feat(runtime): phase 3 — pluggable run queue (rq-mutex / rq-mpmc / rq-striped)
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.
This commit is contained in:
Claude
@@ -0,0 +1,467 @@
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//! The run queue, selected at COMPILE TIME by mutually-exclusive cargo
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//! features (no runtime dispatch — the scheduler's pop loop is the hottest
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//! code in the runtime):
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//!
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//! - `rq-mutex` (default) — `Mutex<VecDeque>`. The control/baseline:
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//! strictly FIFO, trivially correct, one global lock.
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//! - `rq-mpmc` — a single hand-rolled Vyukov bounded MPMC ring (per-cell
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//! sequence numbers). Strict FIFO, lock-free, one hot
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//! enqueue/dequeue cache-line pair.
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//! - `rq-striped` — M Vyukov rings with fetch-add ticket distribution.
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//! *Relaxed* FIFO: ordering across stripes is bounded-skewed
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//! (≈ one ring's worth of reordering per stripe), in exchange
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//! for spreading the hot line M ways. Predicted winner at
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//! high core counts; phase 4's shootout decides.
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//!
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//! Select non-default variants with `--no-default-features --features rq-…`
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//! (cargo features are additive, so the default must be switched off).
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//!
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//! All variants are compiled unconditionally (so every build runs every
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//! variant's unit tests); the feature only picks which one the runtime uses
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//! via the [`RunQueue`] alias.
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//!
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//! # Contract (shared by all variants)
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//!
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//! - **Occupancy is bounded by `max_actors`.** A pid is in the queue at most
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//! once (pushes pair 1:1 with transitions into `Queued`; only the
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//! scheduler transitions `Queued → Running` — see the state-machine docs
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//! in `runtime.rs`), and at most `max_actors` actors exist. The bounded
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//! rings are sized ≥ `max_actors`, so **`push` is infallible**; a full
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//! ring is an invariant violation and panics loudly rather than spinning.
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//! - **Preemption must be disabled around every push/pop** (debug-asserted).
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//! For the mutex variant this is the usual no-switch/no-unwind-under-lock
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//! rule. For the rings it is *load-bearing in a sharper way*: a producer
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//! suspended between claiming a cell and publishing its sequence number
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//! stalls every consumer behind that cell — on a busy runtime that is a
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//! livelock, since the suspended actor's own resume entry sits behind the
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//! hole. Callers get this for free: every queue op happens inside
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//! `with_runtime`/`try_with_runtime` (NoPreempt for their span since
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//! phase 2) or on a scheduler thread between resumes (preemption off).
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//! - **`pop() == None` is a snapshot, not a fence.** A push that is mid-
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//! publish (or in a stripe the probe already passed) may be missed; the
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//! caller's idle path sleeps ≤ 100µs and retries, so the cost is a bounded
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//! latency blip, never a lost entry. Termination does not lean on this:
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//! the all-clear is `live_actors == 0` (+ io quiescent), and `live == 0`
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//! already implies the queue holds nothing actionable — see the argument
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//! in `schedule_loop`.
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//! - `len()` is approximate (stats only).
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use crate::pid::Pid;
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use std::cell::UnsafeCell;
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use std::mem::MaybeUninit;
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use std::sync::atomic::{AtomicUsize, Ordering};
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// ---------------------------------------------------------------------------
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// Feature selection
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// ---------------------------------------------------------------------------
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#[cfg(not(any(feature = "rq-mutex", feature = "rq-mpmc", feature = "rq-striped")))]
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compile_error!(
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"smarm: no run queue selected. Enable exactly one of the features \
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`rq-mutex` (default), `rq-mpmc`, `rq-striped`."
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);
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#[cfg(all(feature = "rq-mutex", feature = "rq-mpmc"))]
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compile_error!(
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"smarm: features `rq-mutex` and `rq-mpmc` are mutually exclusive \
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(use --no-default-features to drop the default `rq-mutex`)."
