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//! This module contains `WorkerMonitor` and related types. It purposes includes:
//!
//! - allowing workers to park,
//! - letting the last parked worker take action, and
//! - letting workers and mutators notify workers when workers are given things to do.
use std::sync::{Condvar, Mutex};
use super::{
worker::WorkerShouldExit,
worker_goals::{WorkerGoal, WorkerGoals},
};
/// The result type of the `on_last_parked` call-back in `WorkMonitor::park_and_wait`.
/// It decides how many workers should wake up after `on_last_parked`.
pub(crate) enum LastParkedResult {
/// The last parked worker should wait, too, until more work packets are added.
ParkSelf,
/// The last parked worker should unpark and find work packet to do.
WakeSelf,
/// Wake up all parked GC workers.
WakeAll,
}
/// A data structure for synchronizing workers with each other and with mutators.
///
/// Unlike `GCWorkerShared`, there is only one instance of `WorkerMonitor`.
///
/// - It allows workers to park and unpark.
/// - It allows mutators to notify workers to schedule a GC.
pub(crate) struct WorkerMonitor {
/// The synchronized part.
sync: Mutex<WorkerMonitorSync>,
/// Workers wait on this when idle. Notified if workers have things to do. That include:
/// - any work packets available, and
/// - any field in `sync.goals.requests` set to true.
workers_have_anything_to_do: Condvar,
}
/// The synchronized part of `WorkerMonitor`.
struct WorkerMonitorSync {
/// Count parked workers.
parker: WorkerParker,
/// Current and requested goals.
goals: WorkerGoals,
}
/// This struct counts the number of workers parked and identifies the last parked worker.
struct WorkerParker {
/// The total number of workers.
worker_count: usize,
/// Number of parked workers.
parked_workers: usize,
}
impl WorkerParker {
fn new(worker_count: usize) -> Self {
Self {
worker_count,
parked_workers: 0,
}
}
/// Increase the packed-workers counter.
/// Called before a worker is parked.
///
/// Return true if all the workers are parked.
fn inc_parked_workers(&mut self) -> bool {
let old = self.parked_workers;
debug_assert!(old < self.worker_count);
let new = old + 1;
self.parked_workers = new;
new == self.worker_count
}
/// Decrease the packed-workers counter.
/// Called after a worker is resumed from the parked state.
fn dec_parked_workers(&mut self) {
let old = self.parked_workers;
debug_assert!(old <= self.worker_count);
debug_assert!(old > 0);
let new = old - 1;
self.parked_workers = new;
}
}
impl WorkerMonitor {
pub fn new(worker_count: usize) -> Self {
Self {
sync: Mutex::new(WorkerMonitorSync {
parker: WorkerParker::new(worker_count),
goals: Default::default(),
}),
workers_have_anything_to_do: Default::default(),
}
}
/// Make a request. Can be called by a mutator to request the workers to work towards the
/// given `goal`.
pub fn make_request(&self, goal: WorkerGoal) {
let mut guard = self.sync.lock().unwrap();
let newly_requested = guard.goals.set_request(goal);
if newly_requested {
self.notify_work_available(false);
}
}
/// Wake up workers when more work packets are made available for workers,
/// or a mutator has requested the GC workers to schedule a GC.
pub fn notify_work_available(&self, all: bool) {
if all {
self.workers_have_anything_to_do.notify_all();
} else {
self.workers_have_anything_to_do.notify_one();
}
}
/// Park a worker and wait on the CondVar `workers_have_anything_to_do`.
///
/// If it is the last worker parked, `on_last_parked` will be called.
/// The argument of `on_last_parked` is true if `sync.gc_requested` is `true`.
/// The return value of `on_last_parked` will determine whether this worker and other workers
/// will wake up or block waiting.
///
/// This function returns `Ok(())` if the current worker should continue working,
/// or `Err(WorkerShouldExit)` if the current worker should exit now.
pub fn park_and_wait<F>(
&self,
ordinal: usize,
on_last_parked: F,
) -> Result<(), WorkerShouldExit>
where
F: FnOnce(&mut WorkerGoals) -> LastParkedResult,
{
let mut sync = self.sync.lock().unwrap();
// Park this worker
let all_parked = sync.parker.inc_parked_workers();
trace!(
"Worker {} parked. parked/total: {}/{}. All parked: {}",
ordinal,
sync.parker.parked_workers,
sync.parker.worker_count,
all_parked
);
let mut should_wait = false;
if all_parked {
trace!("Worker {} is the last worker parked.", ordinal);
let result = on_last_parked(&mut sync.goals);
match result {
LastParkedResult::ParkSelf => {
should_wait = true;
}
LastParkedResult::WakeSelf => {
// Continue without waiting.
}
LastParkedResult::WakeAll => {
self.notify_work_available(true);
}
}
} else {
should_wait = true;
}
if should_wait {
// Notes on CondVar usage:
//
// Conditional variables are usually tested in a loop while holding a mutex
//
// lock();
// while condition() {
// condvar.wait();
// }
// unlock();
//
// The actual condition for this `self.workers_have_anything_to_do.wait(sync)` is:
//
// 1. any work packet is available, or
// 2. a goal (such as doing GC) is requested
//
// But it is not used like the typical use pattern shown above, mainly because work
// packets can be added without holding the mutex `self.sync`. This means one worker
// can add a new work packet (no mutex needed) right after another worker finds no work
// packets are available and then park. In other words, condition (1) can suddenly
// become true after a worker sees it is false but before the worker blocks waiting on
// the CondVar. If this happens, the last parked worker will block forever and never
// get notified. This may happen if mutators or the previously existing "coordinator
// thread" can add work packets.
