Fallback to Parking in std::sync::mpsc Channels
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| // Because writes to the channel are also sequentially consistent, | ||
| // this creates a total order between storing a value and registering a waiter. | ||
| let state = self.state.load(Ordering::SeqCst); | ||
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| // If a notification is already set, the waker thread will take care | ||
| // of further notifications. Otherwise we have to notify if there are waiters. | ||
| if (state >> WAKER) > 0 && (state & NOTIFIED == 0) { | ||
| return self |
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given the fetch_update() is likely to be a a compare_exchange_weak loop already, could it go straight into it instead of doing this extra load check before it?
| /// on the channel, as well as creates a chain that makes up for lost wakeups, | ||
| /// allowing threads to park even if the channel is in an inconsistent state. | ||
| struct WakerState { | ||
| state: AtomicU64, |
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Thoughts on making this an AtomicUsize for 32bit platforms?
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Currently, the queues used by the
mpscchannel flavors are not lock-free. The linearization point of a send or receive is moving the tail or head index respectively, meaning that senders and receivers must spin if they see an updated head/tail despite the corresponding value not being written yet. Blocking in such a case is not possible as sends might notify a receiver even though they are not yet visible due to a lagging sender holding up the channel, or vice versa, leading to missed wakeups.The current solution of unbounded spinning can lead to issues, especially on platforms with a single core or custom thread priorities. One way to allow falling back to blocking is if waiters that see more values in the channel after being woken up notify any other waiters, making up for any missed notifications. However, this ends up being quite expensive and can lead to many unnecessary wakeups. The solution implemented by this PR is wakeup throttling, as used by tokio and other userspace schedulers. The new algorithm is in
sync/mpmc/waker.rs.Should resolve #114851 and #112723. An alternative solution is to introduce a non-linearizable version of
try_recv, but fixing the unbounded spinning is important regardless. This PR leaves a couple cases of unbounded spinning when discarding the messages in the unbounded channel that can be addressed in a later PR, those should not be a large concern.There are a couple other cosmetic changes included in this PR that were not previously ported from crossbeam. Hopefully they shouldn't distract from the primary changes. Downstream patch: crossbeam-rs/crossbeam#1105.
r? @Amanieu