Add utility type WakerSet to the sync module (#390)
* Add utility type Registry to the sync module * Remove unused import * Split unregister into complete and cancel * Refactoring and renaming * Split remove() into complete() and cancel() * Rename to WakerSet * Ignore clippy warning * Ignore another clippy warning * Use stronger SeqCst orderingpull/433/head
Stjepan Glavina
5 years ago
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GitHub
parent
3dd59d7056
commit
87de4e1598
@ -0,0 +1,42 @@
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#![feature(test)]
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extern crate test;
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use std::sync::Arc;
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use async_std::sync::Mutex;
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use async_std::task;
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use test::Bencher;
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#[bench]
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fn create(b: &mut Bencher) {
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b.iter(|| Mutex::new(()));
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}
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#[bench]
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fn contention(b: &mut Bencher) {
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b.iter(|| task::block_on(run(10, 1000)));
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}
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#[bench]
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fn no_contention(b: &mut Bencher) {
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b.iter(|| task::block_on(run(1, 10000)));
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}
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async fn run(task: usize, iter: usize) {
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let m = Arc::new(Mutex::new(()));
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let mut tasks = Vec::new();
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for _ in 0..task {
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let m = m.clone();
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tasks.push(task::spawn(async move {
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for _ in 0..iter {
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let _ = m.lock().await;
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}
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}));
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}
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for t in tasks {
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t.await;
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}
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}
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@ -0,0 +1,11 @@
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#![feature(test)]
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extern crate test;
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use async_std::task;
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use test::Bencher;
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#[bench]
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fn block_on(b: &mut Bencher) {
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b.iter(|| task::block_on(async {}));
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}
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@ -0,0 +1,200 @@
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//! A common utility for building synchronization primitives.
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//!
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//! When an async operation is blocked, it needs to register itself somewhere so that it can be
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//! notified later on. The `WakerSet` type helps with keeping track of such async operations and
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//! notifying them when they may make progress.
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use std::cell::UnsafeCell;
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use std::ops::{Deref, DerefMut};
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use std::sync::atomic::{AtomicUsize, Ordering};
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use crossbeam_utils::Backoff;
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use slab::Slab;
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use crate::task::{Context, Waker};
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/// Set when the entry list is locked.
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#[allow(clippy::identity_op)]
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const LOCKED: usize = 1 << 0;
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/// Set when there are tasks for `notify_one()` to wake.
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const NOTIFY_ONE: usize = 1 << 1;
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/// Set when there are tasks for `notify_all()` to wake.
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const NOTIFY_ALL: usize = 1 << 2;
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/// Inner representation of `WakerSet`.
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struct Inner {
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/// A list of entries in the set.
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///
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/// Each entry has an optional waker associated with the task that is executing the operation.
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/// If the waker is set to `None`, that means the task has been woken up but hasn't removed
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/// itself from the `WakerSet` yet.
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///
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/// The key of each entry is its index in the `Slab`.
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entries: Slab<Option<Waker>>,
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/// The number of entries that have the waker set to `None`.
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none_count: usize,
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}
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/// A set holding wakers.
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pub struct WakerSet {
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/// Holds three bits: `LOCKED`, `NOTIFY_ONE`, and `NOTIFY_ALL`.
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flag: AtomicUsize,
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/// A set holding wakers.
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inner: UnsafeCell<Inner>,
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}
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impl WakerSet {
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/// Creates a new `WakerSet`.
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#[inline]
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pub fn new() -> WakerSet {
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WakerSet {
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flag: AtomicUsize::new(0),
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inner: UnsafeCell::new(Inner {
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entries: Slab::new(),
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none_count: 0,
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}),
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}
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}
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/// Inserts a waker for a blocked operation and returns a key associated with it.
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pub fn insert(&self, cx: &Context<'_>) -> usize {
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let w = cx.waker().clone();
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self.lock().entries.insert(Some(w))
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}
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/// Updates the waker of a previously inserted entry.
