use std::sync::atomic::{AtomicUsize, Ordering}; pub(super) struct AtomicCounters { /// Packs together a number of counters. The counters are ordered as /// follows, from least to most significant bits (here, we assuming /// that [`THREADS_BITS`] is equal to 10): /// /// * Bits 0..10: Stores the number of **sleeping threads** /// * Bits 10..20: Stores the number of **inactive threads** /// * Bits 20..: Stores the **job event counter** (JEC) /// /// This uses 10 bits ([`THREADS_BITS`]) to encode the number of threads. Note /// that the total number of bits (and hence the number of bits used for the /// JEC) will depend on whether we are using a 32- or 64-bit architecture. value: AtomicUsize, } #[derive(Copy, Clone)] pub(super) struct Counters { word: usize, } /// A value read from the **Jobs Event Counter**. /// See the [`README.md`](README.md) for more /// coverage of how the jobs event counter works. #[derive(Copy, Clone, Debug, PartialEq, PartialOrd)] pub(super) struct JobsEventCounter(usize); impl JobsEventCounter { pub(super) const DUMMY: JobsEventCounter = JobsEventCounter(std::usize::MAX); #[inline] pub(super) fn as_usize(self) -> usize { self.0 } /// The JEC "is sleepy" if the last thread to increment it was in the /// process of becoming sleepy. This is indicated by its value being *even*. /// When new jobs are posted, they check if the JEC is sleepy, and if so /// they incremented it. #[inline] pub(super) fn is_sleepy(self) -> bool { (self.as_usize() & 1) == 0 } /// The JEC "is active" if the last thread to increment it was posting new /// work. This is indicated by its value being *odd*. When threads get /// sleepy, they will check if the JEC is active, and increment it. #[inline] pub(super) fn is_active(self) -> bool { !self.is_sleepy() } } /// Number of bits used for the thread counters. #[cfg(target_pointer_width = "64")] const THREADS_BITS: usize = 16; #[cfg(target_pointer_width = "32")] const THREADS_BITS: usize = 8; /// Bits to shift to select the sleeping threads /// (used with `select_bits`). #[allow(clippy::erasing_op)] const SLEEPING_SHIFT: usize = 0 * THREADS_BITS; /// Bits to shift to select the inactive threads /// (used with `select_bits`). #[allow(clippy::identity_op)] const INACTIVE_SHIFT: usize = 1 * THREADS_BITS; /// Bits to shift to select the JEC /// (use JOBS_BITS). const JEC_SHIFT: usize = 2 * THREADS_BITS; /// Max value for the thread counters. pub(crate) const THREADS_MAX: usize = (1 << THREADS_BITS) - 1; /// Constant that can be added to add one sleeping thread. const ONE_SLEEPING: usize = 1; /// Constant that can be added to add one inactive thread. /// An inactive thread is either idle, sleepy, or sleeping. const ONE_INACTIVE: usize = 1 << INACTIVE_SHIFT; /// Constant that can be added to add one to the JEC. const ONE_JEC: usize = 1 << JEC_SHIFT; impl AtomicCounters { #[inline] pub(super) fn new() -> AtomicCounters { AtomicCounters { value: AtomicUsize::new(0), } } /// Load and return the current value of the various counters. /// This value can then be given to other method which will /// attempt to update the counters via compare-and-swap. #[inline] pub(super) fn load(&self, ordering: Ordering) -> Counters { Counters::new(self.value.load(ordering)) } #[inline] fn try_exchange(&self, old_value: Counters, new_value: Counters, ordering: Ordering) -> bool { self.value .compare_exchange(old_value.word, new_value.word, ordering, Ordering::Relaxed) .is_ok() } /// Adds an inactive thread. This cannot fail. /// /// This should be invoked when a thread enters its idle loop looking /// for work. It is decremented when work is found. Note that it is /// not decremented if the thread transitions from idle to sleepy or sleeping; /// so the number of inactive threads is always greater-than-or-equal /// to the number of sleeping threads. #[inline] pub(super) fn add_inactive_thread(&self) { self.value.fetch_add(ONE_INACTIVE, Ordering::SeqCst); } /// Increments the jobs event counter if `increment_when`, when applied to /// the current value, is true. Used to toggle the JEC from even (sleepy) to /// odd (active) or vice versa. Returns the