async_io/driver.rs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291
use std::cell::{Cell, RefCell};
use std::future::Future;
use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
use std::sync::Arc;
use std::task::Waker;
use std::task::{Context, Poll};
use std::thread;
use std::time::{Duration, Instant};
use async_lock::OnceCell;
use futures_lite::pin;
use parking::Parker;
use crate::reactor::Reactor;
/// Number of currently active `block_on()` invocations.
static BLOCK_ON_COUNT: AtomicUsize = AtomicUsize::new(0);
/// Unparker for the "async-io" thread.
fn unparker() -> &'static parking::Unparker {
static UNPARKER: OnceCell<parking::Unparker> = OnceCell::new();
UNPARKER.get_or_init_blocking(|| {
let (parker, unparker) = parking::pair();
// Spawn a helper thread driving the reactor.
//
// Note that this thread is not exactly necessary, it's only here to help push things
// forward if there are no `Parker`s around or if `Parker`s are just idling and never
// parking.
thread::Builder::new()
.name("async-io".to_string())
.spawn(move || main_loop(parker))
.expect("cannot spawn async-io thread");
unparker
})
}
/// Initializes the "async-io" thread.
pub(crate) fn init() {
let _ = unparker();
}
/// The main loop for the "async-io" thread.
fn main_loop(parker: parking::Parker) {
let span = tracing::trace_span!("async_io::main_loop");
let _enter = span.enter();
// The last observed reactor tick.
let mut last_tick = 0;
// Number of sleeps since this thread has called `react()`.
let mut sleeps = 0u64;
loop {
let tick = Reactor::get().ticker();
if last_tick == tick {
let reactor_lock = if sleeps >= 10 {
// If no new ticks have occurred for a while, stop sleeping and spinning in
// this loop and just block on the reactor lock.
Some(Reactor::get().lock())
} else {
Reactor::get().try_lock()
};
if let Some(mut reactor_lock) = reactor_lock {
tracing::trace!("waiting on I/O");
reactor_lock.react(None).ok();
last_tick = Reactor::get().ticker();
sleeps = 0;
}
} else {
last_tick = tick;
}
if BLOCK_ON_COUNT.load(Ordering::SeqCst) > 0 {
// Exponential backoff from 50us to 10ms.
let delay_us = [50, 75, 100, 250, 500, 750, 1000, 2500, 5000]
.get(sleeps as usize)
.unwrap_or(&10_000);
tracing::trace!("sleeping for {} us", delay_us);
if parker.park_timeout(Duration::from_micros(*delay_us)) {
tracing::trace!("notified");
// If notified before timeout, reset the last tick and the sleep counter.
last_tick = Reactor::get().ticker();
sleeps = 0;
} else {
sleeps += 1;
}
}
}
}
/// Blocks the current thread on a future, processing I/O events when idle.
///
/// # Examples
///
/// ```
/// use async_io::Timer;
/// use std::time::Duration;
///
/// async_io::block_on(async {
/// // This timer will likely be processed by the current
/// // thread rather than the fallback "async-io" thread.
/// Timer::after(Duration::from_millis(1)).await;
/// });
/// ```
pub fn block_on<T>(future: impl Future<Output = T>) -> T {
let span = tracing::trace_span!("async_io::block_on");
let _enter = span.enter();
// Increment `BLOCK_ON_COUNT` so that the "async-io" thread becomes less aggressive.
BLOCK_ON_COUNT.fetch_add(1, Ordering::SeqCst);
// Make sure to decrement `BLOCK_ON_COUNT` at the end and wake the "async-io" thread.
let _guard = CallOnDrop(|| {
BLOCK_ON_COUNT.fetch_sub(1, Ordering::SeqCst);
unparker().unpark();
});
// Creates a parker and an associated waker that unparks it.
fn parker_and_waker() -> (Parker, Waker, Arc<AtomicBool>) {
// Parker and unparker for notifying the current thread.
let (p, u) = parking::pair();
// This boolean is set to `true` when the current thread is blocked on I/O.
let io_blocked = Arc::new(AtomicBool::new(false));
// Prepare the waker.
let waker = BlockOnWaker::create(io_blocked.clone(), u);
(p, waker, io_blocked)
}
thread_local! {
// Cached parker and waker for efficiency.
