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Branch: new-scheduler
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use std::cell::Cell;
use std::io;
use std::iter;
use std::ptr;
use std::sync::atomic::{self, AtomicBool, Ordering};
use std::sync::{Arc, Mutex};
use std::thread;
use std::time::Duration;
use crossbeam_deque::{Injector, Steal, Stealer, Worker};
use crossbeam_utils::thread::scope;
use once_cell::unsync::OnceCell;
use crate::rt::Reactor;
use crate::task::Runnable;
use crate::utils::{abort_on_panic, random, Spinlock};
thread_local! {
/// A reference to the current machine, if the current thread runs tasks.
static MACHINE: OnceCell<Arc<Machine>> = OnceCell::new();
/// This flag is set to true whenever `task::yield_now()` is invoked.
static YIELD_NOW: Cell<bool> = Cell::new(false);
}
/// Scheduler state.
struct Scheduler {
/// Set to `true` every time before a machine blocks polling the reactor.
progress: bool,
/// Set to `true` while a machine is polling the reactor.
polling: bool,
/// Idle processors.
processors: Vec<Processor>,
/// Running machines.
machines: Vec<Arc<Machine>>,
}
/// An async runtime.
pub struct Runtime {
/// The reactor.
reactor: Reactor,
/// The global queue of tasks.
injector: Injector<Runnable>,
/// Handles to local queues for stealing work.
stealers: Vec<Stealer<Runnable>>,
/// The scheduler state.
sched: Mutex<Scheduler>,
}
impl Runtime {
/// Creates a new runtime.
pub fn new() -> Runtime {
let cpus = num_cpus::get().max(1);
let processors: Vec<_> = (0..cpus).map(|_| Processor::new()).collect();
let stealers = processors.iter().map(|p| p.worker.stealer()).collect();
Runtime {
reactor: Reactor::new().unwrap(),
injector: Injector::new(),
stealers,
sched: Mutex::new(Scheduler {
processors,
machines: Vec::new(),
progress: false,
polling: false,
}),
}
}
/// Returns a reference to the reactor.
pub fn reactor(&self) -> &Reactor {
&self.reactor
}
/// Flushes the task slot so that tasks get run more fairly.
pub fn yield_now(&self) {
YIELD_NOW.with(|flag| flag.set(true));
}
/// Schedules a task.
pub fn schedule(&self, task: Runnable) {
MACHINE.with(|machine| {
// If the current thread is a worker thread, schedule it onto the current machine.
// Otherwise, push it into the global task queue.
match machine.get() {
None => {
self.injector.push(task);
self.notify();
}
Some(m) => m.schedule(&self, task),
}
});
}
/// Runs the runtime on the current thread.
pub fn run(&self) {
scope(|s| {
let mut idle = 0;
let mut delay = 0;
loop {
// Get a list of new machines to start, if any need to be started.
for m in self.make_machines() {
idle = 0;
s.builder()
.name("async-std/machine".to_string())
.spawn(move |_| {
abort_on_panic(|| {
let _ = MACHINE.with(|machine| machine.set(m.clone()));
m.run(self);
})
})
.expect("cannot start a machine thread");
}
// Sleep for a bit longer if the scheduler state hasn't changed in a while.
if idle > 10 {
delay = (delay * 2).min(10_000);
} else {
idle += 1;
delay = 1000;
}
thread::sleep(Duration::from_micros(delay));
}
})
.unwrap();
}
/// Returns a list of machines that need to be started.
fn make_machines(&self) -> Vec<Arc<Machine>> {
let mut sched = self.sched.lock().unwrap();
let mut to_start = Vec::new();
// If there is a machine that is stuck on a task and not making any progress, steal its
// processor and set up a new machine to take over.
