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).
Shootout (6d9f369 harness, 24-core sweep) landed in RFC 005's World 3:
queue variants indistinguishable in rq_runtime; per-wake latency is the
ceiling. Record the queue-topology closure (rq-mutex frozen as default,
benchmark-driven reopening only), define v0.9 as the wake-path latency
cycle (RFC 005 slot -> bench -> RFC 004 spinning workers -> bench +
interaction pass), and add per-switch cost to Later pending a profiling
spike + RFC.
Completed cycles compacted to the two trailing minors (v0.7, v0.8);
earlier history lives in git.
- Replace v0.3 content throughout; bump version label to v0.8
- New SVG: Krebs-cycle scheduler loop (6 stations, actor handoff as
energy exchange, finalize leaving the cycle)
- New SVG: gen_server dual-plane (model vs implementation mapping)
- New SVG: slot state machine with packed word, park-epoch and all
transitions (loom theorems annotated)
- Updated SVG: module map (22 modules, 4 layers incl. slot_state,
run_queue, raw_mutex, sync_shim, monitor, link, registry, trace)
- Updated SVG: dep graph (runtime hub, OTP layer on public surface)
- New sections: Slot State & Park-Epoch, Run Queue variants,
GenServer, OTP (monitors/links/supervisors/registry)
- Updated: init (slab allocation, counter-based termination), spawn
(closure in slot AtomicPtr, stack pool, live_actors ordering),
preemption (stop sentinel + StopSentinel/Outcome::Stopped),
IO (epoch-matched completions, fd-leak fix), gotchas (lost-wakeup
and global-mutex flipped to green; 4 new cards)
- Yield-sources table grown to 9 rows (select, recv_timeout,
call, request_stop)
- Fix: swap local css2.css font ref for Google Fonts CDN
A stopped actor unwinding out of wait_fd's park leaked its waiters entry
and the kernel-side EPOLLONESHOT registration; the stale entry then failed
every future wait_*() on that fd with AlreadyExists (the defensive bare DEL
in epoll_register sits behind the contains_key check, so it never ran).
Fix where the invariant breaks: a drop guard in wait_fd, armed after a
successful register and forgotten on the normal wake (where FdReady already
removed + DEL'd). On unwind it cleans up iff the entry is still this wait's
(pid, epoch) — an entry consumed by a racing FdReady means the fd may carry
another actor's fresh registration, which must be left alone. Closes the
v0.2 fd-hygiene TODO.
Monitors are created at runtime (you watch a worker you just spawned in a
handler), so down arms can't ride the static info list. init grows a
&ServerCtx parameter (breaking; no-op default) whose clonable Watcher hands
Monitors to the loop over a control arm; the loop selects the live down
arms ahead of everything else. Arm priority: downs → control → infos →
inbox. A delivered Down retires its arm (monitors are one-shot); a state
that never clones the Watcher closes the control arm after init and the
loop falls back to the plain-inbox park.
type Info + handle_info (no-op default) on GenServer; ServerBuilder
(with_info/under/start) so start variants don't multiply — start/start_under
stay as wrappers. The loop selects [infos.., inbox] in priority order when
info arms exist, and keeps the plain recv() park when none do. Closed info
arms are silently removed (closed-arm-is-ready-forever); a closed inbox
still means graceful shutdown.
benches/README.md: catalog of the two bench families (cross-runtime
comparisons + the v0.5 run-queue shootout), how to run the shootout
(scripts/bench_rq.sh, SMARM_BENCH_* knobs, the --no-default-features dance,
summary.csv/RQCSV format), honest-numbers caveats (core count is the
experiment; striped losing at low thread counts is expected), and the
conventions for adding a bench.
tests/README.md: catalog grouped by layer (low-level units / feature areas /
regression+stress), the run matrix (debug-first — that's where the invariant
asserts live — three queue variants, release, loom, trace), why loom tests
live in src/ rather than tests/, and the house conventions: one runtime per
test, oversubscription on purpose, regression tests validated against the
reintroduced bug, odds-stacking for stochastic tests, ordering-not-duration
time assertions, and new-invariant = assert + loom model.
Also fixes a stale header in tests/mutex.rs that claimed 'loom::Mutex' —
it tests smarm::Mutex, and the old name is actively confusing now that loom
is real in this repo.
State machine extracted to src/slot_state.rs as a standalone unit (StateWord:
publish_queued / try_claim / yield_return / park_return / unpark / set_done /
reclaim, plus the Status view for cold paths). runtime.rs keeps the protocol
rationale and consumes the mechanism; every transition self-asserts its
precondition per the assert-the-invariants rule.
src/sync_shim.rs: std vs loom indirection (atomics + UnsafeCell with the
with/with_mut access API) for the two loom-modeled modules. Loom models run
the PRODUCTION code, not a replica:
- slot_state: no-lost-wakeup (park vs unpark), two-unparkers-one-enqueue,
stale-unpark-never-hits-reused-slot (the ABA theorem), unpark-vs-claim.
