New scheduler resilient to blocking
Stjepan Glavina
4 years ago
parent
9311fd7fae
commit
36d24cd0e1
@ -0,0 +1,23 @@
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//! The runtime.
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use std::thread;
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use once_cell::sync::Lazy;
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use crate::utils::abort_on_panic;
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pub use reactor::{Reactor, Watcher};
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pub use runtime::Runtime;
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mod reactor;
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mod runtime;
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/// The global runtime.
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pub static RUNTIME: Lazy<Runtime> = Lazy::new(|| {
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thread::Builder::new()
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.name("async-std/runtime".to_string())
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.spawn(|| abort_on_panic(|| RUNTIME.run()))
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.expect("cannot start a runtime thread");
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Runtime::new()
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});
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@ -0,0 +1,449 @@
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use std::cell::Cell;
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use std::io;
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use std::iter;
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use std::ptr;
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use std::sync::atomic::{self, AtomicBool, Ordering};
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use std::sync::{Arc, Mutex};
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use std::thread;
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use std::time::Duration;
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use crossbeam_deque::{Injector, Steal, Stealer, Worker};
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use crossbeam_utils::thread::scope;
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use once_cell::unsync::OnceCell;
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use crate::rt::Reactor;
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use crate::task::Runnable;
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use crate::utils::{abort_on_panic, random, Spinlock};
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thread_local! {
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/// A reference to the current machine, if the current thread runs tasks.
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static MACHINE: OnceCell<Arc<Machine>> = OnceCell::new();
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/// This flag is set to true whenever `task::yield_now()` is invoked.
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static YIELD_NOW: Cell<bool> = Cell::new(false);
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}
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/// Scheduler state.
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struct Scheduler {
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/// Set to `true` every time before a machine blocks polling the reactor.
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progress: bool,
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/// Set to `true` while a machine is polling the reactor.
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polling: bool,
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/// Idle processors.
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processors: Vec<Processor>,
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/// Running machines.
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machines: Vec<Arc<Machine>>,
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}
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/// An async runtime.
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pub struct Runtime {
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/// The reactor.
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reactor: Reactor,
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/// The global queue of tasks.
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injector: Injector<Runnable>,
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/// Handles to local queues for stealing work.
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stealers: Vec<Stealer<Runnable>>,
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/// The scheduler state.
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sched: Mutex<Scheduler>,
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}
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impl Runtime {
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/// Creates a new runtime.
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pub fn new() -> Runtime {
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let cpus = num_cpus::get().max(1);
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let processors: Vec<_> = (0..cpus).map(|_| Processor::new()).collect();
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let stealers = processors.iter().map(|p| p.worker.stealer()).collect();
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Runtime {
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reactor: Reactor::new().unwrap(),
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injector: Injector::new(),
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stealers,
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sched: Mutex::new(Scheduler {
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processors,
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machines: Vec::new(),
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progress: false,
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polling: false,
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}),
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}
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}
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/// Returns a reference to the reactor.
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pub fn reactor(&self) -> &Reactor {
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&self.reactor
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}
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/// Flushes the task slot so that tasks get run more fairly.
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pub fn yield_now(&self) {
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YIELD_NOW.with(|flag| flag.set(true));
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}
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/// Schedules a task.
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pub fn schedule(&self, task: Runnable) {
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MACHINE.with(|machine| {
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// If the current thread is a worker thread, schedule it onto the current machine.
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// Otherwise, push it into the global task queue.
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match machine.get() {
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None => {
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self.injector.push(task);
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self.notify();
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}
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Some(m) => m.schedule(&self, task),
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}
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});
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}
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/// Runs the runtime on the current thread.
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pub fn run(&self) {
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scope(|s| {
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let mut idle = 0;
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let mut delay = 0;
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loop {
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// Get a list of new machines to start, if any need to be started.
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for m in self.make_machines() {
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idle = 0;
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s.builder()
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.name("async-std/machine".to_string())
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.spawn(move |_| {
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abort_on_panic(|| {
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let _ = MACHINE.with(|machine| machine.set(m.clone()));
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m.run(self);
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})
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})
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.expect("cannot start a machine thread");
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}
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// Sleep for a bit longer if the scheduler state hasn't changed in a while.
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if idle > 10 {
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delay = (delay * 2).min(10_000);
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} else {
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idle += 1;
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delay = 1000;
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}
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thread::sleep(Duration::from_micros(delay));
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}
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})
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.unwrap();
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}
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/// Returns a list of machines that need to be started.