for m in &mut sched.machines {
if !m.progress.swap(false, Ordering::SeqCst) {
let opt_p = m.processor.try_lock().and_then(|mut p| p.take());
if let Some(p) = opt_p {
*m = Arc::new(Machine::new(p));
to_start.push(m.clone());
}
}
}
// If no machine has been polling the reactor in a while, that means the runtime is
// overloaded with work and we need to start another machine.
if !sched.polling {
if !sched.progress {
if let Some(p) = sched.processors.pop() {
let m = Arc::new(Machine::new(p));
to_start.push(m.clone());
sched.machines.push(m);
}
}
sched.progress = false;
}
to_start
}
/// Unparks a thread polling the reactor.
fn notify(&self) {
atomic::fence(Ordering::SeqCst);
self.reactor.notify().unwrap();
}
/// Attempts to poll the reactor without blocking on it.
///
/// Returns `Ok(true)` if at least one new task was woken.
///
/// This function might not poll the reactor at all so do not rely on it doing anything. Only
/// use for optimization.
fn quick_poll(&self) -> io::Result<bool> {
if let Ok(sched) = self.sched.try_lock() {
if !sched.polling {
return self.reactor.poll(Some(Duration::from_secs(0)));
}
}
Ok(false)
}
}
/// A thread running a processor.
struct Machine {
/// Holds the processor until it gets stolen.
processor: Spinlock<Option<Processor>>,
/// Gets set to `true` before running every task to indicate the machine is not stuck.
progress: AtomicBool,
}
impl Machine {
/// Creates a new machine running a processor.
fn new(p: Processor) -> Machine {
Machine {
processor: Spinlock::new(Some(p)),
progress: AtomicBool::new(true),
}
}
/// Schedules a task onto the machine.
fn schedule(&self, rt: &Runtime, task: Runnable) {
match self.processor.lock().as_mut() {
None => {
rt.injector.push(task);
rt.notify();
}
Some(p) => p.schedule(rt, task),
}
}
/// Finds the next runnable task.
fn find_task(&self, rt: &Runtime) -> Steal<Runnable> {
let mut retry = false;
// First try finding a task in the local queue or in the global queue.
if let Some(p) = self.processor.lock().as_mut() {
if let Some(task) = p.pop_task() {
return Steal::Success(task);
}
match p.steal_from_global(rt) {
Steal::Empty => {}
Steal::Retry => retry = true,
Steal::Success(task) => return Steal::Success(task),
}
}
// Try polling the reactor, but don't block on it.
let progress = rt.quick_poll().unwrap();
// Try finding a task in the local queue, which might hold tasks woken by the reactor. If
// the local queue is still empty, try stealing from other processors.
if let Some(p) = self.processor.lock().as_mut() {
if progress {
if let Some(task) = p.pop_task() {
return Steal::Success(task);
}
}
match p.steal_from_others(rt) {
Steal::Empty => {}
Steal::Retry => retry = true,
Steal::Success(task) => return Steal::Success(task),
}
}
if retry { Steal::Retry } else { Steal::Empty }
}
/// Runs the machine on the current thread.
fn run(&self, rt: &Runtime) {
/// Number of yields when no runnable task is found.
const YIELDS: u32 = 3;
/// Number of short sleeps when no runnable task in found.
const SLEEPS: u32 = 10;
/// Number of runs in a row before the global queue is inspected.
const RUNS: u32 = 64;
// The number of times the thread found work in a row.
let mut runs = 0;
// The number of times the thread didn't find work in a row.
let mut fails = 0;
loop {
// let the scheduler know this machine is making progress.
self.progress.store(true, Ordering::SeqCst);
// Check if `task::yield_now()` was invoked and flush the slot if so.