- run_queue: mpmc exactly-once through a lap wraparound, push/pop race,
striped two-producer drain.
RUSTFLAGS="--cfg loom" cargo test --lib --release — 7 models, all pass.
RawMutex deliberately not loom-modeled (futexes can't be; textbook mutex3
with stress + unwind coverage).
Assertion sweep (invariants now checked at the point of reliance):
- enqueue debug-asserts the word reads EXACTLY (gen, Queued) — the
at-most-once-enqueued invariant the ring capacity proof leans on.
- RawMutex enforces the leaf rule mechanically: debug-build thread-local
held-count, panics at the acquisition that violates it.
- live_actors underflow (double finalize) asserted.
- StateWord transition preconditions asserted (yield/park/done/reclaim/
publish/claim).
Audit: with_runtime, RawMutex guards, run-queue ops, and trace::record all
gate preemption (and thereby the stop sentinel) for their span; trace was
already self-gating. Sole remaining exception is the channel std MutexGuard
— the documented first post-v0.5 fast-follow.
Review fixes to the branched-in work:
- slot_state::status_for mapped (matching gen, Vacant) to Stale instead of
unreachable!: Pid::new is public, so a forged/never-issued pid (e.g.
monitor(Pid::new(5,0)) on a fresh slab) could reach it — "no such actor"
is the correct total answer; issued pids still can't get there.
- Tightened enqueue's assert from Status::Live to the exact (gen, Queued)
word its own comment argues for.
Validated: 22 suites in debug (all asserts + leaf counter live) for
rq-mutex/rq-mpmc/rq-striped, release for rq-mutex, smarm-trace build +
stress, loom 7/7.
- 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).
src/run_queue.rs: the run queue extracted behind a compile-time-selected
type alias; mutually-exclusive cargo features with compile_error! guards
(zero or >1 selected). No runtime dispatch. All variants compile in every
build so their unit tests always run; the feature only picks the alias.
- rq-mutex (default): Mutex<VecDeque>, the control/baseline.
- rq-mpmc: hand-rolled Vyukov bounded MPMC ring, per-cell sequence numbers,
padded enqueue/dequeue counters. Strict FIFO, lock-free.
- rq-striped: M Vyukov rings (M = thread_count rounded up to pow2),
fetch-add ticket distribution, probe-from-home on push. Relaxed FIFO with
stripe-bounded skew; Σcapacity ≈ 2×max_actors so the probe terminates.
Capacity soundness: occupancy ≤ max_actors by the at-most-once-enqueued
invariant + slab cap, rings sized ≥ that bound, so push is infallible; a
full ring panics as a double-enqueue invariant violation rather than spin.
Two contracts the extraction made explicit (documented + debug-asserted):
- Queue ops require preemption disabled: a producer suspended between
claiming a cell and publishing its sequence stalls every consumer behind
it — livelock, since the suspended actor's own resume entry is behind the
hole. Structurally guaranteed since the phase-2 with_runtime NoPreempt fix.
- pop()==None is a snapshot, not a fence. Termination is counter-first:
every queue entry's target stays Queued (hence live) until that entry is
popped, so live==0 alone implies nothing actionable is or can be queued;
argument rewritten at the schedule_loop site. SharedState and with_shared
are deleted — nothing global is mutex-guarded on the run path under the
ring variants.
Validated: 22 suites green per variant (release for all three; debug with
live asserts for all three), ring unit tests (FIFO, lap wraparound,
4p/4c exactly-once, skewed drain) in every build, both compile_error!
guards verified to fire.
The slot table split (ROADMAP_v0.5 phase 2): slot lookup is lock-free, the
run path (yield/park/unpark/pop/resume) takes zero locks beyond the queue
mutex itself, and SharedState shrinks to { run_queue }.
Core pieces:
- src/raw_mutex.rs: hand-rolled 3-state futex mutex (Drepper mutex3),
non-poisoning; the guard enters NoPreempt, which both bounds hold times and
structurally closes the unwind-under-lock hole (the stop sentinel shares
the gate). Used for per-slot cold data, the free list, and the stack pool.
- Fixed slab Box<[Slot]> (default max_actors = 16_384, ~4 MiB, align(128)
against false sharing). Slots never move -> stable addresses, lock-free
index. Exhaustion panics loudly, naming Config::max_actors(n) as the fix.
Unbounded/segmented slab stays deferred (see ROADMAP).
- Per-slot AtomicU64 packing (generation << 32 | state), states
Vacant/Queued/Running/RunningNotified/Parked/Done. Every transition CASes
the packed word, so the generation check is atomic with the transition: no
ABA, no spurious unparks on recycled slots. RunningNotified replaces the
pending_unpark bool (the lost-wakeup window is a state, not a flag) and
uniformly fixes a LATENT LOST WAKEUP in the old Blocking-IO completion
path, which set the result for a still-Running actor without flagging it.
