Cycle 3: Thread Parking for Instant Worker Wakeup — Development History

Commit: acacc1b · 4 files · 59 insertions, 10 deletions

Motivation

Before this change, idle workers used thread::sleep with a fixed timeout to wait for new work. This meant an idle worker wouldn't notice new messages until its sleep timer expired — up to 1ms of unnecessary latency on the idle-to-active transition. Under bursty workloads, this sleep-based backoff wastes both time and power.

Competitor Analysis

Runtime	Idle Strategy	Wakeup Mechanism
Tokio	Parker state machine (notified/sleeping/empty)	`unpark()` via atomic CAS
Linux	NO_HZ adaptive ticks (stop tick when idle)	Interrupt on new work
Go	`notewakeup` / futex	OS-level wake
BEAM	Scheduler sleep + signal	Thread signal
Swactor (before)	`thread::sleep(1ms)`	Timer expiry only

Tokio's parker uses a 3-state machine (notified → sleeping → empty) with atomic transitions. The key insight: unpark() is a no-op if the thread isn't parked, so callers pay zero cost on the hot path.

Implementation

Replaced thread::sleep with thread::park_timeout in worker run loop
Workers register thread::current() via OnceLock<Thread> on startup
send_to and spawn call Thread::unpark() on target worker after enqueuing work
Cross-worker sends from WorkerContext also unpark the target
Zero new dependencies — uses only std::sync::OnceLock + std::thread::park_timeout

Key files modified: src/worker.rs, src/runtime.rs, src/delivery.rs

Design Decisions

OnceLock<Thread> for thread handle storage — set-once semantics match the worker lifecycle (one thread per worker, never changes). Simpler than Mutex<Option<Thread>>.
park_timeout instead of park — timeout ensures workers periodically wake even without explicit unpark, preventing permanent sleep if an unpark is missed.
Unpark on send_to and spawn — these are the two operations that create work for a worker. The cost is a single atomic store (no-op if thread is already running).
No condvar — thread::park/unpark is simpler and avoids the spurious wakeup complexity of condition variables. Tokio's parker validates this approach.

Tests Added

1 new test (51 → 52 total):

mt_parked_worker_wakes_on_send — verifies that a parked worker processes a message immediately after send (not after timeout)

Result

52 tests pass
All workspace crates compile
Idle-to-active latency reduced from up to 1ms to near-zero
No overhead on hot path — unpark() is a no-op when thread isn't parked

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