time: lazily init timers on first poll (#6512)

benches/Cargo.toml

@@ -90,3 +90,8 @@ harness = false
 name = "time_now"
 path = "time_now.rs"
 harness = false
+
+[[bench]]
+name = "time_timeout"
+path = "time_timeout.rs"
+harness = false

benches/time_timeout.rs

@@ -0,0 +1,109 @@
+use std::time::{Duration, Instant};
+
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use tokio::{
+    runtime::Runtime,
+    time::{sleep, timeout},
+};
+
+// a very quick async task, but might timeout
+async fn quick_job() -> usize {
+    1
+}
+
+fn build_run_time(workers: usize) -> Runtime {
+    if workers == 1 {
+        tokio::runtime::Builder::new_current_thread()
+            .enable_all()
+            .build()
+            .unwrap()
+    } else {
+        tokio::runtime::Builder::new_multi_thread()
+            .enable_all()
+            .worker_threads(workers)
+            .build()
+            .unwrap()
+    }
+}
+
+fn single_thread_scheduler_timeout(c: &mut Criterion) {
+    do_timeout_test(c, 1, "single_thread_timeout");
+}
+
+fn multi_thread_scheduler_timeout(c: &mut Criterion) {
+    do_timeout_test(c, 8, "multi_thread_timeout-8");
+}
+
+fn do_timeout_test(c: &mut Criterion, workers: usize, name: &str) {
+    let runtime = build_run_time(workers);
+    c.bench_function(name, |b| {
+        b.iter_custom(|iters| {
+            let start = Instant::now();
+            runtime.block_on(async {
+                black_box(spawn_timeout_job(iters as usize, workers).await);
+            });
+            start.elapsed()
+        })
+    });
+}
+
+async fn spawn_timeout_job(iters: usize, procs: usize) {
+    let mut handles = Vec::with_capacity(procs);
+    for _ in 0..procs {
+        handles.push(tokio::spawn(async move {
+            for _ in 0..iters / procs {
+                let h = timeout(Duration::from_secs(1), quick_job());
+                assert_eq!(black_box(h.await.unwrap()), 1);
+            }
+        }));
+    }
+    for handle in handles {
+        handle.await.unwrap();
+    }
+}
+
+fn single_thread_scheduler_sleep(c: &mut Criterion) {
+    do_sleep_test(c, 1, "single_thread_sleep");
+}
+
+fn multi_thread_scheduler_sleep(c: &mut Criterion) {
+    do_sleep_test(c, 8, "multi_thread_sleep-8");
+}
+
+fn do_sleep_test(c: &mut Criterion, workers: usize, name: &str) {
+    let runtime = build_run_time(workers);
+
+    c.bench_function(name, |b| {
+        b.iter_custom(|iters| {
+            let start = Instant::now();
+            runtime.block_on(async {
+                black_box(spawn_sleep_job(iters as usize, workers).await);
+            });
+            start.elapsed()
+        })
+    });
+}
+
+async fn spawn_sleep_job(iters: usize, procs: usize) {
+    let mut handles = Vec::with_capacity(procs);
+    for _ in 0..procs {
+        handles.push(tokio::spawn(async move {
+            for _ in 0..iters / procs {
+                let _h = black_box(sleep(Duration::from_secs(1)));
+            }
+        }));
+    }
+    for handle in handles {
+        handle.await.unwrap();
+    }
+}
+
+criterion_group!(
+    timeout_benchmark,
+    single_thread_scheduler_timeout,
+    multi_thread_scheduler_timeout,
+    single_thread_scheduler_sleep,
+    multi_thread_scheduler_sleep
+);
+
+criterion_main!(timeout_benchmark);

tokio/src/runtime/time/entry.rs

@@ -293,7 +293,7 @@ pub(crate) struct TimerEntry {
     ///
     /// This is manipulated only under the inner mutex. TODO: Can we use loom
     /// cells for this?
-    inner: StdUnsafeCell<TimerShared>,
+    inner: StdUnsafeCell<Option<TimerShared>>,
     /// Deadline for the timer. This is used to register on the first
     /// poll, as we can't register prior to being pinned.
     deadline: Instant,
@@ -469,35 +469,48 @@ unsafe impl linked_list::Link for TimerShared {
 
 impl TimerEntry {
     #[track_caller]
-    pub(crate) fn new(handle: &scheduler::Handle, deadline: Instant) -> Self {
+    pub(crate) fn new(handle: scheduler::Handle, deadline: Instant) -> Self {
         // Panic if the time driver is not enabled
         let _ = handle.driver().time();
 