//
// However, after the "coordinator thread" was removed, only GC worker threads can add
// work packets during GC. Parked workers (except the last parked worker) cannot make
// more work packets availble (by adding new packets or opening buckets). For this
// reason, the **last** parked worker can be sure that after it finds no packets
// available, no other workers can add another work packet (because they all parked).
// So the **last** parked worker can open more buckets or declare GC finished.
//
// Condition (2), i.e. goals added to `sync.goals`, is guarded by the monitor `sync`.
// When a mutator adds a goal via `WorkerMonitor::make_request`, it will notify a
// worker; and the last parked worker always checks it before waiting. So this
// condition will not be set without any worker noticing.
//
// Note that generational barriers may add `ProcessModBuf` work packets when not in GC.
// This is benign because those work packets are not executed immediately, and are
// guaranteed to be executed in the next GC.
// Notes on spurious wake-up:
//
// 1. The condition variable `workers_have_anything_to_do` is guarded by `self.sync`.
// Because the last parked worker is holding the mutex `self.sync` when executing
// `on_last_parked`, no workers can unpark (even if they spuriously wake up) during
// `on_last_parked` because they cannot re-acquire the mutex `self.sync`.
//
// 2. Workers may spuriously wake up and unpark when `on_last_parked` is not being
// executed (including the case when the last parked worker is waiting here, too).
// If one or more GC workers spuriously wake up, they will check for work packets,
// and park again if not available. The last parked worker will ensure the two
// conditions listed above are both false before blocking. If either condition is
// true, the last parked worker will take action.
sync = self.workers_have_anything_to_do.wait(sync).unwrap();
}
// Unpark this worker.
sync.parker.dec_parked_workers();
trace!(
"Worker {} unparked. parked/total: {}/{}.",
ordinal,
sync.parker.parked_workers,
sync.parker.worker_count,
);
// If the current goal is `StopForFork`, the worker thread should exit.
if matches!(sync.goals.current(), Some(WorkerGoal::StopForFork)) {
return Err(WorkerShouldExit);
}
Ok(())
}
/// Called when all workers have exited.
pub fn on_all_workers_exited(&self) {
let mut sync = self.sync.try_lock().unwrap();
sync.goals.on_current_goal_completed();
}
}
#[cfg(test)]
mod tests {
use std::sync::{
atomic::{AtomicBool, AtomicUsize, Ordering},
Arc,
};
use super::WorkerMonitor;
/// Test if the `WorkerMonitor::park_and_wait` method calls the `on_last_parked` callback
/// properly.
#[test]
fn test_last_worker_park_wake_all() {
let number_threads = 4;
let worker_monitor = Arc::new(WorkerMonitor::new(number_threads));
let on_last_parked_called = AtomicUsize::new(0);
let should_unpark = AtomicBool::new(false);
std::thread::scope(|scope| {
for ordinal in 0..number_threads {
let worker_monitor = worker_monitor.clone();
let on_last_parked_called = &on_last_parked_called;
let should_unpark = &should_unpark;
scope.spawn(move || {
// This emulates the use pattern in the scheduler, i.e. checking the condition
// ("Is there any work packets available") without holding a mutex.
while !should_unpark.load(Ordering::SeqCst) {
println!("Thread {} parking...", ordinal);
worker_monitor
.park_and_wait(ordinal, |_goals| {
println!("Thread {} is the last thread parked.", ordinal);
on_last_parked_called.fetch_add(1, Ordering::SeqCst);
should_unpark.store(true, Ordering::SeqCst);
super::LastParkedResult::WakeAll
})
.unwrap();
println!("Thread {} unparked.", ordinal);
}
});
}
});
// `on_last_parked` should only be called once.
assert_eq!(on_last_parked_called.load(Ordering::SeqCst), 1);
}
/// Like `test_last_worker_park_wake_all`, but only wake up the last parked worker when it
/// parked.
#[test]
fn test_last_worker_park_wake_self() {
let number_threads = 4;
let worker_monitor = Arc::new(WorkerMonitor::new(number_threads));
let on_last_parked_called = AtomicUsize::new(0);
let threads_running = AtomicUsize::new(0);
let should_unpark = AtomicBool::new(false);
std::thread::scope(|scope| {
for ordinal in 0..number_threads {
let worker_monitor = worker_monitor.clone();
let on_last_parked_called = &on_last_parked_called;
let threads_running = &threads_running;
let should_unpark = &should_unpark;
scope.spawn(move || {
let mut i_am_the_last_parked_worker = false;
// Record the number of threads entering the following `while` loop.
threads_running.fetch_add(1, Ordering::SeqCst);
while !should_unpark.load(Ordering::SeqCst) {
println!("Thread {} parking...", ordinal);
worker_monitor
.park_and_wait(ordinal, |_goals| {
println!("Thread {} is the last thread parked.", ordinal);
on_last_parked_called.fetch_add(1, Ordering::SeqCst);
should_unpark.store(true, Ordering::SeqCst);
i_am_the_last_parked_worker = true;
super::LastParkedResult::WakeSelf
})
.unwrap();
println!("Thread {} unparked.", ordinal);
}
threads_running.fetch_sub(1, Ordering::SeqCst);
if i_am_the_last_parked_worker {
println!("The last parked worker woke up");
// Only the current worker should wake and leave the `while` loop above.
assert_eq!(threads_running.load(Ordering::SeqCst), number_threads - 1);
should_unpark.store(true, Ordering::SeqCst);
worker_monitor.notify_work_available(true);
}
});
}
});
// `on_last_parked` should only be called once.
assert_eq!(on_last_parked_called.load(Ordering::SeqCst), 1);
}
}