static CACHE: RefCell<(Parker, Waker, Arc<AtomicBool>)> = RefCell::new(parker_and_waker());
// Indicates that the current thread is polling I/O, but not necessarily blocked on it.
static IO_POLLING: Cell<bool> = const { Cell::new(false) };
}
struct BlockOnWaker {
io_blocked: Arc<AtomicBool>,
unparker: parking::Unparker,
}
impl BlockOnWaker {
fn create(io_blocked: Arc<AtomicBool>, unparker: parking::Unparker) -> Waker {
Waker::from(Arc::new(BlockOnWaker {
io_blocked,
unparker,
}))
}
}
impl std::task::Wake for BlockOnWaker {
fn wake_by_ref(self: &Arc<Self>) {
if self.unparker.unpark() {
// Check if waking from another thread and if currently blocked on I/O.
if !IO_POLLING.with(Cell::get) && self.io_blocked.load(Ordering::SeqCst) {
Reactor::get().notify();
}
}
}
fn wake(self: Arc<Self>) {
self.wake_by_ref()
}
}
CACHE.with(|cache| {
// Try grabbing the cached parker and waker.
let tmp_cached;
let tmp_fresh;
let (p, waker, io_blocked) = match cache.try_borrow_mut() {
Ok(cache) => {
// Use the cached parker and waker.
tmp_cached = cache;
&*tmp_cached
}
Err(_) => {
// Looks like this is a recursive `block_on()` call.
// Create a fresh parker and waker.
tmp_fresh = parker_and_waker();
&tmp_fresh
}
};
pin!(future);
let cx = &mut Context::from_waker(waker);
loop {
// Poll the future.
if let Poll::Ready(t) = future.as_mut().poll(cx) {
// Ensure the cached parker is reset to the unnotified state for future block_on calls,
// in case this future called wake and then immediately returned Poll::Ready.
p.park_timeout(Duration::from_secs(0));
tracing::trace!("completed");
return t;
}
// Check if a notification was received.
if p.park_timeout(Duration::from_secs(0)) {
tracing::trace!("notified");
// Try grabbing a lock on the reactor to process I/O events.
if let Some(mut reactor_lock) = Reactor::get().try_lock() {
// First let wakers know this parker is processing I/O events.
IO_POLLING.with(|io| io.set(true));
let _guard = CallOnDrop(|| {
IO_POLLING.with(|io| io.set(false));
});
// Process available I/O events.
reactor_lock.react(Some(Duration::from_secs(0))).ok();
}
continue;
}
// Try grabbing a lock on the reactor to wait on I/O.
if let Some(mut reactor_lock) = Reactor::get().try_lock() {
// Record the instant at which the lock was grabbed.
let start = Instant::now();
loop {
// First let wakers know this parker is blocked on I/O.
IO_POLLING.with(|io| io.set(true));
io_blocked.store(true, Ordering::SeqCst);
let _guard = CallOnDrop(|| {
IO_POLLING.with(|io| io.set(false));
io_blocked.store(false, Ordering::SeqCst);
});
// Check if a notification has been received before `io_blocked` was updated
// because in that case the reactor won't receive a wakeup.
if p.park_timeout(Duration::from_secs(0)) {
tracing::trace!("notified");
break;
}
// Wait for I/O events.
tracing::trace!("waiting on I/O");
reactor_lock.react(None).ok();
// Check if a notification has been received.
if p.park_timeout(Duration::from_secs(0)) {
tracing::trace!("notified");
break;
}
// Check if this thread been handling I/O events for a long time.
if start.elapsed() > Duration::from_micros(500) {
tracing::trace!("stops hogging the reactor");
// This thread is clearly processing I/O events for some other threads
// because it didn't get a notification yet. It's best to stop hogging the
// reactor and give other threads a chance to process I/O events for
// themselves.
drop(reactor_lock);
// Unpark the "async-io" thread in case no other thread is ready to start
// processing I/O events. This way we prevent a potential latency spike.
unparker().unpark();
// Wait for a notification.
p.park();
break;
}
}
} else {
// Wait for an actual notification.
tracing::trace!("sleep until notification");
p.park();
}
}
})
}
/// Runs a closure when dropped.
struct CallOnDrop<F: Fn()>(F);
impl<F: Fn()> Drop for CallOnDrop<F> {
fn drop(&mut self) {
(self.0)();
}
}