tokio/runtime/scheduler/multi_thread/idle.rs
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//! Coordinates idling workers
use crate::loom::sync::atomic::AtomicUsize;
use crate::runtime::scheduler::multi_thread::Shared;
use std::fmt;
use std::sync::atomic::Ordering::{self, SeqCst};
pub(super) struct Idle {
/// Tracks both the number of searching workers and the number of unparked
/// workers.
///
/// Used as a fast-path to avoid acquiring the lock when needed.
state: AtomicUsize,
/// Total number of workers.
num_workers: usize,
}
/// Data synchronized by the scheduler mutex
pub(super) struct Synced {
/// Sleeping workers
sleepers: Vec<usize>,
}
const UNPARK_SHIFT: usize = 16;
const UNPARK_MASK: usize = !SEARCH_MASK;
const SEARCH_MASK: usize = (1 << UNPARK_SHIFT) - 1;
#[derive(Copy, Clone)]
struct State(usize);
impl Idle {
pub(super) fn new(num_workers: usize) -> (Idle, Synced) {
let init = State::new(num_workers);
let idle = Idle {
state: AtomicUsize::new(init.into()),
num_workers,
};
let synced = Synced {
sleepers: Vec::with_capacity(num_workers),
};
(idle, synced)
}
/// If there are no workers actively searching, returns the index of a
/// worker currently sleeping.
pub(super) fn worker_to_notify(&self, shared: &Shared) -> Option<usize> {
// If at least one worker is spinning, work being notified will
// eventually be found. A searching thread will find **some** work and
// notify another worker, eventually leading to our work being found.
//
// For this to happen, this load must happen before the thread
// transitioning `num_searching` to zero. Acquire / Release does not
// provide sufficient guarantees, so this load is done with `SeqCst` and
// will pair with the `fetch_sub(1)` when transitioning out of
// searching.
if !self.notify_should_wakeup() {
return None;
}
// Acquire the lock
let mut lock = shared.synced.lock();
// Check again, now that the lock is acquired
if !self.notify_should_wakeup() {
return None;
}
// A worker should be woken up, atomically increment the number of
// searching workers as well as the number of unparked workers.
State::unpark_one(&self.state, 1);
// Get the worker to unpark
let ret = lock.idle.sleepers.pop();
debug_assert!(ret.is_some());
ret
}
/// Returns `true` if the worker needs to do a final check for submitted
/// work.
pub(super) fn transition_worker_to_parked(
&self,
shared: &Shared,
worker: usize,
is_searching: bool,
) -> bool {
// Acquire the lock
let mut lock = shared.synced.lock();
// Decrement the number of unparked threads
let ret = State::dec_num_unparked(&self.state, is_searching);
// Track the sleeping worker
lock.idle.sleepers.push(worker);
ret
}
pub(super) fn transition_worker_to_searching(&self) -> bool {
let state = State::load(&self.state, SeqCst);
if 2 * state.num_searching() >= self.num_workers {
return false;
}
// It is possible for this routine to allow more than 50% of the workers
// to search. That is OK. Limiting searchers is only an optimization to
// prevent too much contention.
State::inc_num_searching(&self.state, SeqCst);
true
}
/// A lightweight transition from searching -> running.
///
/// Returns `true` if this is the final searching worker. The caller
/// **must** notify a new worker.
pub(super) fn transition_worker_from_searching(&self) -> bool {
State::dec_num_searching(&self.state)
}
/// Unpark a specific worker. This happens if tasks are submitted from
/// within the worker's park routine.
///
/// Returns `true` if the worker was parked before calling the method.
pub(super) fn unpark_worker_by_id(&self, shared: &Shared, worker_id: usize) -> bool {
let mut lock = shared.synced.lock();
let sleepers = &mut lock.idle.sleepers;
for index in 0..sleepers.len() {
if sleepers[index] == worker_id {
sleepers.swap_remove(index);
// Update the state accordingly while the lock is held.
State::unpark_one(&self.state, 0);
return true;
}
}
false
}
/// Returns `true` if `worker_id` is contained in the sleep set.
pub(super) fn is_parked(&self, shared: &Shared, worker_id: usize) -> bool {
let lock = shared.synced.lock();
lock.idle.sleepers.contains(&worker_id)
}
fn notify_should_wakeup(&self) -> bool {
let state = State(self.state.fetch_add(0, SeqCst));
state.num_searching() == 0 && state.num_unparked() < self.num_workers
}
}
impl State {
fn new(num_workers: usize) -> State {
// All workers start in the unparked state
let ret = State(num_workers << UNPARK_SHIFT);
debug_assert_eq!(num_workers, ret.num_unparked());
debug_assert_eq!(0, ret.num_searching());
ret
}
fn load(cell: &AtomicUsize, ordering: Ordering) -> State {
State(cell.load(ordering))
}
fn unpark_one(cell: &AtomicUsize, num_searching: usize) {
cell.fetch_add(num_searching | (1 << UNPARK_SHIFT), SeqCst);
}
fn inc_num_searching(cell: &AtomicUsize, ordering: Ordering) {
cell.fetch_add(1, ordering);
}
/// Returns `true` if this is the final searching worker
fn dec_num_searching(cell: &AtomicUsize) -> bool {
let state = State(cell.fetch_sub(1, SeqCst));
state.num_searching() == 1
}
/// Track a sleeping worker
///
/// Returns `true` if this is the final searching worker.
fn dec_num_unparked(cell: &AtomicUsize, is_searching: bool) -> bool {
let mut dec = 1 << UNPARK_SHIFT;
if is_searching {
dec += 1;
}
let prev = State(cell.fetch_sub(dec, SeqCst));
is_searching && prev.num_searching() == 1
}
/// Number of workers currently searching
fn num_searching(self) -> usize {
self.0 & SEARCH_MASK
}
/// Number of workers currently unparked
fn num_unparked(self) -> usize {
(self.0 & UNPARK_MASK) >> UNPARK_SHIFT
}
}
impl From<usize> for State {
fn from(src: usize) -> State {
State(src)
}
}
impl From<State> for usize {
fn from(src: State) -> usize {
src.0
}
}
impl fmt::Debug for State {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt.debug_struct("worker::State")
.field("num_unparked", &self.num_unparked())
.field("num_searching", &self.num_searching())
.finish()
}
}
#[test]
fn test_state() {
assert_eq!(0, UNPARK_MASK & SEARCH_MASK);
assert_eq!(0, !(UNPARK_MASK | SEARCH_MASK));
let state = State::new(10);
assert_eq!(10, state.num_unparked());
assert_eq!(0, state.num_searching());
}