Spawn more than one blocking thread (#475)

* Spawn more than 1 blocking thread

* Fix a bug

* Fix check when the thread is last sleeping

src/task/spawn_blocking.rs

@@ -1,4 +1,4 @@
-use std::sync::atomic::{AtomicU64, Ordering};
+use std::sync::atomic::{AtomicUsize, Ordering};
 use std::thread;
 use std::time::Duration;
 
@@ -8,8 +8,6 @@ use once_cell::sync::Lazy;
 use crate::task::{JoinHandle, Task};
 use crate::utils::{abort_on_panic, random};
 
-type Runnable = async_task::Task<Task>;
-
 /// Spawns a blocking task.
 ///
 /// The task will be spawned onto a thread pool specifically dedicated to blocking tasks. This
@@ -44,14 +42,16 @@ where
     F: FnOnce() -> T + Send + 'static,
     T: Send + 'static,
 {
+    let schedule = |task| POOL.sender.send(task).unwrap();
     let (task, handle) = async_task::spawn(async { f() }, schedule, Task::new(None));
     task.schedule();
     JoinHandle::new(handle)
 }
 
-const MAX_THREADS: u64 = 10_000;
+type Runnable = async_task::Task<Task>;
 
-static DYNAMIC_THREAD_COUNT: AtomicU64 = AtomicU64::new(0);
+/// The number of sleeping worker threads.
+static SLEEPING: AtomicUsize = AtomicUsize::new(0);
 
 struct Pool {
     sender: Sender<Runnable>,
@@ -59,78 +59,52 @@ struct Pool {
 }
 
 static POOL: Lazy<Pool> = Lazy::new(|| {
-    for _ in 0..2 {
-        thread::Builder::new()
-            .name("async-std/blocking".to_string())
-            .spawn(|| {
-                abort_on_panic(|| {
-                    for task in &POOL.receiver {
-                        task.run();
-                    }
-                })
-            })
-            .expect("cannot start a thread driving blocking tasks");
-    }
+    // Start a single worker thread waiting for the first task.
+    start_thread();
 
-    // We want to use an unbuffered channel here to help
-    // us drive our dynamic control. In effect, the
-    // kernel's scheduler becomes the queue, reducing
-    // the number of buffers that work must flow through
-    // before being acted on by a core. This helps keep
-    // latency snappy in the overall async system by
-    // reducing bufferbloat.
     let (sender, receiver) = unbounded();
     Pool { sender, receiver }
 });
 
-// Create up to MAX_THREADS dynamic blocking task worker threads.
-// Dynamic threads will terminate themselves if they don't
-// receive any work after between one and ten seconds.
-fn maybe_create_another_blocking_thread() {
-    // We use a `Relaxed` atomic operation because
-    // it's just a heuristic, and would not lose correctness
-    // even if it's random.
-    let workers = DYNAMIC_THREAD_COUNT.load(Ordering::Relaxed);
-    if workers >= MAX_THREADS {
-        return;
-    }
-
-    let n_to_spawn = std::cmp::min(2 + (workers / 10), 10);
+fn start_thread() {
+    SLEEPING.fetch_add(1, Ordering::SeqCst);
 
-    for _ in 0..n_to_spawn {
-        // We want to avoid having all threads terminate at
-        // exactly the same time, causing thundering herd
-        // effects. We want to stagger their destruction over
-        // 10 seconds or so to make the costs fade into
-        // background noise.
-        //
-        // Generate a simple random number of milliseconds
-        let rand_sleep_ms = u64::from(random(10_000));
+    // Generate a random duration of time between 1 second and 10 seconds. If the thread doesn't
+    // receive the next task in this duration of time, it will stop running.
+    let timeout = Duration::from_millis(1000 + u64::from(random(9_000)));
 
     thread::Builder::new()
         .name("async-std/blocking".to_string())
         .spawn(move || {
-                let wait_limit = Duration::from_millis(1000 + rand_sleep_ms);
-
-                DYNAMIC_THREAD_COUNT.fetch_add(1, Ordering::Relaxed);
-                while let Ok(task) = POOL.receiver.recv_timeout(wait_limit) {
-                    abort_on_panic(|| task.run());
+            loop {
+                let task = match POOL.receiver.recv_timeout(timeout) {
+                    Ok(task) => task,
+                    Err(_) => {
+                        // Check whether this is the last sleeping thread.
+                        if SLEEPING.fetch_sub(1, Ordering::SeqCst) == 1 {
+                            // If so, then restart the thread to make sure there is always at least
+                            // one sleeping thread.
+                            if SLEEPING.compare_and_swap(0, 1, Ordering::SeqCst) == 0 {
+                                continue;
                             }
-                DYNAMIC_THREAD_COUNT.fetch_sub(1, Ordering::Relaxed);
-            })
-            .expect("cannot start a dynamic thread driving blocking tasks");
                         }
+
+                        // Stop the thread.
+                        return;
                     }
+                };
 
-// Enqueues work, attempting to send to the threadpool in a
-// nonblocking way and spinning up another worker thread if
-// there is not a thread ready to accept the work.
-pub(crate) fn schedule(task: Runnable) {
-    if let Err(err) = POOL.sender.try_send(task) {
-        // We were not able to send to the channel without
-        // blocking. Try to spin up another thread and then
-        // retry sending while blocking.
-        maybe_create_another_blocking_thread();
-        POOL.sender.send(err.into_inner()).unwrap();
+                // If there are no sleeping threads, then start one to make sure there is always at
+                // least one sleeping thread.
+                if SLEEPING.fetch_sub(1, Ordering::SeqCst) == 1 {
+                    start_thread();
                 }
+
+                // Run the task.
+                abort_on_panic(|| task.run());
+
+                SLEEPING.fetch_add(1, Ordering::SeqCst);
+            }
+        })
+        .expect("cannot start a blocking thread");
 }

src/utils.rs

@@ -25,7 +25,13 @@ pub fn random(n: u32) -> u32 {
     use std::num::Wrapping;
 
     thread_local! {
-        static RNG: Cell<Wrapping<u32>> = Cell::new(Wrapping(1_406_868_647));
+        static RNG: Cell<Wrapping<u32>> = {
+            // Take the address of a local value as seed.
+            let mut x = 0i32;
+            let r = &mut x;
+            let addr = r as *mut i32 as usize;
+            Cell::new(Wrapping(addr as u32))
+        }
     }
 
     RNG.with(|rng| {