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);
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#[cfg(all(feature = "rq-mutex", feature = "rq-striped"))]
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compile_error!(
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"smarm: features `rq-mutex` and `rq-striped` are mutually exclusive \
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(use --no-default-features to drop the default `rq-mutex`)."
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);
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#[cfg(all(feature = "rq-mpmc", feature = "rq-striped"))]
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compile_error!("smarm: features `rq-mpmc` and `rq-striped` are mutually exclusive.");
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#[cfg(feature = "rq-mutex")]
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pub(crate) type RunQueue = MutexQueue;
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#[cfg(feature = "rq-mpmc")]
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pub(crate) type RunQueue = MpmcRing;
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#[cfg(feature = "rq-striped")]
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pub(crate) type RunQueue = StripedRing;
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#[inline]
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fn assert_no_preempt() {
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debug_assert!(
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!crate::preempt::PREEMPTION_ENABLED.with(|c| c.get()),
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"run-queue op with preemption enabled — a switch mid-op stalls or \
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corrupts the queue; route through with_runtime or scheduler context"
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);
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}
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// ---------------------------------------------------------------------------
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// rq-mutex — the baseline
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// ---------------------------------------------------------------------------
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#[allow(dead_code)]
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pub(crate) struct MutexQueue {
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q: std::sync::Mutex<std::collections::VecDeque<Pid>>,
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}
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#[allow(dead_code)]
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impl MutexQueue {
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pub(crate) fn new(_threads: usize, max_actors: usize) -> Self {
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Self {
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// Pre-size: the queue can never outgrow the slab, and one
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// allocation at init beats reallocating under the lock later.
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q: std::sync::Mutex::new(std::collections::VecDeque::with_capacity(max_actors)),
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}
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}
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pub(crate) fn push(&self, pid: Pid) {
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assert_no_preempt();
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self.q.lock().unwrap().push_back(pid);
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}
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pub(crate) fn pop(&self) -> Option<Pid> {
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assert_no_preempt();
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self.q.lock().unwrap().pop_front()
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}
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pub(crate) fn len(&self) -> u64 {
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self.q.lock().unwrap().len() as u64
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}
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}
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// ---------------------------------------------------------------------------
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// rq-mpmc — Vyukov bounded MPMC ring
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// ---------------------------------------------------------------------------
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//
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// Dmitry Vyukov's bounded MPMC queue: each cell carries a sequence number.
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// A producer may write cell `i` when `seq == pos` (its turn); it publishes
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// with `seq = pos + 1`. A consumer may read when `seq == pos + 1`; it
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// releases the cell to the next lap with `seq = pos + capacity`. Producers
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// and consumers each contend on one counter; cell handoff is a per-cell
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// Acquire/Release pair, so unrelated push/pop pairs don't serialize.
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/// Pad to a cache-line pair so the producer and consumer counters (and the
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/// cells) don't false-share.
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#[repr(align(128))]
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struct CachePadded<T>(T);
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struct Cell {
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seq: AtomicUsize,
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pid: UnsafeCell<MaybeUninit<Pid>>,
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}
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#[allow(dead_code)]
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pub(crate) struct MpmcRing {
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buf: Box<[Cell]>,
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mask: usize,
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enqueue_pos: CachePadded<AtomicUsize>,
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dequeue_pos: CachePadded<AtomicUsize>,
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}
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// SAFETY: cells are handed off between threads via the per-cell seq
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// (Release on publish, Acquire on claim); Pid is Copy + Send.
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unsafe impl Send for MpmcRing {}
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unsafe impl Sync for MpmcRing {}
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#[allow(dead_code)]
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impl MpmcRing {
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pub(crate) fn new(_threads: usize, max_actors: usize) -> Self {
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Self::with_capacity(max_actors)
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}
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fn with_capacity(min_cap: usize) -> Self {
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// Occupancy ≤ max_actors (queue contract), so capacity = the next
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// power of two ≥ max_actors can never overflow. (≥ 2 so mask works.)
