Add dynamic threadpool

src/task/blocking.rs

@@ -2,15 +2,21 @@
 
 use std::fmt;
 use std::pin::Pin;
+use std::sync::atomic::{AtomicU64, Ordering};
 use std::thread;
+use std::time::Duration;
 
-use crossbeam_channel::{unbounded, Receiver, Sender};
+use crossbeam_channel::{bounded, Receiver, Sender};
 use lazy_static::lazy_static;
 
 use crate::future::Future;
 use crate::task::{Context, Poll};
 use crate::utils::abort_on_panic;
 
+const MAX_THREADS: u64 = 10_000;
+
+static DYNAMIC_THREAD_COUNT: AtomicU64 = AtomicU64::new(0);
+
 struct Pool {
     sender: Sender<async_task::Task<()>>,
     receiver: Receiver<async_task::Task<()>>,
@@ -29,11 +35,69 @@ lazy_static! {
                 .expect("cannot start a thread driving blocking tasks");
         }
 
-        let (sender, receiver) = unbounded();
+        // 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) = bounded(0);
         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 one second.
+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;
+    }
+
+    thread::Builder::new()
+        .name("async-blocking-driver-dynamic".to_string())
+        .spawn(|| {
+            let wait_limit = Duration::from_secs(1);
+
+            DYNAMIC_THREAD_COUNT.fetch_add(1, Ordering::Relaxed);
+            while let Ok(task) = POOL.receiver.recv_timeout(wait_limit) {
+                abort_on_panic(|| task.run());
+            }
+            DYNAMIC_THREAD_COUNT.fetch_sub(1, Ordering::Relaxed);
+        })
+        .expect("cannot start a dynamic thread driving blocking tasks");
+}
+
+// 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.
+fn schedule(t: async_task::Task<()>) {
+    let first_try_result = POOL.sender.try_send(t);
+    match first_try_result {
+        Ok(()) => {
+            // NICEEEE
+        }
+        Err(crossbeam::channel::TrySendError::Full(t)) => {
+            // 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(t).unwrap()
+        }
+        Err(crossbeam::channel::TrySendError::Disconnected(_)) => {
+            panic!(
+                "unable to send to blocking threadpool \
+                due to receiver disconnection"
+            );
+        }
+    }
+}
+
 /// Spawns a blocking task.
 ///
 /// The task will be spawned onto a thread pool specifically dedicated to blocking tasks.
@@ -42,7 +106,6 @@ where
     F: Future<Output = R> + Send + 'static,
     R: Send + 'static,
 {
-    let schedule = |t| POOL.sender.send(t).unwrap();
     let (task, handle) = async_task::spawn(future, schedule, ());
     task.schedule();
     JoinHandle(handle)