diff --git a/futures-util/src/stream/futures_unordered/mod.rs b/futures-util/src/stream/futures_unordered/mod.rs index 46a0ae5bde..fdbd53de8e 100644 --- a/futures-util/src/stream/futures_unordered/mod.rs +++ b/futures-util/src/stream/futures_unordered/mod.rs @@ -6,7 +6,6 @@ use crate::task::AtomicWaker; use alloc::sync::{Arc, Weak}; use core::cell::UnsafeCell; -use core::cmp; use core::fmt::{self, Debug}; use core::iter::FromIterator; use core::marker::PhantomData; @@ -31,33 +30,6 @@ use self::task::Task; mod ready_to_run_queue; use self::ready_to_run_queue::{Dequeue, ReadyToRunQueue}; -/// Constant used for a `FuturesUnordered` to determine how many times it is -/// allowed to poll underlying futures without yielding. -/// -/// A single call to `poll_next` may potentially do a lot of work before -/// yielding. This happens in particular if the underlying futures are awoken -/// frequently but continue to return `Pending`. This is problematic if other -/// tasks are waiting on the executor, since they do not get to run. This value -/// caps the number of calls to `poll` on underlying futures a single call to -/// `poll_next` is allowed to make. -/// -/// The value itself is chosen somewhat arbitrarily. It needs to be high enough -/// that amortize wakeup and scheduling costs, but low enough that we do not -/// starve other tasks for long. -/// -/// See also https://github.com/rust-lang/futures-rs/issues/2047. -/// -/// Note that using the length of the `FuturesUnordered` instead of this value -/// may cause problems if the number of futures is large. -/// See also https://github.com/rust-lang/futures-rs/pull/2527. -/// -/// Additionally, polling the same future twice per iteration may cause another -/// problem. So, when using this value, it is necessary to limit the max value -/// based on the length of the `FuturesUnordered`. -/// (e.g., `cmp::min(self.len(), YIELD_EVERY)`) -/// See also https://github.com/rust-lang/futures-rs/pull/2333. -const YIELD_EVERY: usize = 32; - /// A set of futures which may complete in any order. /// /// This structure is optimized to manage a large number of futures. @@ -149,6 +121,7 @@ impl FuturesUnordered { next_ready_to_run: AtomicPtr::new(ptr::null_mut()), queued: AtomicBool::new(true), ready_to_run_queue: Weak::new(), + woken: AtomicBool::new(false), }); let stub_ptr = Arc::as_ptr(&stub); let ready_to_run_queue = Arc::new(ReadyToRunQueue { @@ -195,6 +168,7 @@ impl FuturesUnordered { next_ready_to_run: AtomicPtr::new(ptr::null_mut()), queued: AtomicBool::new(true), ready_to_run_queue: Arc::downgrade(&self.ready_to_run_queue), + woken: AtomicBool::new(false), }); // Reset the `is_terminated` flag if we've previously marked ourselves @@ -411,12 +385,12 @@ impl Stream for FuturesUnordered { type Item = Fut::Output; fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll> { - // See YIELD_EVERY docs　for more. - let yield_every = cmp::min(self.len(), YIELD_EVERY); + let len = self.len(); // Keep track of how many child futures we have polled, // in case we want to forcibly yield. let mut polled = 0; + let mut yielded = 0; // Ensure `parent` is correctly set. self.ready_to_run_queue.waker.register(cx.waker()); @@ -527,7 +501,11 @@ impl Stream for FuturesUnordered { // the internal allocation, appropriately accessing fields and // deallocating the task if need be. let res = { - let waker = Task::waker_ref(bomb.task.as_ref().unwrap()); + let task = bomb.task.as_ref().unwrap(); + // We are only interested in whether the future is awoken before it + // finishes polling, so reset the flag here. + task.woken.store(false, Relaxed); + let waker = Task::waker_ref(task); let mut cx = Context::from_waker(&waker); // Safety: We won't move the future ever again @@ -540,12 +518,17 @@ impl Stream for FuturesUnordered { match res { Poll::Pending => { let task = bomb.task.take().unwrap(); + // If the future was awoken during polling, we assume + // the future wanted to explicitly yield. + yielded += task.woken.load(Relaxed) as usize; bomb.queue.link(task); - if polled == yield_every { - // We have polled a large number of futures in a row without yielding. - // To ensure we do not starve other tasks waiting on the executor, - // we yield here, but immediately wake ourselves up to continue. + // If a future yields, we respect it and yield here. + // If all futures have been polled, we also yield here to + // avoid starving other tasks waiting on the executor. + // (polling the same future twice per iteration may cause + // the problem: https://github.com/rust-lang/futures-rs/pull/2333) + if yielded >= 2 || polled == len { cx.waker().wake_by_ref(); return Poll::Pending; } diff --git a/futures-util/src/stream/futures_unordered/task.rs b/futures-util/src/stream/futures_unordered/task.rs index 5216199831..ec2114effa 100644 --- a/futures-util/src/stream/futures_unordered/task.rs +++ b/futures-util/src/stream/futures_unordered/task.rs @@ -1,6 +1,6 @@ use alloc::sync::{Arc, Weak}; use core::cell::UnsafeCell; -use core::sync::atomic::Ordering::{self, SeqCst}; +use core::sync::atomic::Ordering::{self, Relaxed, SeqCst}; use core::sync::atomic::{AtomicBool, AtomicPtr}; use super::abort::abort; @@ -31,6 +31,11 @@ pub(super) struct Task { // Whether or not this task is currently in the ready to run queue pub(super) queued: AtomicBool, + + // Whether the future was awoken during polling + // It is possible for this flag to be set to true after the polling, + // but it will be ignored. + pub(super) woken: AtomicBool, } // `Task` can be sent across threads safely because it ensures that @@ -48,6 +53,8 @@ impl ArcWake for Task { None => return, }; + arc_self.woken.store(true, Relaxed); + // It's our job to enqueue this task it into the ready to run queue. To // do this we set the `queued` flag, and if successful we then do the // actual queueing operation, ensuring that we're only queued once. diff --git a/futures/tests/stream_futures_unordered.rs b/futures/tests/stream_futures_unordered.rs index 7c86c894fa..8ddd62f4a8 100644 --- a/futures/tests/stream_futures_unordered.rs +++ b/futures/tests/stream_futures_unordered.rs @@ -342,7 +342,7 @@ fn polled_only_once_at_most_per_iteration() { let mut tasks = FuturesUnordered::from_iter(vec![F::default(); 33]); assert!(tasks.poll_next_unpin(cx).is_pending()); - assert_eq!(32, tasks.iter().filter(|f| f.polled).count()); + assert_eq!(33, tasks.iter().filter(|f| f.polled).count()); let mut tasks = FuturesUnordered::::new(); assert_eq!(Poll::Ready(None), tasks.poll_next_unpin(cx));