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+use crate::primitive::hint;
+use core::cell::Cell;
+use core::fmt;
+
+const SPIN_LIMIT: u32 = 6;
+const YIELD_LIMIT: u32 = 10;
+
+/// Performs exponential backoff in spin loops.
+///
+/// Backing off in spin loops reduces contention and improves overall performance.
+///
+/// This primitive can execute *YIELD* and *PAUSE* instructions, yield the current thread to the OS
+/// scheduler, and tell when is a good time to block the thread using a different synchronization
+/// mechanism. Each step of the back off procedure takes roughly twice as long as the previous
+/// step.
+///
+/// # Examples
+///
+/// Backing off in a lock-free loop:
+///
+/// ```
+/// use crossbeam_utils::Backoff;
+/// use std::sync::atomic::AtomicUsize;
+/// use std::sync::atomic::Ordering::SeqCst;
+///
+/// fn fetch_mul(a: &AtomicUsize, b: usize) -> usize {
+/// let backoff = Backoff::new();
+/// loop {
+/// let val = a.load(SeqCst);
+/// if a.compare_exchange(val, val.wrapping_mul(b), SeqCst, SeqCst).is_ok() {
+/// return val;
+/// }
+/// backoff.spin();
+/// }
+/// }
+/// ```
+///
+/// Waiting for an [`AtomicBool`] to become `true`:
+///
+/// ```
+/// use crossbeam_utils::Backoff;
+/// use std::sync::atomic::AtomicBool;
+/// use std::sync::atomic::Ordering::SeqCst;
+///
+/// fn spin_wait(ready: &AtomicBool) {
+/// let backoff = Backoff::new();
+/// while !ready.load(SeqCst) {
+/// backoff.snooze();
+/// }
+/// }
+/// ```
+///
+/// Waiting for an [`AtomicBool`] to become `true` and parking the thread after a long wait.
+/// Note that whoever sets the atomic variable to `true` must notify the parked thread by calling
+/// [`unpark()`]:
+///
+/// ```
+/// use crossbeam_utils::Backoff;
+/// use std::sync::atomic::AtomicBool;
+/// use std::sync::atomic::Ordering::SeqCst;
+/// use std::thread;
+///
+/// fn blocking_wait(ready: &AtomicBool) {
+/// let backoff = Backoff::new();
+/// while !ready.load(SeqCst) {
+/// if backoff.is_completed() {
+/// thread::park();
+/// } else {
+/// backoff.snooze();
+/// }
+/// }
+/// }
+/// ```
+///
+/// [`is_completed`]: Backoff::is_completed
+/// [`std::thread::park()`]: std::thread::park
+/// [`Condvar`]: std::sync::Condvar
+/// [`AtomicBool`]: std::sync::atomic::AtomicBool
+/// [`unpark()`]: std::thread::Thread::unpark
+pub struct Backoff {
+ step: Cell<u32>,
+}
+
+impl Backoff {
+ /// Creates a new `Backoff`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use crossbeam_utils::Backoff;
+ ///
+ /// let backoff = Backoff::new();
+ /// ```
+ #[inline]
+ pub fn new() -> Self {
+ Backoff { step: Cell::new(0) }
+ }
+
+ /// Resets the `Backoff`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use crossbeam_utils::Backoff;
+ ///
+ /// let backoff = Backoff::new();
+ /// backoff.reset();
+ /// ```
+ #[inline]
+ pub fn reset(&self) {
+ self.step.set(0);
+ }
+
+ /// Backs off in a lock-free loop.
+ ///
+ /// This method should be used when we need to retry an operation because another thread made
+ /// progress.
+ ///
+ /// The processor may yield using the *YIELD* or *PAUSE* instruction.
+ ///
+ /// # Examples
+ ///
+ /// Backing off in a lock-free loop:
+ ///
+ /// ```
+ /// use crossbeam_utils::Backoff;
+ /// use std::sync::atomic::AtomicUsize;
+ /// use std::sync::atomic::Ordering::SeqCst;
+ ///
+ /// fn fetch_mul(a: &AtomicUsize, b: usize) -> usize {
+ /// let backoff = Backoff::new();
+ /// loop {
+ /// let val = a.load(SeqCst);
+ /// if a.compare_exchange(val, val.wrapping_mul(b), SeqCst, SeqCst).is_ok() {
+ /// return val;
+ /// }
+ /// backoff.spin();
+ /// }
+ /// }
+ ///
+ /// let a = AtomicUsize::new(7);
+ /// assert_eq!(fetch_mul(&a, 8), 7);
+ /// assert_eq!(a.load(SeqCst), 56);
+ /// ```
+ #[inline]
+ pub fn spin(&self) {
+ for _ in 0..1 << self.step.get().min(SPIN_LIMIT) {
+ hint::spin_loop();
+ }
+
+ if self.step.get() <= SPIN_LIMIT {
+ self.step.set(self.step.get() + 1);
+ }
+ }
+
+ /// Backs off in a blocking loop.
