smarm/ROADMAP_v0.5.md

smarm v0.5 — Runtime decomposition & run-path scaling

Goal: dismantle the single Mutex<SharedState> along the run path so the runtime scales to dozens of cores, with crash isolation hardened so a torn write or a stray cancellation can never poison shared runtime state.

Guiding rules established this cycle:

• Lock count that matters is hot-path locks. Cold lifecycle paths (spawn/join/monitor/link/finalize) may keep a small per-slot lock; the run path (yield/park/unpark/pop/resume) targets zero locks.
• Lock ordering: io-before-shared, and slot locks are leaves (never hold two slot locks at once). Any new lock states its position in this order.
• No unwind inside a runtime critical section. The stop sentinel only fires at lock-free observation points.
• Never hold a thread-local guard across a possible switch point. An actor can be preempted at any allocation and resume on a different OS thread; a live RefCell borrow (or std MutexGuard) then pairs its acquire/release across two threads' locals — count underflow / UB. with_runtime, with_shared, and every RawMutex guard disable preemption for exactly this reason (phase 2 found this the hard way).

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Phase 1 — State decomposition: easy peel-offs ✅ DONE

Split independent concerns out of SharedState so the global lock guards less.

• next_monitor_id → AtomicU64 on RuntimeInner (lock-free id minting).
• timers → own Mutex<Timers> (only drain-winner + blocking prims touch it).
• io → own Mutex<Option<IoThread>> (completion queue already self-locked).
• pending_closures: Vec → folded into Slot::pending_closure (per-actor data).
• Termination check split: read io liveness before shared; ordering argument documented in schedule_loop.
• Poison fix: gate check_cancelled() in maybe_preempt behind PREEMPTION_ENABLED, so a cancellation sentinel can never unwind while a std::sync::Mutex (shared/channel) is held. Regression test: tests/poison_stop.rs.

SharedState now holds only: slots, free_list, run_queue, root_pid.

Phase 2 — Slot table split ✅ DONE

Make slot lookup lock-free and per-slot state independently mutable.

• Fixed slab: Box<[Slot]>, slots never move → stable addresses, lock-free index. Assert on exhaustion with a panic message naming Config::max_actors(n) as the fix. Default max_actors = 16_384. (Mental note: must become unbounded or configurable-bounded later — see "Deferred" below. Do not let the fixed cap calcify into an assumption.)
• Generation packed INTO the state word (better than the planned separate AtomicU32): one AtomicU64 = (gen << 32) | state, so the gen check is atomic with every transition — no ABA, no spurious unparks, by CAS.
• Per-slot CAS state machine Vacant/Queued/Running/RunningNotified/ Parked/Done replacing state + pending_unpark. Also fixed a latent lost wakeup in the Blocking-IO completion path (result set for a still-Running actor without a flag).
• sp → relaxed AtomicUsize; stop flag + first-resume closure as AtomicPtrs — resume path fully lock-free.
• Per-slot raw non-poisoning futex mutex (src/raw_mutex.rs) for the cold collections. Guard enters NoPreempt.
• Free list → RawMutex<Vec<u32>> (a leaf lock). Treiber/striped only if the phase-4 spawn-storm bench shows contention — revisit then.
• finalize_actor link cascade locks peers one at a time; acyclicity argument written at the site. link() registers target-first with the race argument at the site.
• live_actors atomic; termination = io_out == 0 (read pre-queue-lock) && queue empty && live == 0; decrement-last ordering documented as the correctness crux at the site.

Phase 3 — Pluggable run queue ✅ DONE (shootout = phase 4 harness)

• RunQueue alias in src/run_queue.rs, compile-time selected, compile_error! guards for zero / >1 features. No runtime dispatch. All variants compile in every build (unit tests always run); the feature only picks the alias. Non-default variants: --no-default-features --features rq-…. - rq-mutex — Mutex<VecDeque>, the control/baseline. - rq-mpmc — single hand-rolled Vyukov bounded MPMC ring (per-cell seq numbers). Strict FIFO; "one hot cache line". - rq-striped — M Vyukov rings, fetch-add ticket distribution. Relaxed FIFO, reordering bounded by ~M. Predicted winner @20c.
• Bounded rings sound via slab cap + at-most-once-enqueued (occupancy ≤ max_actors); mpmc capacity = next_pow2(max_actors), striped Σcapacity ≈ 2×max_actors with probe-from-home-stripe. A full ring panics as an invariant violation (double enqueue), never spins.
• All hand-rolled, dependency-free.
• Two contracts surfaced by the extraction, documented + debug-asserted in run_queue.rs: queue ops require preemption disabled (a producer suspended mid-publish stalls every consumer behind its cell — livelock); and pop-None is a snapshot, not a fence — termination is counter-first (live == 0 alone implies the queue holds nothing actionable; argument rewritten at the schedule_loop site).

Phase 4 — Bench harness

• Raw-structure microbench: N threads, push/pop throughput vs thread count, sweeping producer:consumer ratios. Isolates the data structure.
• Runtime-level: yield-storm, ping-pong-pairs, spawn-storm, all sweeping scheduler count. Reuses existing benches/ harness style.
• Driver script rebuilding per rq-* feature to compare variants in one go.
• Sandbox validates correctness via oversubscribed Config::exact(N) on 1 core; real contention numbers come from the 20-core box.

Phase 5 — Safety hardening & model checking

• loom (dev-dependency only, x86 Linux) model tests for the slot state machine and each ring variant.
• NoPreempt / no-unwind-under-lock audit across all internal critical sections; assert the invariant in debug builds where feasible.

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Fast follow (post-v0.5, written down so it isn't lost)

• Channel mutex migration. channel::Inner<T> is Arc<Mutex<_>> of the same poison class as the old shared lock; recv_match even runs a user predicate under it. The Phase-1 check_cancelled gating already removes the unwind source globally, so channels are poison-safe today — but phase 2 surfaced a second, sharper reason to migrate: the std MutexGuard is held with preemption ENABLED, so a timeslice switch inside a channel critical section migrates the actor and unlocks the pthread mutex from a different OS thread — technically UB (Linux futexes happen to tolerate it). The RawMutex guard disables preemption and is cross-thread-release sound by construction. Migrate as the first post-v0.5 change.

Deferred / explicitly out of scope for v0.5

• Unbounded / configurable-bounded actor count. v0.5 ships a fixed slab with a loud assert. Revisit with a segmented slab (array of AtomicPtr<Segment>, doubling segment sizes, append-only) once we actually hit the cap or a workload demands it.
• Idle-wakeup eventcount. Idle scheduler threads keep the current 100µs poll-sleep. A futex-based eventcount is a later optimization if benches show idle latency matters.
• User-facing safe data structures (ArcSwap-style cells, structurally- shared persistent maps). Context for the runtime work, not in scope here.
• IO fd hygiene on actor death (pre-existing v0.2 TODO in io.rs).