alg: Push tasks directly to the local runner

This commit adds an optimization where a thread-local variable contains the queue of the
current runner. Rather than pushing to the global queue and hoping that a local queue
eventually picks it up, tasks are pushed directly to this local queue if available.

This has led to speedups of up to 70% in some cases and up to 10% in other workloads.

src/lib.rs

@@ -20,6 +20,7 @@
 
 #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
 
+use std::cell::RefCell;
 use std::fmt;
 use std::future::Future;
 use std::marker::PhantomData;
@@ -229,29 +230,56 @@ impl<'a> Executor<'a> {
         let runner = Runner::new(self.state());
         let mut rng = fastrand::Rng::new();
 
-        // A future that runs tasks forever.
-        let run_forever = async {
-            loop {
-                for _ in 0..200 {
-                    let runnable = runner.runnable(&mut rng).await;
-                    runnable.run();
-                }
-                future::yield_now().await;
-            }
-        };
+        // Set the local queue while we're running.
+        LocalQueue::set(self.state(), &runner.local, {
+            let runner = &runner;
+            async move {
+                // A future that runs tasks forever.
+                let run_forever = async {
+                    loop {
+                        for _ in 0..200 {
+                            let runnable = runner.runnable(&mut rng).await;
+                            runnable.run();
+                        }
+                        future::yield_now().await;
+                    }
+                };
 
-        // Run `future` and `run_forever` concurrently until `future` completes.
-        future.or(run_forever).await
+                // Run `future` and `run_forever` concurrently until `future` completes.
+                future.or(run_forever).await
+            }
+        })
+        .await
     }
 
     /// Returns a function that schedules a runnable task when it gets woken up.
     fn schedule(&self) -> impl Fn(Runnable) + Send + Sync + 'static {
         let state = self.state().clone();
 
-        // TODO(stjepang): If possible, push into the current local queue and notify the ticker.
+        // If possible, push into the current local queue and notify the ticker.
         move |runnable| {
-            state.queue.push(runnable).unwrap();
-            state.notify();
+            let mut runnable = Some(runnable);
+
+            // Try to push into the local queue.
+            LocalQueue::with(|local_queue| {
+                // Make sure that we don't accidentally push to an executor that isn't ours.
+                if !std::ptr::eq(local_queue.state, &*state) {
+                    return;
+                }
+
+                if let Err(e) = local_queue.queue.push(runnable.take().unwrap()) {
+                    runnable = Some(e.into_inner());
+                    return;
+                }
+
+                local_queue.waker.wake_by_ref();
+            });
+
+            // If the local queue push failed, just push to the global queue.
+            if let Some(runnable) = runnable {
+                state.queue.push(runnable).unwrap();
+                state.notify();
+            }
         }
     }
 
@@ -819,6 +847,97 @@ impl Drop for Runner<'_> {
     }
 }
 
+/// The state of the currently running local queue.
+struct LocalQueue {
+    /// The pointer to the state of the executor.
+    ///
+    /// Used to make sure we don't push runnables to the wrong executor.
+    state: *const State,
+
+    /// The concurrent queue.
+    queue: Arc<ConcurrentQueue<Runnable>>,
+
+    /// The waker for the runnable.
+    waker: Waker,
+}
+
+impl LocalQueue {
+    /// Run a function with the current local queue.
+    fn with<R>(f: impl FnOnce(&LocalQueue) -> R) -> Option<R> {
+        std::thread_local! {
+            /// The current local queue.
+            static LOCAL_QUEUE: RefCell<Option<LocalQueue>> = RefCell::new(None);
+        }
+
+        impl LocalQueue {
+            /// Run a function with a set local queue.
+            async fn set<F>(
+                state: &State,
+                queue: &Arc<ConcurrentQueue<Runnable>>,
+                fut: F,
+            ) -> F::Output
+            where
+                F: Future,
+            {
+                // Store the local queue and the current waker.
+                let mut old = with_waker(|waker| {
+                    LOCAL_QUEUE.with(move |slot| {
+                        slot.borrow_mut().replace(LocalQueue {
+                            state: state as *const State,
+                            queue: queue.clone(),
+                            waker: waker.clone(),
+                        })
+                    })
+                })
+                .await;
+
+                // Restore the old local queue on drop.
+                let _guard = CallOnDrop(move || {
+                    let old = old.take();
+                    let _ = LOCAL_QUEUE.try_with(move |slot| {
+                        *slot.borrow_mut() = old;
+                    });
+                });
+
+                // Pin the future.
+                futures_lite::pin!(fut);
+
+                // Run it such that the waker is updated every time it's polled.
+                future::poll_fn(move |cx| {
+                    LOCAL_QUEUE
+                        .try_with({
+                            let waker = cx.waker();
+                            move |slot| {
+                                let mut slot = slot.borrow_mut();
+                                let qaw = slot.as_mut().expect("missing local queue");
+
+                                // If we've been replaced, just ignore the slot.
+                                if !Arc::ptr_eq(&qaw.queue, queue) {
+                                    return;
+                                }
+
+                                // Update the waker, if it has changed.
+                                if !qaw.waker.will_wake(waker) {
+                                    qaw.waker = waker.clone();
+                                }
+                            }
+                        })
+                        .ok();
+
+                    // Poll the future.
+                    fut.as_mut().poll(cx)
+                })
+                .await
+            }
+        }
+
+        LOCAL_QUEUE
+            .try_with(|local_queue| local_queue.borrow().as_ref().map(f))
+            .ok()
+            .flatten()
+    }
+}
+
 /// Steals some items from one queue into another.
 fn steal<T>(src: &ConcurrentQueue<T>, dest: &ConcurrentQueue<T>) {
     // Half of `src`'s length rounded up.
@@ -911,10 +1030,19 @@ fn debug_executor(executor: &Executor<'_>, name: &str, f: &mut fmt::Formatter<'_
 }
 
 /// Runs a closure when dropped.
-struct CallOnDrop<F: Fn()>(F);
+struct CallOnDrop<F: FnMut()>(F);
 
-impl<F: Fn()> Drop for CallOnDrop<F> {
+impl<F: FnMut()> Drop for CallOnDrop<F> {
     fn drop(&mut self) {
         (self.0)();
     }
 }
+
+/// Run a closure with the current waker.
+fn with_waker<F: FnOnce(&Waker) -> R, R>(f: F) -> impl Future<Output = R> {
+    let mut f = Some(f);
+    future::poll_fn(move |cx| {
+        let f = f.take().unwrap();
+        Poll::Ready(f(cx.waker()))
+    })
+}