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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()
}
}