YIELD_NOW.with(|flag| {
if flag.replace(false) {
if let Some(p) = self.processor.lock().as_mut() {
p.flush_slot(rt);
}
}
});
// After a number of runs in a row, do some work to ensure no task is left behind
// indefinitely. Poll the reactor, steal tasks from the global queue, and flush the
// task slot.
if runs >= RUNS {
runs = 0;
rt.quick_poll().unwrap();
if let Some(p) = self.processor.lock().as_mut() {
if let Steal::Success(task) = p.steal_from_global(rt) {
p.schedule(rt, task);
}
p.flush_slot(rt);
}
}
// Try to find a runnable task.
if let Steal::Success(task) = self.find_task(rt) {
task.run();
runs += 1;
fails = 0;
continue;
}
fails += 1;
// Check if the processor was stolen.
if self.processor.lock().is_none() {
break;
}
// Yield the current thread a few times.
if fails <= YIELDS {
thread::yield_now();
continue;
}
// Put the current thread to sleep a few times.
if fails <= YIELDS + SLEEPS {
let opt_p = self.processor.lock().take();
thread::sleep(Duration::from_micros(10));
*self.processor.lock() = opt_p;
continue;
}
let mut sched = rt.sched.lock().unwrap();
// One final check for available tasks while the scheduler is locked.
if let Some(task) = iter::repeat_with(|| self.find_task(rt))
.find(|s| !s.is_retry())
.and_then(|s| s.success())
{
self.schedule(rt, task);
continue;
}
// If another thread is already blocked on the reactor, there is no point in keeping
// the current thread around since there is too little work to do.
if sched.polling {
break;
}
// Take out the machine associated with the current thread.
let m = match sched
.machines
.iter()
.position(|elem| ptr::eq(&**elem, self))
{
None => break, // The processor was stolen.
Some(pos) => sched.machines.swap_remove(pos),
};
// Unlock the schedule poll the reactor until new I/O events arrive.
sched.polling = true;
drop(sched);
rt.reactor.poll(None).unwrap();
// Lock the scheduler again and re-register the machine.
sched = rt.sched.lock().unwrap();
sched.polling = false;
sched.machines.push(m);
sched.progress = true;
runs = 0;
fails = 0;
}
// When shutting down the thread, take the processor out if still available.
let opt_p = self.processor.lock().take();
// Return the processor to the scheduler and remove the machine.
if let Some(p) = opt_p {
let mut sched = rt.sched.lock().unwrap();
sched.processors.push(p);
sched.machines.retain(|elem| !ptr::eq(&**elem, self));
}
}
}
struct Processor {
/// The local task queue.
worker: Worker<Runnable>,
/// Contains the next task to run as an optimization that skips the queue.
slot: Option<Runnable>,
}
impl Processor {
/// Creates a new processor.
fn new() -> Processor {
Processor {
worker: Worker::new_fifo(),
slot: None,
}
}
/// Schedules a task to run on this processor.
fn schedule(&mut self, rt: &Runtime, task: Runnable) {
match self.slot.replace(task) {
None => {}
Some(task) => {
self.worker.push(task);
rt.notify();
}
}
}
/// Flushes a task from the slot into the local queue.
fn flush_slot(&mut self, rt: &Runtime) {
if let Some(task) = self.slot.take() {
self.worker.push(task);
rt.notify();
}
}
/// Pops a task from this processor.
fn pop_task(&mut self) -> Option<Runnable> {
self.slot.take().or_else(|| self.worker.pop())
}
/// Steals a task from the global queue.
fn steal_from_global(&mut self, rt: &Runtime) -> Steal<Runnable> {
rt.injector.steal_batch_and_pop(&self.worker)
}
/// Steals a task from other processors.
fn steal_from_others(&mut self, rt: &Runtime) -> Steal<Runnable> {
// Pick a random starting point in the list of queues.
let len = rt.stealers.len();
let start = random(len as u32) as usize;
// Create an iterator over stealers that starts from the chosen point.
let (l, r) = rt.stealers.split_at(start);
let stealers = r.iter().chain(l.iter());
// Try stealing a batch of tasks from each queue.
stealers
.map(|s| s.steal_batch_and_pop(&self.worker))
.collect()
}
}
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