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let cap = min_cap.next_power_of_two().max(2);
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let buf: Box<[Cell]> = (0..cap)
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.map(|i| Cell {
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seq: AtomicUsize::new(i),
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pid: UnsafeCell::new(MaybeUninit::uninit()),
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})
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.collect();
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Self {
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buf,
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mask: cap - 1,
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enqueue_pos: CachePadded(AtomicUsize::new(0)),
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dequeue_pos: CachePadded(AtomicUsize::new(0)),
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}
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}
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pub(crate) fn push(&self, pid: Pid) {
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assert_no_preempt();
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assert!(
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self.try_push(pid),
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"smarm: run queue overflow — occupancy exceeded the slab bound, \
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which the at-most-once-enqueued invariant forbids. This is a \
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runtime bug (double enqueue), not a capacity tuning problem."
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);
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}
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/// One full claim attempt; `false` only if the ring is full.
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fn try_push(&self, pid: Pid) -> bool {
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let mut pos = self.enqueue_pos.0.load(Ordering::Relaxed);
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loop {
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let cell = &self.buf[pos & self.mask];
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let seq = cell.seq.load(Ordering::Acquire);
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let diff = seq as isize - pos as isize;
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if diff == 0 {
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// Our turn: claim the position.
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match self.enqueue_pos.0.compare_exchange_weak(
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pos, pos + 1, Ordering::Relaxed, Ordering::Relaxed,
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) {
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Ok(_) => {
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// SAFETY: the claim gives us exclusive write access
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// to this cell until we publish below.
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unsafe { (*cell.pid.get()).write(pid) };
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cell.seq.store(pos + 1, Ordering::Release);
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return true;
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}
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Err(actual) => pos = actual,
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}
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} else if diff < 0 {
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return false; // full — a whole lap behind
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} else {
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pos = self.enqueue_pos.0.load(Ordering::Relaxed);
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}
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}
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}
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pub(crate) fn pop(&self) -> Option<Pid> {
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assert_no_preempt();
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let mut pos = self.dequeue_pos.0.load(Ordering::Relaxed);
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loop {
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let cell = &self.buf[pos & self.mask];
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let seq = cell.seq.load(Ordering::Acquire);
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let diff = seq as isize - (pos + 1) as isize;
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if diff == 0 {
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match self.dequeue_pos.0.compare_exchange_weak(
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pos, pos + 1, Ordering::Relaxed, Ordering::Relaxed,
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) {
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Ok(_) => {
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// SAFETY: the claim gives us exclusive read access;
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// the producer's Release publish made `pid` visible
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// to our Acquire load of `seq`.
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let pid = unsafe { (*cell.pid.get()).assume_init_read() };
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// Release the cell for the next lap.
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cell.seq.store(pos + self.mask + 1, Ordering::Release);
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return Some(pid);
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}
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Err(actual) => pos = actual,
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}
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} else if diff < 0 {
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// Empty (or the producer at this cell hasn't published yet —
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// a snapshot miss the caller's idle-retry loop absorbs).
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return None;
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} else {
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pos = self.dequeue_pos.0.load(Ordering::Relaxed);
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}
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}
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}
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pub(crate) fn len(&self) -> u64 {
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let e = self.enqueue_pos.0.load(Ordering::Relaxed);
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let d = self.dequeue_pos.0.load(Ordering::Relaxed);
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e.saturating_sub(d) as u64
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}
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}
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// ---------------------------------------------------------------------------
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// rq-striped — M Vyukov rings, ticket-distributed
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// ---------------------------------------------------------------------------
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//
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// Producers fetch-add a ticket and start probing at stripe `ticket % M`;
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// consumers do the same with their own ticket. Under symmetric load the
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// tickets spread producers and consumers uniformly, so each stripe sees
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// ~1/M of the traffic and the single hot cache-line pair becomes M cooler
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// ones. FIFO is relaxed: two pushes that land in different stripes can be
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// popped in either order, with skew bounded by stripe occupancy imbalance.
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//
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// Push probes forward from its home stripe until a `try_push` succeeds.