+ ///
+ /// This method should be used when we need to wait for another thread to make progress.
+ ///
+ /// The processor may yield using the *YIELD* or *PAUSE* instruction and the current thread
+ /// may yield by giving up a timeslice to the OS scheduler.
+ ///
+ /// In `#[no_std]` environments, this method is equivalent to [`spin`].
+ ///
+ /// If possible, use [`is_completed`] to check when it is advised to stop using backoff and
+ /// block the current thread using a different synchronization mechanism instead.
+ ///
+ /// [`spin`]: Backoff::spin
+ /// [`is_completed`]: Backoff::is_completed
+ ///
+ /// # Examples
+ ///
+ /// Waiting for an [`AtomicBool`] to become `true`:
+ ///
+ /// ```
+ /// use crossbeam_utils::Backoff;
+ /// use std::sync::Arc;
+ /// use std::sync::atomic::AtomicBool;
+ /// use std::sync::atomic::Ordering::SeqCst;
+ /// use std::thread;
+ /// use std::time::Duration;
+ ///
+ /// fn spin_wait(ready: &AtomicBool) {
+ /// let backoff = Backoff::new();
+ /// while !ready.load(SeqCst) {
+ /// backoff.snooze();
+ /// }
+ /// }
+ ///
+ /// let ready = Arc::new(AtomicBool::new(false));
+ /// let ready2 = ready.clone();
+ ///
+ /// thread::spawn(move || {
+ /// thread::sleep(Duration::from_millis(100));
+ /// ready2.store(true, SeqCst);
+ /// });
+ ///
+ /// assert_eq!(ready.load(SeqCst), false);
+ /// spin_wait(&ready);
+ /// assert_eq!(ready.load(SeqCst), true);
+ /// # std::thread::sleep(std::time::Duration::from_millis(500)); // wait for background threads closed: https://github.com/rust-lang/miri/issues/1371
+ /// ```
+ ///
+ /// [`AtomicBool`]: std::sync::atomic::AtomicBool
+ #[inline]
+ pub fn snooze(&self) {
+ if self.step.get() <= SPIN_LIMIT {
+ for _ in 0..1 << self.step.get() {
+ hint::spin_loop();
+ }
+ } else {
+ #[cfg(not(feature = "std"))]
+ for _ in 0..1 << self.step.get() {
+ hint::spin_loop();
+ }
+
+ #[cfg(feature = "std")]
+ ::std::thread::yield_now();
+ }
+
+ if self.step.get() <= YIELD_LIMIT {
+ self.step.set(self.step.get() + 1);
+ }
+ }
+
+ /// Returns `true` if exponential backoff has completed and blocking the thread is advised.
+ ///
+ /// # Examples
+ ///
+ /// Waiting for an [`AtomicBool`] to become `true` and parking the thread after a long wait:
+ ///
+ /// ```
+ /// use crossbeam_utils::Backoff;
+ /// use std::sync::Arc;
+ /// use std::sync::atomic::AtomicBool;
+ /// use std::sync::atomic::Ordering::SeqCst;
+ /// use std::thread;
+ /// use std::time::Duration;
+ ///
+ /// fn blocking_wait(ready: &AtomicBool) {
+ /// let backoff = Backoff::new();
+ /// while !ready.load(SeqCst) {
+ /// if backoff.is_completed() {
+ /// thread::park();
+ /// } else {
+ /// backoff.snooze();
+ /// }
+ /// }
+ /// }
+ ///
+ /// let ready = Arc::new(AtomicBool::new(false));
+ /// let ready2 = ready.clone();
+ /// let waiter = thread::current();
+ ///
+ /// thread::spawn(move || {
+ /// thread::sleep(Duration::from_millis(100));
+ /// ready2.store(true, SeqCst);
+ /// waiter.unpark();
+ /// });
+ ///
+ /// assert_eq!(ready.load(SeqCst), false);
+ /// blocking_wait(&ready);
+ /// assert_eq!(ready.load(SeqCst), true);
+ /// # std::thread::sleep(std::time::Duration::from_millis(500)); // wait for background threads closed: https://github.com/rust-lang/miri/issues/1371
+ /// ```
+ ///
+ /// [`AtomicBool`]: std::sync::atomic::AtomicBool
+ #[inline]
+ pub fn is_completed(&self) -> bool {
+ self.step.get() > YIELD_LIMIT
+ }
+}
+
+impl fmt::Debug for Backoff {
+ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+ f.debug_struct("Backoff")
+ .field("step", &self.step)
+ .field("is_completed", &self.is_completed())
+ .finish()
+ }
+}
+
+impl Default for Backoff {
+ fn default() -> Backoff {
+ Backoff::new()
+ }
+}
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