Reduce contention in watch channel #5464
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Darksonn
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A-tokio
We need the `Notified` future to be created before the first poll. Otherwise, we wont catch `notify_waiters` calls between the creation and first poll of the future.
carllerche
reviewed
Feb 17, 2023
| /// This function implements the case where randomness is not available. | ||
| #[cfg(not(all(not(loom), feature = "sync", any(feature = "rt", feature = "macros"))))] | ||
| pub(super) fn notified(&self) -> Notified<'_> { | ||
| let i = self.next.fetch_add(1, Relaxed) % 8; |
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Could you use TaskID here or something similar? Not a big deal either way.
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We can't use task or thread ids here, because they require the context thread-local to be available, which it isn't when we use the fallback.
carllerche
approved these changes
Feb 17, 2023
|
@tijsvd Would you be willing to submit a PR that adds your benchmark to Tokio's benchmarks? |
Yes I'll look into it. I'm now looking at some weird effects in a changed version of this benchmark, where it looks like the time of the sleep between test runs hugely influences the outcome. Positive results of this PR are still measurable, but I want to understand this sleep effect better before submitting anything. |
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Ok, the sleep time influences the test run just because of CPU frequency stepping. And because it ran so fast, I increased the amount of "work" by a factor of 10. Now when running the test hot, I could hardly measure a difference between pre and post PR. This was due to the probability of mutex contention being lower and/or the effect of contention being smaller. Bringing back the amount of work to its original allows for running the test hot (criterion style), and shows a significant improvement in this PR (the variant with pre-allocated waker slots still outperforms it by a margin). |
I saw quite a bit of variance running benches; you might want to check moving to Criterion instead, it's generally a straightforward conversion. In my experience, Criterion outcomes are a bit more stable due to proper warm-up. |
Noah-Kennedy
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Mar 1, 2023
# 1.26.0 (March 1st, 2023) ### Fixed - sync: don't leak tracing spans in mutex guards ([#5469]) - sync: drop wakers after unlocking the mutex in Notify ([#5471]) - sync: drop wakers outside lock in semaphore ([#5475]) - macros: fix empty `join!` and `try_join!` ([#5504]) ### Added - fs: add `fs::try_exists` ([#4299]) - net: add types for named unix pipes ([#5351]) - sync: add `MappedOwnedMutexGuard` ([#5474]) ### Documented - task: clarify what happens to spawned work during runtime shutdown ([#5394]) - task: clarify `process::Command` docs (#5406) ([#5413]) - sync: add doc aliases for `blocking_*` methods ([#5448]) - task: fix wording with 'unsend' ([#5452]) - signal: updated Documentation for Signals ([#5459]) - sync: fix docs for Send/Sync bounds in broadcast ([#5480]) - io: improve AsyncFd example ([#5481]) - tokio: document supported platforms ([#5483]) - runtime: document the nature of the main future ([#5494]) - sync: document drop behavior for channels ([#5497]) - time: document immediate completion guarantee for timeouts ([#5509]) - runtime: remove extra period in docs ([#5511]) ### Changed - net: use Message Read Mode for named pipes ([#5350]) - chore: update windows-sys to 0.45 ([#5386]) - sync: mark lock guards with `#[clippy::has_significant_drop]` ([#5422]) - sync: reduce contention in watch channel ([#5464]) - time: remove cache padding in timer entries ([#5468]) - time: Improve `Instant::now()` perf with test-util ([#5513]) ### Internal Changes - tests: port proptest fuzz harnesses to use cargo-fuzz ([#5392]) - time: don't store deadline twice in sleep entries ([#5410]) - rt: remove Arc from Clock ([#5434]) - sync: make `notify_waiters` calls atomic ([#5458]) - net: refactor named pipe builders to not use bitfields ([#5477]) - io: use `poll_fn` in `copy_bidirectional` ([#5486]) - fs: add more tests for filesystem functionality ([#5493]) - net: fix test compilation failure ([#5506]) - io: ignore SplitByUtf8BoundaryIfWindows test on miri ([#5507]) ### Unstable - metrics: add a new metric for budget exhaustion yields ([#5517]) [#4299]: #4299 [#5350]: #5350 [#5351]: #5351 [#5386]: #5386 [#5392]: #5392 [#5394]: #5394 [#5410]: #5410 [#5413]: #5413 [#5422]: #5422 [#5434]: #5434 [#5448]: #5448 [#5452]: #5452 [#5458]: #5458 [#5459]: #5459 [#5464]: #5464 [#5468]: #5468 [#5469]: #5469 [#5471]: #5471 [#5474]: #5474 [#5475]: #5475 [#5477]: #5477 [#5480]: #5480 [#5481]: #5481 [#5483]: #5483 [#5486]: #5486 [#5493]: #5493 [#5494]: #5494 [#5497]: #5497 [#5504]: #5504 [#5506]: #5506 [#5507]: #5507 [#5509]: #5509 [#5511]: #5511 [#5513]: #5513 [#5517]: #5517
Noah-Kennedy