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fn make_machines(&self) -> Vec<Arc<Machine>> {
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let mut sched = self.sched.lock().unwrap();
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let mut to_start = Vec::new();
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// If there is a machine that is stuck on a task and not making any progress, steal its
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// processor and set up a new machine to take over.
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for m in &mut sched.machines {
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if !m.progress.swap(false, Ordering::SeqCst) {
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let opt_p = m.processor.try_lock().and_then(|mut p| p.take());
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if let Some(p) = opt_p {
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*m = Arc::new(Machine::new(p));
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to_start.push(m.clone());
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}
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}
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}
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// If no machine has been polling the reactor in a while, that means the runtime is
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// overloaded with work and we need to start another machine.
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if !sched.polling {
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if !sched.progress {
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if let Some(p) = sched.processors.pop() {
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let m = Arc::new(Machine::new(p));
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to_start.push(m.clone());
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sched.machines.push(m);
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}
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}
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sched.progress = false;
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}
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to_start
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}
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/// Unparks a thread polling the reactor.
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fn notify(&self) {
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atomic::fence(Ordering::SeqCst);
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self.reactor.notify().unwrap();
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}
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/// Attempts to poll the reactor without blocking on it.
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///
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/// Returns `Ok(true)` if at least one new task was woken.
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///
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/// This function might not poll the reactor at all so do not rely on it doing anything. Only
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/// use for optimization.
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fn quick_poll(&self) -> io::Result<bool> {
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if let Ok(sched) = self.sched.try_lock() {
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if !sched.polling {
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return self.reactor.poll(Some(Duration::from_secs(0)));
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}
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}
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Ok(false)
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}
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}
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/// A thread running a processor.
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struct Machine {
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/// Holds the processor until it gets stolen.
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processor: Spinlock<Option<Processor>>,
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/// Gets set to `true` before running every task to indicate the machine is not stuck.
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progress: AtomicBool,
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}
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impl Machine {
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/// Creates a new machine running a processor.
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fn new(p: Processor) -> Machine {
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Machine {
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processor: Spinlock::new(Some(p)),
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progress: AtomicBool::new(true),
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}
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}
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/// Schedules a task onto the machine.
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fn schedule(&self, rt: &Runtime, task: Runnable) {
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match self.processor.lock().as_mut() {
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None => {
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rt.injector.push(task);
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rt.notify();
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}
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Some(p) => p.schedule(rt, task),
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}
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}
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/// Finds the next runnable task.
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fn find_task(&self, rt: &Runtime) -> Steal<Runnable> {
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let mut retry = false;
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// First try finding a task in the local queue or in the global queue.
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if let Some(p) = self.processor.lock().as_mut() {
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if let Some(task) = p.pop_task() {
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return Steal::Success(task);
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}
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match p.steal_from_global(rt) {
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Steal::Empty => {}
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Steal::Retry => retry = true,
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Steal::Success(task) => return Steal::Success(task),
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}
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}
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// Try polling the reactor, but don't block on it.
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let progress = rt.quick_poll().unwrap();
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// Try finding a task in the local queue, which might hold tasks woken by the reactor. If
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// the local queue is still empty, try stealing from other processors.
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if let Some(p) = self.processor.lock().as_mut() {
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if progress {
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if let Some(task) = p.pop_task() {
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return Steal::Success(task);
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}
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}
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match p.steal_from_others(rt) {
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Steal::Empty => {}
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Steal::Retry => retry = true,
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Steal::Success(task) => return Steal::Success(task),
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}
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}
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if retry { Steal::Retry } else { Steal::Empty }
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}
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/// Runs the machine on the current thread.
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fn run(&self, rt: &Runtime) {
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/// Number of yields when no runnable task is found.
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const YIELDS: u32 = 3;
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/// Number of short sleeps when no runnable task in found.
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const SLEEPS: u32 = 10;
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/// Number of runs in a row before the global queue is inspected.
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const RUNS: u32 = 64;
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// The number of times the thread found work in a row.
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let mut runs = 0;
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// The number of times the thread didn't find work in a row.
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let mut fails = 0;
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loop {
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// let the scheduler know this machine is making progress.
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self.progress.store(true, Ordering::SeqCst);
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// Check if `task::yield_now()` was invoked and flush the slot if so.