-        let driver = handle.clone();
-
         Self {
-            driver,
-            inner: StdUnsafeCell::new(TimerShared::new()),
+            driver: handle,
+            inner: StdUnsafeCell::new(None),
             deadline,
             registered: false,
             _m: std::marker::PhantomPinned,
         }
     }
 
+    fn is_inner_init(&self) -> bool {
+        unsafe { &*self.inner.get() }.is_some()
+    }
+
+    // This lazy initialization is for performance purposes.
     fn inner(&self) -> &TimerShared {
-        unsafe { &*self.inner.get() }
+        let inner = unsafe { &*self.inner.get() };
+        if inner.is_none() {
+            unsafe {
+                *self.inner.get() = Some(TimerShared::new());
+            }
+        }
+        return inner.as_ref().unwrap();
     }
 
     pub(crate) fn deadline(&self) -> Instant {
         self.deadline
     }
 
     pub(crate) fn is_elapsed(&self) -> bool {
-        !self.inner().state.might_be_registered() && self.registered
+        self.is_inner_init() && !self.inner().state.might_be_registered() && self.registered
     }
 
     /// Cancels and deregisters the timer. This operation is irreversible.
     pub(crate) fn cancel(self: Pin<&mut Self>) {
+        // Avoid calling the `clear_entry` method, because it has not been initialized yet.
+        if !self.is_inner_init() {
+            return;
+        }
         // We need to perform an acq/rel fence with the driver thread, and the
         // simplest way to do so is to grab the driver lock.
         //
@@ -524,8 +537,9 @@ impl TimerEntry {
     }
 
     pub(crate) fn reset(mut self: Pin<&mut Self>, new_time: Instant, reregister: bool) {
-        unsafe { self.as_mut().get_unchecked_mut() }.deadline = new_time;
-        unsafe { self.as_mut().get_unchecked_mut() }.registered = reregister;
+        let this = unsafe { self.as_mut().get_unchecked_mut() };
+        this.deadline = new_time;
+        this.registered = reregister;
 
         let tick = self.driver().time_source().deadline_to_tick(new_time);
 

tokio/src/runtime/time/tests/mod.rs

@@ -49,7 +49,7 @@ fn single_timer() {
         let handle_ = handle.clone();
         let jh = thread::spawn(move || {
             let entry = TimerEntry::new(
-                &handle_.inner,
+                handle_.inner.clone(),
                 handle_.inner.driver().clock().now() + Duration::from_secs(1),
             );
             pin!(entry);
@@ -83,7 +83,7 @@ fn drop_timer() {
         let handle_ = handle.clone();
         let jh = thread::spawn(move || {
             let entry = TimerEntry::new(
-                &handle_.inner,
+                handle_.inner.clone(),
                 handle_.inner.driver().clock().now() + Duration::from_secs(1),
             );
             pin!(entry);
@@ -117,7 +117,7 @@ fn change_waker() {
         let handle_ = handle.clone();
         let jh = thread::spawn(move || {
             let entry = TimerEntry::new(
-                &handle_.inner,
+                handle_.inner.clone(),
                 handle_.inner.driver().clock().now() + Duration::from_secs(1),
             );
             pin!(entry);
@@ -157,7 +157,7 @@ fn reset_future() {
         let start = handle.inner.driver().clock().now();
 
         let jh = thread::spawn(move || {
-            let entry = TimerEntry::new(&handle_.inner, start + Duration::from_secs(1));
+            let entry = TimerEntry::new(handle_.inner.clone(), start + Duration::from_secs(1));
             pin!(entry);
 
             let _ = entry
@@ -219,7 +219,7 @@ fn poll_process_levels() {
 
     for i in 0..normal_or_miri(1024, 64) {
         let mut entry = Box::pin(TimerEntry::new(
-            &handle.inner,
+            handle.inner.clone(),
             handle.inner.driver().clock().now() + Duration::from_millis(i),
         ));
 
@@ -253,7 +253,7 @@ fn poll_process_levels_targeted() {
     let handle = rt.handle();
 
     let e1 = TimerEntry::new(
-        &handle.inner,
+        handle.inner.clone(),
         handle.inner.driver().clock().now() + Duration::from_millis(193),
     );
     pin!(e1);

tokio/src/time/sleep.rs

@@ -254,12 +254,11 @@ impl Sleep {
         location: Option<&'static Location<'static>>,
     ) -> Sleep {
         use crate::runtime::scheduler;
-
         let handle = scheduler::Handle::current();
-        let entry = TimerEntry::new(&handle, deadline);
-
+        let entry = TimerEntry::new(handle, deadline);
         #[cfg(all(tokio_unstable, feature = "tracing"))]
         let inner = {
+            let handle = scheduler::Handle::current();
             let clock = handle.driver().clock();
             let handle = &handle.driver().time();
             let time_source = handle.time_source();