time: reduce timer drop contention with per-worker timers #7668
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This benchmark demonstrates the mutex contention issue described in tokio-rs#6504, specifically focusing on the drop path for timers that are registered but never fire. The benchmark creates 10,000 sleep timers, polls each once to initialize and register it with the timer wheel, then drops them before they fire. This simulates the common case of timeouts that don't fire (e.g., operations that complete before their timeout). Baseline results show severe contention: the 8-worker case is only ~1.5x faster than single-threaded. Refs: tokio-rs#6504
Reduces lock contention in timer operations by registering timers in a per-worker HashMap for the multi-threaded runtime, while falling back to the global timer wheel for current_thread runtime and block_in_place. Benchmark results (benches/time_drop_sleep_contention.rs): - Single-threaded: 33.3ms → 32.7ms (no regression) - Multi-threaded (8 workers): 21.6ms → 16.0ms (25.9% faster) Refs tokio-rs#6504
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Darksonn
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Oct 10, 2025
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| fn fire_expired_timers(&mut self, now: Instant) { | ||
| self.timers.retain(|&deadline, wakers| { | ||
| (now < deadline) || { | ||
| wakers.drain(..).for_each(Waker::wake); | ||
| false | ||
| } | ||
| }); |
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This is a very expensive operation.
ADD-SP
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Oct 11, 2025
| /// This is called from TimerEntry::poll_elapsed when a timer is registered. | ||
| /// The waker will be fired when fire_expired_timers() is called with a time | ||
| /// >= deadline. | ||
| pub(crate) fn register_timer(&mut self, deadline: Instant, waker: Waker) { |
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Apart from issues of O(n) time complexity, how to de-register a timer?
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Ref #6504
Reduces lock contention in timer operations by registering timers in a per-worker HashMap for the multi-threaded runtime, while falling back to the global timer wheel for current_thread runtime and block_in_place contexts.
Benchmark
Added
benches/time_drop_sleep_contention.rsto measure timer drop performance: