield is not
a wake) — the one-slice starvation bound, zero new counters. Idle and
AllDone are only reachable with an empty local slot by pop order; an
occupied slot elsewhere holds a Queued (live) actor, so the counter-first
termination argument is untouched.
Observability: per-thread slot_hits / slot_displacements (reset at run()
start so post-run stats() reads are per-run), SlotPush/SlotPop trace events.
Bench plan (roadmap v0.9 item 2): rq_runtime gains the slot on/off
dimension (SMARM_BENCH_SLOT, default "0 1") — ping-pong-pairs is the
target metric, yield-storm the regression guard, spawn-storm the
neutrality check. RQCSV grows a slot column; RQSLOT lines carry the
counters; bench_rq.sh aggregates both. Tests pin the push policy through
the counters (actor-context hits, spawn/join bypass, displacement,
default-off, per-run reset).
- benches/rq_micro.rs: raw-structure microbench, threads x producer:consumer
ratio sweep. Benches all three queue types in one binary (they compile in
every build; only the runtime alias is feature-selected), so no rebuild
dance. Queues sized to the op count so the occupancy contract is met.
- benches/rq_runtime.rs: whole-runtime benches with the selected variant:
yield-storm (pure queue churn), ping-pong-pairs (park/unpark latency),
spawn-storm (slab + free list + queue under churn). Scheduler-count sweep.
- scripts/bench_rq.sh: rebuilds rq_runtime per rq-* feature, runs rq_micro
once, aggregates RQCSV lines into bench_results/summary.csv.
- All knobs via SMARM_BENCH_* env vars; house table format + machine lines.
- run_queue module is now #[doc(hidden)] pub (types + push/pop/len +
MpmcRing::with_capacity) solely so the external bench binary can drive the
raw structures.
docs(roadmap): phase 4 ticked (harness done; numbers from the 20-core box).
New fast-follow per review: assert the invariants we lean on — debug_assert!
on hot paths, loud assert!/panic on cold ones, at the point of reliance;
sweep existing code during the phase-5 audit, adopt as house style.
Validated end-to-end at smoke scale on the 1-core sandbox: full driver run,
24-row summary.csv across micro (3 structures x sweeps) and runtime
(3 variants x 3 benches x thread sweep).
Claude