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YIELD_NOW.with(|flag| {
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if flag.replace(false) {
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if let Some(p) = self.processor.lock().as_mut() {
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p.flush_slot(rt);
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}
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}
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});
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// After a number of runs in a row, do some work to ensure no task is left behind
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// indefinitely. Poll the reactor, steal tasks from the global queue, and flush the
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// task slot.
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if runs >= RUNS {
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runs = 0;
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rt.quick_poll().unwrap();
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if let Some(p) = self.processor.lock().as_mut() {
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if let Steal::Success(task) = p.steal_from_global(rt) {
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p.schedule(rt, task);
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}
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p.flush_slot(rt);
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}
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}
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// Try to find a runnable task.
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if let Steal::Success(task) = self.find_task(rt) {
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task.run();
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runs += 1;
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fails = 0;
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continue;
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}
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fails += 1;
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// Check if the processor was stolen.
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if self.processor.lock().is_none() {
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break;
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}
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// Yield the current thread a few times.
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if fails <= YIELDS {
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thread::yield_now();
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continue;
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}
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// Put the current thread to sleep a few times.
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if fails <= YIELDS + SLEEPS {
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let opt_p = self.processor.lock().take();
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thread::sleep(Duration::from_micros(10));
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*self.processor.lock() = opt_p;
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continue;
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}
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let mut sched = rt.sched.lock().unwrap();
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// One final check for available tasks while the scheduler is locked.
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if let Some(task) = iter::repeat_with(|| self.find_task(rt))
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.find(|s| !s.is_retry())
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.and_then(|s| s.success())
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{
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self.schedule(rt, task);
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continue;
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}
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// If another thread is already blocked on the reactor, there is no point in keeping
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// the current thread around since there is too little work to do.
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if sched.polling {
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break;
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}
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// Take out the machine associated with the current thread.
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let m = match sched
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.machines
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.iter()
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.position(|elem| ptr::eq(&**elem, self))
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{
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None => break, // The processor was stolen.
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Some(pos) => sched.machines.swap_remove(pos),
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};
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// Unlock the schedule poll the reactor until new I/O events arrive.
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sched.polling = true;
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drop(sched);
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rt.reactor.poll(None).unwrap();
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// Lock the scheduler again and re-register the machine.
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sched = rt.sched.lock().unwrap();
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sched.polling = false;
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sched.machines.push(m);
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sched.progress = true;
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runs = 0;
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fails = 0;
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}
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// When shutting down the thread, take the processor out if still available.
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let opt_p = self.processor.lock().take();
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// Return the processor to the scheduler and remove the machine.
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if let Some(p) = opt_p {
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let mut sched = rt.sched.lock().unwrap();
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sched.processors.push(p);
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sched.machines.retain(|elem| !ptr::eq(&**elem, self));
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}
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}
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}
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struct Processor {
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/// The local task queue.
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worker: Worker<Runnable>,
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/// Contains the next task to run as an optimization that skips the queue.
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slot: Option<Runnable>,
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}
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impl Processor {
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/// Creates a new processor.
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fn new() -> Processor {
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Processor {
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worker: Worker::new_fifo(),
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slot: None,
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}
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}
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/// Schedules a task to run on this processor.
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fn schedule(&mut self, rt: &Runtime, task: Runnable) {
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match self.slot.replace(task) {
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None => {}
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Some(task) => {
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self.worker.push(task);
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rt.notify();
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}
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}
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}
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/// Flushes a task from the slot into the local queue.
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fn flush_slot(&mut self, rt: &Runtime) {
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if let Some(task) = self.slot.take() {
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self.worker.push(task);
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rt.notify();
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}
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}
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/// Pops a task from this processor.
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fn pop_task(&mut self) -> Option<Runnable> {
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self.slot.take().or_else(|| self.worker.pop())
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}
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/// Steals a task from the global queue.
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fn steal_from_global(&mut self, rt: &Runtime) -> Steal<Runnable> {
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rt.injector.steal_batch_and_pop(&self.worker)
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}
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/// Steals a task from other processors.
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fn steal_from_others(&mut self, rt: &Runtime) -> Steal<Runnable> {
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// Pick a random starting point in the list of queues.
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let len = rt.stealers.len();
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let start = random(len as u32) as usize;
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// Create an iterator over stealers that starts from the chosen point.
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let (l, r) = rt.stealers.split_at(start);
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let stealers = r.iter().chain(l.iter());
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// Try stealing a batch of tasks from each queue.
