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</pre><pre class="rust"><code><span class="kw">use </span><span class="kw">crate</span>::primitive::sync::atomic;
<span class="kw">use </span>core::cell::Cell;
<span class="kw">use </span>core::fmt;
<span class="kw">const </span>SPIN_LIMIT: u32 = <span class="number">6</span>;
<span class="kw">const </span>YIELD_LIMIT: u32 = <span class="number">10</span>;
<span class="doccomment">/// 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: &amp;AtomicUsize, b: usize) -&gt; 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: &amp;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: &amp;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
</span><span class="kw">pub struct </span>Backoff {
step: Cell&lt;u32&gt;,
}
<span class="kw">impl </span>Backoff {
<span class="doccomment">/// Creates a new `Backoff`.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::Backoff;
///
/// let backoff = Backoff::new();
/// ```
</span><span class="attribute">#[inline]
</span><span class="kw">pub fn </span>new() -&gt; <span class="self">Self </span>{
Backoff { step: Cell::new(<span class="number">0</span>) }
}
<span class="doccomment">/// Resets the `Backoff`.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::Backoff;
///
/// let backoff = Backoff::new();
/// backoff.reset();
/// ```
</span><span class="attribute">#[inline]
</span><span class="kw">pub fn </span>reset(<span class="kw-2">&amp;</span><span class="self">self</span>) {
<span class="self">self</span>.step.set(<span class="number">0</span>);
}
<span class="doccomment">/// 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: &amp;AtomicUsize, b: usize) -&gt; 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(&amp;a, 8), 7);
/// assert_eq!(a.load(SeqCst), 56);
/// ```
</span><span class="attribute">#[inline]
</span><span class="kw">pub fn </span>spin(<span class="kw-2">&amp;</span><span class="self">self</span>) {
<span class="kw">for _ in </span><span class="number">0</span>..<span class="number">1 </span>&lt;&lt; <span class="self">self</span>.step.get().min(SPIN_LIMIT) {
<span class="comment">// TODO(taiki-e): once we bump the minimum required Rust version to 1.49+,
// use [`core::hint::spin_loop`] instead.
</span><span class="attribute">#[allow(deprecated)]
</span>atomic::spin_loop_hint();
}
<span class="kw">if </span><span class="self">self</span>.step.get() &lt;= SPIN_LIMIT {
<span class="self">self</span>.step.set(<span class="self">self</span>.step.get() + <span class="number">1</span>);
}
}
<span class="doccomment">/// 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: &amp;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(&amp;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
</span><span class="attribute">#[inline]
</span><span class="kw">pub fn </span>snooze(<span class="kw-2">&amp;</span><span class="self">self</span>) {
<span class="kw">if </span><span class="self">self</span>.step.get() &lt;= SPIN_LIMIT {
<span class="kw">for _ in </span><span class="number">0</span>..<span class="number">1 </span>&lt;&lt; <span class="self">self</span>.step.get() {
<span class="comment">// TODO(taiki-e): once we bump the minimum required Rust version to 1.49+,
// use [`core::hint::spin_loop`] instead.
</span><span class="attribute">#[allow(deprecated)]
</span>atomic::spin_loop_hint();
}
} <span class="kw">else </span>{
<span class="attribute">#[cfg(not(feature = <span class="string">&quot;std&quot;</span>))]
</span><span class="kw">for _ in </span><span class="number">0</span>..<span class="number">1 </span>&lt;&lt; <span class="self">self</span>.step.get() {
<span class="comment">// TODO(taiki-e): once we bump the minimum required Rust version to 1.49+,
// use [`core::hint::spin_loop`] instead.
</span><span class="attribute">#[allow(deprecated)]
</span>atomic::spin_loop_hint();
}
<span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
</span>::std::thread::yield_now();
}
<span class="kw">if </span><span class="self">self</span>.step.get() &lt;= YIELD_LIMIT {
<span class="self">self</span>.step.set(<span class="self">self</span>.step.get() + <span class="number">1</span>);
}
}
<span class="doccomment">/// 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: &amp;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(&amp;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
</span><span class="attribute">#[inline]
</span><span class="kw">pub fn </span>is_completed(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
<span class="self">self</span>.step.get() &gt; YIELD_LIMIT
}
}
<span class="kw">impl </span>fmt::Debug <span class="kw">for </span>Backoff {
<span class="kw">fn </span>fmt(<span class="kw-2">&amp;</span><span class="self">self</span>, f: <span class="kw-2">&amp;mut </span>fmt::Formatter&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; fmt::Result {
f.debug_struct(<span class="string">&quot;Backoff&quot;</span>)
.field(<span class="string">&quot;step&quot;</span>, <span class="kw-2">&amp;</span><span class="self">self</span>.step)
.field(<span class="string">&quot;is_completed&quot;</span>, <span class="kw-2">&amp;</span><span class="self">self</span>.is_completed())
.finish()
}
}
<span class="kw">impl </span>Default <span class="kw">for </span>Backoff {
<span class="kw">fn </span>default() -&gt; Backoff {
Backoff::new()
}
}
</code></pre></div>
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