added a commit
that referenced
this pull request
Mar 1, 2023
# 1.26.0 (March 1st, 2023) ### Fixed - macros: fix empty `join!` and `try_join!` ([#5504]) - sync: don't leak tracing spans in mutex guards ([#5469]) - sync: drop wakers after unlocking the mutex in Notify ([#5471]) - sync: drop wakers outside lock in semaphore ([#5475]) ### Added - fs: add `fs::try_exists` ([#4299]) - net: add types for named unix pipes ([#5351]) - sync: add `MappedOwnedMutexGuard` ([#5474]) ### Changed - chore: update windows-sys to 0.45 ([#5386]) - net: use Message Read Mode for named pipes ([#5350]) - sync: mark lock guards with `#[clippy::has_significant_drop]` ([#5422]) - sync: reduce contention in watch channel ([#5464]) - time: remove cache padding in timer entries ([#5468]) - time: Improve `Instant::now()` perf with test-util ([#5513]) ### Internal Changes - io: use `poll_fn` in `copy_bidirectional` ([#5486]) - net: refactor named pipe builders to not use bitfields ([#5477]) - rt: remove Arc from Clock ([#5434]) - sync: make `notify_waiters` calls atomic ([#5458]) - time: don't store deadline twice in sleep entries ([#5410]) ### Unstable - metrics: add a new metric for budget exhaustion yields ([#5517]) ### Documented - io: improve AsyncFd example ([#5481]) - runtime: document the nature of the main future ([#5494]) - runtime: remove extra period in docs ([#5511]) - signal: updated Documentation for Signals ([#5459]) - sync: add doc aliases for `blocking_*` methods ([#5448]) - sync: fix docs for Send/Sync bounds in broadcast ([#5480]) - sync: document drop behavior for channels ([#5497]) - task: clarify what happens to spawned work during runtime shutdown ([#5394]) - task: clarify `process::Command` docs ([#5413]) - task: fix wording with 'unsend' ([#5452]) - time: document immediate completion guarantee for timeouts ([#5509]) - tokio: document supported platforms ([#5483]) [#4299]: #4299 [#5350]: #5350 [#5351]: #5351 [#5386]: #5386 [#5394]: #5394 [#5410]: #5410 [#5413]: #5413 [#5422]: #5422 [#5434]: #5434 [#5448]: #5448 [#5452]: #5452 [#5458]: #5458 [#5459]: #5459 [#5464]: #5464 [#5468]: #5468 [#5469]: #5469 [#5471]: #5471 [#5474]: #5474 [#5475]: #5475 [#5477]: #5477 [#5480]: #5480 [#5481]: #5481 [#5483]: #5483 [#5486]: #5486 [#5494]: #5494 [#5497]: #5497 [#5504]: #5504 [#5509]: #5509 [#5511]: #5511 [#5513]: #5513 [#5517]: #5517
Noah-Kennedy
added a commit
that referenced
this pull request
Mar 1, 2023
# 1.26.0 (March 1st, 2023) ### Fixed - macros: fix empty `join!` and `try_join!` ([#5504]) - sync: don't leak tracing spans in mutex guards ([#5469]) - sync: drop wakers after unlocking the mutex in Notify ([#5471]) - sync: drop wakers outside lock in semaphore ([#5475]) ### Added - fs: add `fs::try_exists` ([#4299]) - net: add types for named unix pipes ([#5351]) - sync: add `MappedOwnedMutexGuard` ([#5474]) ### Changed - chore: update windows-sys to 0.45 ([#5386]) - net: use Message Read Mode for named pipes ([#5350]) - sync: mark lock guards with `#[clippy::has_significant_drop]` ([#5422]) - sync: reduce contention in watch channel ([#5464]) - time: remove cache padding in timer entries ([#5468]) - time: Improve `Instant::now()` perf with test-util ([#5513]) ### Internal Changes - io: use `poll_fn` in `copy_bidirectional` ([#5486]) - net: refactor named pipe builders to not use bitfields ([#5477]) - rt: remove Arc from Clock ([#5434]) - sync: make `notify_waiters` calls atomic ([#5458]) - time: don't store deadline twice in sleep entries ([#5410]) ### Unstable - metrics: add a new metric for budget exhaustion yields ([#5517]) ### Documented - io: improve AsyncFd example ([#5481]) - runtime: document the nature of the main future ([#5494]) - runtime: remove extra period in docs ([#5511]) - signal: updated Documentation for Signals ([#5459]) - sync: add doc aliases for `blocking_*` methods ([#5448]) - sync: fix docs for Send/Sync bounds in broadcast ([#5480]) - sync: document drop behavior for channels ([#5497]) - task: clarify what happens to spawned work during runtime shutdown ([#5394]) - task: clarify `process::Command` docs ([#5413]) - task: fix wording with 'unsend' ([#5452]) - time: document immediate completion guarantee for timeouts ([#5509]) - tokio: document supported platforms ([#5483]) [#4299]: #4299 [#5350]: #5350 [#5351]: #5351 [#5386]: #5386 [#5394]: #5394 [#5410]: #5410 [#5413]: #5413 [#5422]: #5422 [#5434]: #5434 [#5448]: #5448 [#5452]: #5452 [#5458]: #5458 [#5459]: #5459 [#5464]: #5464 [#5468]: #5468 [#5469]: #5469 [#5471]: #5471 [#5474]: #5474 [#5475]: #5475 [#5477]: #5477 [#5480]: #5480 [#5481]: #5481 [#5483]: #5483 [#5486]: #5486 [#5494]: #5494 [#5497]: #5497 [#5504]: #5504 [#5509]: #5509 [#5511]: #5511 [#5513]: #5513 [#5517]: #5517
This was referenced Apr 8, 2025
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See discussion in #5403 for context.
cc @tijsvd
Closes: #5403