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stealers
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.map(|s| s.steal_batch_and_pop(&self.worker))
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.collect()
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}
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}
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@ -1,13 +0,0 @@
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//! Task executor.
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//!
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//! API bindings between `crate::task` and this module are very simple:
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//!
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//! * The only export is the `schedule` function.
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//! * The only import is the `crate::task::Runnable` type.
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pub(crate) use pool::schedule;
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use sleepers::Sleepers;
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mod pool;
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mod sleepers;
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@ -1,179 +0,0 @@
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use std::cell::Cell;
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use std::iter;
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use std::thread;
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use std::time::Duration;
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use crossbeam_deque::{Injector, Stealer, Worker};
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use once_cell::sync::Lazy;
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use once_cell::unsync::OnceCell;
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use crate::task::executor::Sleepers;
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use crate::task::Runnable;
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use crate::utils::{abort_on_panic, random};
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/// The state of an executor.
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struct Pool {
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/// The global queue of tasks.
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injector: Injector<Runnable>,
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/// Handles to local queues for stealing work from worker threads.
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stealers: Vec<Stealer<Runnable>>,
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/// Used for putting idle workers to sleep and notifying them when new tasks come in.
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sleepers: Sleepers,
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}
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/// Global executor that runs spawned tasks.
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static POOL: Lazy<Pool> = Lazy::new(|| {
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let num_threads = num_cpus::get().max(1);
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let mut stealers = Vec::new();
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// Spawn worker threads.
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for _ in 0..num_threads {
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let worker = Worker::new_fifo();
|
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stealers.push(worker.stealer());
|
||||
|
||||
let proc = Processor {
|
||||
worker,
|
||||
slot: Cell::new(None),
|
||||
slot_runs: Cell::new(0),
|
||||
};
|
||||
|
||||
thread::Builder::new()
|
||||
.name("async-std/executor".to_string())
|
||||
.spawn(|| {
|
||||
let _ = PROCESSOR.with(|p| p.set(proc));
|
||||
abort_on_panic(main_loop);
|
||||
})
|
||||
.expect("cannot start a thread driving tasks");
|
||||
}
|
||||
|
||||
Pool {
|
||||
injector: Injector::new(),
|
||||
stealers,
|
||||
sleepers: Sleepers::new(),
|
||||
}
|
||||
});
|
||||
|
||||
/// The state of a worker thread.
|
||||
struct Processor {
|
||||
/// The local task queue.
|
||||
worker: Worker<Runnable>,
|
||||
|
||||
/// Contains the next task to run as an optimization that skips queues.
|
||||
slot: Cell<Option<Runnable>>,
|
||||
|
||||
/// How many times in a row tasks have been taked from the slot rather than the queue.
|
||||
slot_runs: Cell<u32>,
|
||||
}
|
||||
|
||||
thread_local! {
|
||||
/// Worker thread state.
|
||||
static PROCESSOR: OnceCell<Processor> = OnceCell::new();
|
||||
}
|
||||
|
||||
/// Schedules a new runnable task for execution.
|
||||
pub(crate) fn schedule(task: Runnable) {
|
||||
PROCESSOR.with(|proc| {
|
||||
// If the current thread is a worker thread, store it into its task slot or push it into
|
||||
// its local task queue. Otherwise, push it into the global task queue.
|
||||
match proc.get() {
|
||||
Some(proc) => {
|
||||
// Replace the task in the slot.
|
||||
if let Some(task) = proc.slot.replace(Some(task)) {
|
||||
// If the slot already contained a task, push it into the local task queue.
|
||||
proc.worker.push(task);
|
||||
POOL.sleepers.notify_one();
|
||||
}
|
||||
}
|
||||
None => {
|
||||
POOL.injector.push(task);
|
||||
POOL.sleepers.notify_one();
|
||||
}
|
||||
}
|
||||
})
|
||||
}
|
||||
|
||||
/// Main loop running a worker thread.
|
||||
fn main_loop() {
|
||||
/// 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 = 1;
|
||||
|
||||
// The number of times the thread didn't find work in a row.
|
||||
let mut fails = 0;
|
||||
|
||||
loop {
|
||||
// Try to find a runnable task.
|
||||
match find_runnable() {
|
||||
Some(task) => {
|
||||
fails = 0;
|
||||
|
||||
// Run the found task.
|
||||
task.run();
|
||||
}
|
||||
None => {
|
||||
fails += 1;
|
||||
|
||||
// Yield the current thread or put it to sleep.
