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// Copyright 2016 Amanieu d'Antras // // Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or // http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or // http://opensource.org/licenses/MIT>, at your option. This file may not be // copied, modified, or distributed except according to those terms. use crate::thread_parker; use std::sync::atomic::spin_loop_hint; // Wastes some CPU time for the given number of iterations, // using a hint to indicate to the CPU that we are spinning. #[inline] fn cpu_relax(iterations: u32) { for _ in 0..iterations { spin_loop_hint() } } /// A counter used to perform exponential backoff in spin loops. #[derive(Default)] pub struct SpinWait { counter: u32, } impl SpinWait { /// Creates a new `SpinWait`. #[inline] pub fn new() -> Self { Self::default() } /// Resets a `SpinWait` to its initial state. #[inline] pub fn reset(&mut self) { self.counter = 0; } /// Spins until the sleep threshold has been reached. /// /// This function returns whether the sleep threshold has been reached, at /// which point further spinning has diminishing returns and the thread /// should be parked instead. /// /// The spin strategy will initially use a CPU-bound loop but will fall back /// to yielding the CPU to the OS after a few iterations. #[inline] pub fn spin(&mut self) -> bool { if self.counter >= 10 { return false; } self.counter += 1; if self.counter <= 3 { cpu_relax(1 << self.counter); } else { thread_parker::thread_yield(); } true } /// Spins without yielding the thread to the OS. /// /// Instead, the backoff is simply capped at a maximum value. This can be /// used to improve throughput in `compare_exchange` loops that have high /// contention. #[inline] pub fn spin_no_yield(&mut self) { self.counter += 1; if self.counter > 10 { self.counter = 10; } cpu_relax(1 << self.counter); } }