Revision control
Copy as Markdown
Other Tools
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()
}
}