final value of the counters, for /// which `increment_when` is guaranteed to return false. pub(super) fn increment_jobs_event_counter_if( &self, increment_when: impl Fn(JobsEventCounter) -> bool, ) -> Counters { loop { let old_value = self.load(Ordering::SeqCst); if increment_when(old_value.jobs_counter()) { let new_value = old_value.increment_jobs_counter(); if self.try_exchange(old_value, new_value, Ordering::SeqCst) { return new_value; } } else { return old_value; } } } /// Subtracts an inactive thread. This cannot fail. It is invoked /// when a thread finds work and hence becomes active. It returns the /// number of sleeping threads to wake up (if any). /// /// See `add_inactive_thread`. #[inline] pub(super) fn sub_inactive_thread(&self) -> usize { let old_value = Counters::new(self.value.fetch_sub(ONE_INACTIVE, Ordering::SeqCst)); debug_assert!( old_value.inactive_threads() > 0, "sub_inactive_thread: old_value {:?} has no inactive threads", old_value, ); debug_assert!( old_value.sleeping_threads() <= old_value.inactive_threads(), "sub_inactive_thread: old_value {:?} had {} sleeping threads and {} inactive threads", old_value, old_value.sleeping_threads(), old_value.inactive_threads(), ); // Current heuristic: whenever an inactive thread goes away, if // there are any sleeping threads, wake 'em up. let sleeping_threads = old_value.sleeping_threads(); std::cmp::min(sleeping_threads, 2) } /// Subtracts a sleeping thread. This cannot fail, but it is only /// safe to do if you you know the number of sleeping threads is /// non-zero (i.e., because you have just awoken a sleeping /// thread). #[inline] pub(super) fn sub_sleeping_thread(&self) { let old_value = Counters::new(self.value.fetch_sub(ONE_SLEEPING, Ordering::SeqCst)); debug_assert!( old_value.sleeping_threads() > 0, "sub_sleeping_thread: old_value {:?} had no sleeping threads", old_value, ); debug_assert!( old_value.sleeping_threads() <= old_value.inactive_threads(), "sub_sleeping_thread: old_value {:?} had {} sleeping threads and {} inactive threads", old_value, old_value.sleeping_threads(), old_value.inactive_threads(), ); } #[inline] pub(super) fn try_add_sleeping_thread(&self, old_value: Counters) -> bool { debug_assert!( old_value.inactive_threads() > 0, "try_add_sleeping_thread: old_value {:?} has no inactive threads", old_value, ); debug_assert!( old_value.sleeping_threads() < THREADS_MAX, "try_add_sleeping_thread: old_value {:?} has too many sleeping threads", old_value, ); let mut new_value = old_value; new_value.word += ONE_SLEEPING; self.try_exchange(old_value, new_value, Ordering::SeqCst) } } #[inline] fn select_thread(word: usize, shift: usize) -> usize { (word >> shift) & THREADS_MAX } #[inline] fn select_jec(word: usize) -> usize { word >> JEC_SHIFT } impl Counters { #[inline] fn new(word: usize) -> Counters { Counters { word } } #[inline] fn increment_jobs_counter(self) -> Counters { // We can freely add to JEC because it occupies the most significant bits. // Thus it doesn't overflow into the other counters, just wraps itself. Counters { word: self.word.wrapping_add(ONE_JEC), } } #[inline] pub(super) fn jobs_counter(self) -> JobsEventCounter { JobsEventCounter(select_jec(self.word)) } /// The number of threads that are not actively /// executing work. They may be idle, sleepy, or asleep. #[inline] pub(super) fn inactive_threads(self) -> usize { select_thread(self.word, INACTIVE_SHIFT) } #[inline] pub(super) fn awake_but_idle_threads(self) -> usize { debug_assert!( self.sleeping_threads() <= self.inactive_threads(), "sleeping threads: {} > raw idle threads {}", self.sleeping_threads(), self.inactive_threads() ); self.inactive_threads() - self.sleeping_threads() } #[inline] pub(super) fn sleeping_threads(self) -> usize { select_thread(self.word, SLEEPING_SHIFT) } } impl std::fmt::Debug for Counters { fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { let word = format!("{:016x}", self.word); fmt.debug_struct("Counters") .field("word", &word) .field("jobs", &self.jobs_counter().0) .field("inactive", &self.inactive_threads()) .field("sleeping", &self.sleeping_threads()) .finish() } }