- Invariant: a pid is in the run queue at most once (pushes pair 1:1 with
transitions into Queued; only the scheduler does Queued->Running). This is
what makes phase 3's bounded rings sound.
- sp -> relaxed AtomicUsize; stop flag + first-resume closure as AtomicPtrs
(closure double-boxed for a thin pointer, swap-to-take): the resume path is
fully atomic.
- finalize_actor: Done published under the dying slot's cold lock (join's
check-or-register is linearized by it); link cascade locks peers ONE AT A
TIME (cold locks are leaves) with the acyclicity argument written at the
site; link() registers on the target first, then self (stale self-entries
are benign, every walk re-verifies the peer's word).
- Termination by counters: live_actors incremented in spawn pre-enqueue,
decremented at the very END of finalize after all wakeup enqueues; exit on
io_out == 0 (read before the queue lock, phase-1 ordering) && queue empty
&& live == 0. Soundness note at the site: any enqueue targets a live actor.
- spawn boxes the closure and acquires the stack BEFORE any runtime lock: no
allocation ever happens under a global lock anymore.
- with_runtime/try_with_runtime now enter NoPreempt for their full span.
This fixes a bug the rework exposed: install_actor allocated with
preemption enabled while the RUNTIME RefCell borrow was live; a timeslice
preemption there migrates the actor across OS threads and the borrow guard
increments one thread's RefCell count and decrements another's — underflow
to 'permanently mutably borrowed', cascading panics (caught by stress
suite: deterministic non-unwinding-panic abort in lost_wakeup_many_pairs).
The old code was safe only by accident; now it's structural.
- Behavior note: request_stop on a RUNNING target now marks it
RunningNotified, so its next park returns immediately to an observation
point — faster stop observation; parked/queued/done semantics unchanged.
Validated: 22 suites green in release (1/2/8-thread oversubscribed on the
1-core sandbox) and in debug with all debug_asserts live; stress suite x5.
The phase-1 split filled Idle::next_deadline from the timers lock after the
shared lock was released, which turned the stale-PID branch (formerly a 100µs
poll) into a potential sleep-until-timer-deadline while runnable actors sat in
the queue — a hard stall on exact(1). Stale pops are now PopResult::Retry and
loop without sleeping.
test(poison): strengthen regression coverage. The stop-storm test never fired
the sentinel under a lock (its actors only allocate at lock-free points). Add
self_stop_during_spawn_does_not_poison_shared_mutex: a stop-flagged actor whose
next allocation is the Box::new(closure) inside spawn's with_shared. Validated
both ways: passes with the gate, SIGABRTs (unwind-in-allocator) with the gate
removed. Also alloc_interval(1) so every allocation is an observation point.
- next_monitor_id -> AtomicU64 on RuntimeInner
- timers -> own Mutex<Timers>; io -> own Mutex<Option<IoThread>> (lock order: io-before-shared)
- pending_closures Vec folded into Slot::pending_closure
- termination check reads io liveness before shared; ordering argument documented
- poison fix: check_cancelled() no longer fires while preemption is disabled,
so a cancellation unwind can never poison a runtime/channel mutex
- regression test: tests/poison_stop.rs
- ROADMAP_v0.5.md added
Stable Rust emits stack probes inline (subq/movq/jne loop) rather than
calling __rust_probestack, so there's no transparent hook for stack-
frame preemption. Override of __rust_probestack links cleanly but never
runs. Falling back to an explicit check!() that users drop into hot
compute loops.
check!() decrements the same ALLOC_COUNT counter as the heap path, so
both event sources fire timeslice checks at the same rate. Documents
the prep-to-park invariant on maybe_preempt — library code that
registers a wakeup and then parks must keep that window alloc-free and
check-free, or a preemption-driven yield in the middle would lose the
wakeup.
Adds a BinaryHeap of timer entries on SchedulerState. sleep() inserts
an entry and parks; schedule_loop pops due entries each iteration and
unparks them. When the run queue is empty but timers are pending, the
OS thread sleeps until the soonest deadline.
Single-threaded only; thread::sleep is fine because no other thread
can wake us. The IO thread coming next will need a Condvar or pipe
wakeup to break this OS-sleep early.
v0.2 will add stack-frame entries as a second preemption event source.
Both routes share ALLOC_COUNT so the timeslice check rate is the same
whether the actor is alloc-heavy, frame-heavy, or mixed.
Hand-rolled context switching on mmap'd stacks with guard pages,
allocator-driven RDTSC preemption, unbounded MPSC channels, supervision
via per-slot Signal mailboxes, root supervisor as sentinel PID.
Lib + tests + benches clean check/clippy. All 29 tests pass.
Bench: smarm 3.4% over serial baseline, within 160us of tokio
current-thread on prime-counting fan-out.
Claude