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// Σ stripe capacity ≥ 2 × max_actors while occupancy ≤ max_actors, so at
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// every instant at least half the total capacity is free and the probe
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// terminates (in practice on the first stripe).
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#[allow(dead_code)]
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pub(crate) struct StripedRing {
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stripes: Box<[MpmcRing]>,
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/// Stripe count minus one (count is a power of two).
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stripe_mask: usize,
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push_ticket: CachePadded<AtomicUsize>,
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pop_ticket: CachePadded<AtomicUsize>,
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}
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|
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#[allow(dead_code)]
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impl StripedRing {
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pub(crate) fn new(threads: usize, max_actors: usize) -> Self {
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// One stripe per scheduler thread, rounded up to a power of two —
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// more stripes than threads buys nothing (at most `threads` ops are
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// in flight) and costs pop-probe latency when mostly empty.
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let n = threads.max(1).next_power_of_two();
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// Per-stripe capacity: 2 × max_actors / n in total, and never below
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// a floor that keeps degenerate configs (tiny slab, many threads)
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// trivially correct.
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let per = ((2 * max_actors) / n).next_power_of_two().max(8);
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let stripes: Box<[MpmcRing]> = (0..n).map(|_| MpmcRing::with_capacity(per)).collect();
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Self {
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stripes,
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stripe_mask: n - 1,
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push_ticket: CachePadded(AtomicUsize::new(0)),
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pop_ticket: CachePadded(AtomicUsize::new(0)),
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}
|
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}
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|
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pub(crate) fn push(&self, pid: Pid) {
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assert_no_preempt();
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let home = self.push_ticket.0.fetch_add(1, Ordering::Relaxed);
|
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// Probe from the home stripe; capacity headroom (Σ ≥ 2×occupancy)
|
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// guarantees a free stripe exists, so the outer loop terminates.
|
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// The retry-from-home lap handles the racy case where every stripe
|
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// momentarily refused us.
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loop {
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for i in 0..=self.stripe_mask {
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let s = &self.stripes[(home + i) & self.stripe_mask];
|
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if s.try_push(pid) {
|
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return;
|
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}
|
||||
}
|
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std::hint::spin_loop();
|
||||
}
|
||||
}
|
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|
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pub(crate) fn pop(&self) -> Option<Pid> {
|
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assert_no_preempt();
|
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let home = self.pop_ticket.0.fetch_add(1, Ordering::Relaxed);
|
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for i in 0..=self.stripe_mask {
|
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if let Some(pid) = self.stripes[(home + i) & self.stripe_mask].pop() {
|
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return Some(pid);
|
||||
}
|
||||
}
|
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None // snapshot miss possible across stripes; idle-retry absorbs it
|
||||
}
|
||||
|
||||
pub(crate) fn len(&self) -> u64 {
|
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self.stripes.iter().map(|s| s.len()).sum()
|
||||
}
|
||||
}
|
||||
|
||||
// ---------------------------------------------------------------------------
|
||||
// Tests — all variants, in every build (the feature only picks the alias)
|
||||
// ---------------------------------------------------------------------------
|
||||
|
||||
#[cfg(test)]
|
||||
mod tests {
|
||||
use super::*;
|
||||
use std::collections::HashSet;
|
||||
use std::sync::Arc;
|
||||
|
||||
fn pid(i: u32) -> Pid {
|
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Pid::new(i, 0)
|
||||
}
|
||||
|
||||
fn fifo_smoke<Q>(q: &Q, push: impl Fn(&Q, Pid), pop: impl Fn(&Q) -> Option<Pid>) {
|
||||
for i in 0..100 {
|
||||
push(q, pid(i));
|
||||
}
|
||||
for i in 0..100 {
|
||||
assert_eq!(pop(q), Some(pid(i)));
|
||||
}
|
||||
assert_eq!(pop(q), None);
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn mutex_fifo() {
|
||||
let q = MutexQueue::new(1, 1024);
|
||||
fifo_smoke(&q, |q, p| q.push(p), |q| q.pop());
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn mpmc_fifo_single_thread() {
|
||||
let q = MpmcRing::new(1, 1024);
|
||||
fifo_smoke(&q, |q, p| q.push(p), |q| q.pop());
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn mpmc_wraps_many_laps() {
|
||||
let q = MpmcRing::with_capacity(8);
|
||||
for lap in 0..1000u32 {
|
||||
for i in 0..8 {
|
||||
q.push(pid(lap * 8 + i));
|
||||
}
|
||||
for i in 0..8 {
|
||||
assert_eq!(q.pop(), Some(pid(lap * 8 + i)));
|
||||
}
|
||||
}
|
||||
assert_eq!(q.pop(), None);
|
||||
}
|
||||
|
||||
/// N producers, M consumers, every element exactly once. Run on plain OS
|
||||
/// threads (PREEMPTION_ENABLED defaults false, satisfying the contract).