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pub fn update(&self, key: usize, cx: &Context<'_>) {
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let mut inner = self.lock();
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match &mut inner.entries[key] {
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None => {
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// Fill in the waker.
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let w = cx.waker().clone();
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inner.entries[key] = Some(w);
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inner.none_count -= 1;
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}
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Some(w) => {
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// Replace the waker if the existing one is different.
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if !w.will_wake(cx.waker()) {
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*w = cx.waker().clone();
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}
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}
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}
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}
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/// Removes the waker of a completed operation.
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pub fn complete(&self, key: usize) {
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let mut inner = self.lock();
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if inner.entries.remove(key).is_none() {
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inner.none_count -= 1;
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}
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}
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/// Removes the waker of a cancelled operation.
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pub fn cancel(&self, key: usize) {
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let mut inner = self.lock();
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if inner.entries.remove(key).is_none() {
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inner.none_count -= 1;
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// The operation was cancelled and notified so notify another operation instead.
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if let Some((_, opt_waker)) = inner.entries.iter_mut().next() {
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// If there is no waker in this entry, that means it was already woken.
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if let Some(w) = opt_waker.take() {
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w.wake();
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inner.none_count += 1;
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}
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}
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}
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}
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/// Notifies one blocked operation.
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#[inline]
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pub fn notify_one(&self) {
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// Use `SeqCst` ordering to synchronize with `Lock::drop()`.
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if self.flag.load(Ordering::SeqCst) & NOTIFY_ONE != 0 {
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self.notify(false);
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}
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}
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/// Notifies all blocked operations.
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// TODO: Delete this attribute when `crate::sync::channel()` is stabilized.
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#[cfg(feature = "unstable")]
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#[inline]
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pub fn notify_all(&self) {
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// Use `SeqCst` ordering to synchronize with `Lock::drop()`.
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if self.flag.load(Ordering::SeqCst) & NOTIFY_ALL != 0 {
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self.notify(true);
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}
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}
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/// Notifies blocked operations, either one or all of them.
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fn notify(&self, all: bool) {
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let mut inner = &mut *self.lock();
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for (_, opt_waker) in inner.entries.iter_mut() {
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// If there is no waker in this entry, that means it was already woken.
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if let Some(w) = opt_waker.take() {
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w.wake();
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inner.none_count += 1;
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}
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if !all {
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break;
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}
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}
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}
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/// Locks the list of entries.
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#[cold]
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fn lock(&self) -> Lock<'_> {
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let backoff = Backoff::new();
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while self.flag.fetch_or(LOCKED, Ordering::Acquire) & LOCKED != 0 {
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backoff.snooze();
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}
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Lock { waker_set: self }
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}
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}
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/// A guard holding a `WakerSet` locked.
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struct Lock<'a> {
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waker_set: &'a WakerSet,
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}
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impl Drop for Lock<'_> {
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#[inline]
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fn drop(&mut self) {
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let mut flag = 0;
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// If there is at least one entry and all are `Some`, then `notify_one()` has work to do.
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if !self.entries.is_empty() && self.none_count == 0 {
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flag |= NOTIFY_ONE;
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}
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// If there is at least one `Some` entry, then `notify_all()` has work to do.
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if self.entries.len() - self.none_count > 0 {
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flag |= NOTIFY_ALL;
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}
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// Use `SeqCst` ordering to synchronize with `WakerSet::lock_to_notify()`.
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self.waker_set.flag.store(flag, Ordering::SeqCst);
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}
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}
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impl Deref for Lock<'_> {
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type Target = Inner;
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#[inline]
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fn deref(&self) -> &Inner {
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unsafe { &*self.waker_set.inner.get() }
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}
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}
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impl DerefMut for Lock<'_> {
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#[inline]
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fn deref_mut(&mut self) -> &mut Inner {
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unsafe { &mut *self.waker_set.inner.get() }
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}
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}
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