|
||||
if fails <= YIELDS {
|
||||
thread::yield_now();
|
||||
} else if fails <= YIELDS + SLEEPS {
|
||||
thread::sleep(Duration::from_micros(10));
|
||||
} else {
|
||||
POOL.sleepers.wait();
|
||||
fails = 0;
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
/// Find the next runnable task.
|
||||
fn find_runnable() -> Option<Runnable> {
|
||||
/// Maximum number of times the slot can be used in a row.
|
||||
const SLOT_LIMIT: u32 = 16;
|
||||
|
||||
PROCESSOR.with(|proc| {
|
||||
let proc = proc.get().unwrap();
|
||||
|
||||
// Try taking a task from the slot.
|
||||
let runs = proc.slot_runs.get();
|
||||
if runs < SLOT_LIMIT {
|
||||
if let Some(task) = proc.slot.take() {
|
||||
proc.slot_runs.set(runs + 1);
|
||||
return Some(task);
|
||||
}
|
||||
}
|
||||
proc.slot_runs.set(0);
|
||||
|
||||
// Pop a task from the local queue, if not empty.
|
||||
proc.worker.pop().or_else(|| {
|
||||
// Otherwise, we need to look for a task elsewhere.
|
||||
iter::repeat_with(|| {
|
||||
// Try stealing a batch of tasks from the global queue.
|
||||
POOL.injector
|
||||
.steal_batch_and_pop(&proc.worker)
|
||||
// Or try stealing a batch of tasks from one of the other threads.
|
||||
.or_else(|| {
|
||||
// First, pick a random starting point in the list of local queues.
|
||||
let len = POOL.stealers.len();
|
||||
let start = random(len as u32) as usize;
|
||||
|
||||
// Try stealing a batch of tasks from each local queue starting from the
|
||||
// chosen point.
|
||||
let (l, r) = POOL.stealers.split_at(start);
|
||||
let stealers = r.iter().chain(l.iter());
|
||||
stealers
|
||||
.map(|s| s.steal_batch_and_pop(&proc.worker))
|
||||
.collect()
|
||||
})
|
||||
})
|
||||
// Loop while no task was stolen and any steal operation needs to be retried.
|
||||
.find(|s| !s.is_retry())
|
||||
// Extract the stolen task, if there is one.
|
||||
.and_then(|s| s.success())
|
||||
})
|
||||
})
|
||||
}
|
||||
@ -1,52 +0,0 @@
|
||||
use std::sync::atomic::{AtomicBool, Ordering};
|
||||
use std::sync::{Condvar, Mutex};
|
||||
|
||||
/// The place where worker threads go to sleep.
|
||||
///
|
||||
/// Similar to how thread parking works, if a notification comes up while no threads are sleeping,
|
||||
/// the next thread that attempts to go to sleep will pick up the notification immediately.
|
||||
pub struct Sleepers {
|
||||
/// How many threads are currently a sleep.
|
||||
sleep: Mutex<usize>,
|
||||
|
||||
/// A condvar for notifying sleeping threads.
|
||||
wake: Condvar,
|
||||
|
||||
/// Set to `true` if a notification came up while nobody was sleeping.
|
||||
notified: AtomicBool,
|
||||
}
|
||||
|
||||
impl Sleepers {
|
||||
/// Creates a new `Sleepers`.
|
||||
pub fn new() -> Sleepers {
|
||||
Sleepers {
|
||||
sleep: Mutex::new(0),
|
||||
wake: Condvar::new(),
|
||||
notified: AtomicBool::new(false),
|
||||
}
|
||||
}
|
||||
|
||||
/// Puts the current thread to sleep.
|
||||
pub fn wait(&self) {
|
||||
let mut sleep = self.sleep.lock().unwrap();
|
||||
|
||||
if !self.notified.swap(false, Ordering::SeqCst) {
|
||||
*sleep += 1;
|
||||
let _ = self.wake.wait(sleep).unwrap();
|
||||
}
|
||||
}
|
||||
|
||||
/// Notifies one thread.
|
||||
pub fn notify_one(&self) {
|
||||
if !self.notified.load(Ordering::SeqCst) {
|
||||
let mut sleep = self.sleep.lock().unwrap();
|
||||
|
||||
if *sleep > 0 {
|
||||
*sleep -= 1;
|
||||
self.wake.notify_one();
|
||||
} else {
|
||||
self.notified.store(true, Ordering::SeqCst);
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
Loading…
Reference in New Issue