|
||||
fn exactly_once<Q: Send + Sync + 'static>(
|
||||
q: Q,
|
||||
push: fn(&Q, Pid),
|
||||
pop: fn(&Q) -> Option<Pid>,
|
||||
producers: u32,
|
||||
consumers: u32,
|
||||
per_producer: u32,
|
||||
) {
|
||||
let q = Arc::new(q);
|
||||
let total = (producers * per_producer) as usize;
|
||||
let popped = Arc::new(std::sync::Mutex::new(Vec::with_capacity(total)));
|
||||
let remaining = Arc::new(AtomicUsize::new(total));
|
||||
|
||||
let mut hs = Vec::new();
|
||||
for p in 0..producers {
|
||||
let q = q.clone();
|
||||
hs.push(std::thread::spawn(move || {
|
||||
for i in 0..per_producer {
|
||||
push(&q, pid(p * per_producer + i));
|
||||
}
|
||||
}));
|
||||
}
|
||||
for _ in 0..consumers {
|
||||
let q = q.clone();
|
||||
let popped = popped.clone();
|
||||
let remaining = remaining.clone();
|
||||
hs.push(std::thread::spawn(move || {
|
||||
let mut local = Vec::new();
|
||||
while remaining.load(Ordering::Relaxed) > 0 {
|
||||
if let Some(pid) = pop(&q) {
|
||||
remaining.fetch_sub(1, Ordering::Relaxed);
|
||||
local.push(pid);
|
||||
} else {
|
||||
std::hint::spin_loop();
|
||||
}
|
||||
}
|
||||
popped.lock().unwrap().extend(local);
|
||||
}));
|
||||
}
|
||||
for h in hs {
|
||||
h.join().unwrap();
|
||||
}
|
||||
|
||||
let popped = popped.lock().unwrap();
|
||||
assert_eq!(popped.len(), total, "count mismatch");
|
||||
let set: HashSet<u64> = popped.iter().map(|p| ((p.index() as u64) << 32) | p.generation() as u64).collect();
|
||||
assert_eq!(set.len(), total, "duplicate or lost element");
|
||||
assert_eq!(pop(&q), None);
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn mpmc_exactly_once_contended() {
|
||||
exactly_once(MpmcRing::new(8, 4096), |q, p| q.push(p), |q| q.pop(), 4, 4, 1000);
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn striped_exactly_once_contended() {
|
||||
exactly_once(StripedRing::new(8, 4096), |q, p| q.push(p), |q| q.pop(), 4, 4, 1000);
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn striped_drains_after_skewed_load() {
|
||||
// Hammer pushes from one thread (all tickets walk the stripes in
|
||||
// order) and verify a single consumer sees every element.
|
||||
let q = StripedRing::new(4, 64);
|
||||
let mut seen = HashSet::new();
|
||||
for i in 0..64 {
|
||||
q.push(pid(i));
|
||||
}
|
||||
while let Some(p) = q.pop() {
|
||||
assert!(seen.insert(p.index()));
|
||||
}
|
||||
assert_eq!(seen.len(), 64);
|
||||
}
|
||||
}
|
||||
Reference in New Issue
Block a user