use park + unpark instead of spawning

src/rt/runtime.rs

@@ -8,7 +8,11 @@ use std::thread;
 use std::time::Duration;
 
 use crossbeam_deque::{Injector, Steal, Stealer, Worker};
-use crossbeam_utils::thread::scope;
+use crossbeam_utils::{
+    sync::{Parker, Unparker},
+    thread::scope,
+};
+use once_cell::sync::Lazy;
 use once_cell::unsync::OnceCell;
 
 use crate::rt::Reactor;
@@ -24,10 +28,16 @@ thread_local! {
     static YIELD_NOW: Cell<bool> = Cell::new(false);
 }
 
+/// Maximum number of OS threads = processors = machines
+static MAXPROCS: Lazy<usize> = Lazy::new(|| num_cpus::get().max(1));
+
 struct Scheduler {
     /// Set to `true` while a machine is polling the reactor.
     polling: bool,
 
+    /// Available threads.
+    threads: Vec<ThreadState>,
+
     /// Idle processors.
     processors: Vec<Processor>,
 
@@ -35,6 +45,13 @@ struct Scheduler {
     machines: Vec<Arc<Machine>>,
 }
 
+struct ThreadState {
+    unparker: Unparker,
+    parked: Arc<atomic::AtomicBool>,
+    /// Used to transfer the machine into the thread.
+    machine_sender: crossbeam_channel::Sender<Arc<Machine>>,
+}
+
 /// An async runtime.
 pub struct Runtime {
     /// The reactor.
@@ -53,9 +70,9 @@ pub struct Runtime {
 impl Runtime {
     /// Creates a new runtime.
     pub fn new() -> Runtime {
-        let cpus = num_cpus::get().max(1);
-        let processors: Vec<_> = (0..cpus).map(|_| Processor::new()).collect();
+        let processors: Vec<_> = (0..*MAXPROCS).map(|_| Processor::new()).collect();
         let stealers = processors.iter().map(|p| p.worker.stealer()).collect();
+        let threads = Vec::with_capacity(*MAXPROCS);
 
         Runtime {
             reactor: Reactor::new().unwrap(),
@@ -63,7 +80,8 @@ impl Runtime {
             stealers,
             sched: Mutex::new(Scheduler {
                 processors,
-                machines: Vec::new(),
+                machines: Vec::with_capacity(*MAXPROCS),
+                threads,
                 polling: false,
             }),
         }
@@ -102,18 +120,67 @@ impl Runtime {
 
             loop {
                 // Get a list of new machines to start, if any need to be started.
-                for m in self.make_machines() {
+                let machines = self.make_machines();
+                for m in machines {
                     idle = 0;
 
-                    s.builder()
-                        .name("async-std/machine".to_string())
-                        .spawn(move |_| {
-                            abort_on_panic(|| {
-                                let _ = MACHINE.with(|machine| machine.set(m.clone()));
-                                m.run(self);
+                    // println!("getting idle thread");
+                    let mut sched = self.sched.lock().unwrap();
+                    'inner: for thread in &sched.threads {
+                        // grab the first parked thread
+                        if thread
+                            .parked
+                            .compare_and_swap(true, false, Ordering::Acquire)
+                        {
+                            // transfer the machine
+                            thread
+                                .machine_sender
+                                .send(m)
+                                .expect("failed to send machine to thread");
+                            // unpark the thread
+                            thread.unparker.unpark();
+                            break 'inner;
+                        }
+                    }
+
+                    // no idle thread available, check if we can spawn one
+                    if sched.threads.len() < *MAXPROCS {
+                        // we can spawn one, lets do it
+                        let parked = Arc::new(atomic::AtomicBool::new(true));
+                        let parked2 = parked.clone();
+                        let (machine_sender, machine_recv) = crossbeam_channel::bounded(1);
+                        let parker = Parker::new();
+                        let unparker = parker.unparker().clone();
+
+                        s.builder()
+                            .name("async-std/machine".to_string())
+                            .spawn(move |_| {
+                                abort_on_panic(|| {
+                                    loop {
+                                        while parked2.load(Ordering::Acquire) {
+                                            parker.park();
+                                        }
+                                        // when this thread is unparked, retrieve machine
+                                        let m: Arc<Machine> =
+                                            machine_recv.recv().expect("failed to receive machine");
+                                        // store it in the thread local
+                                        let _ = MACHINE.with(|machine| machine.set(m.clone()));
+                                        // run it
+                                        m.run(self);
+                                        // when run ends, go into parked mode again
+                                        parked2.store(false, Ordering::Relaxed);
+                                    }
+                                })
                             })
-                        })
-                        .expect("cannot start a machine thread");
+                            .expect("cannot start a machine thread");
+
+                        sched.threads.push(ThreadState {
+                            unparker,
+                            parked,
+                            machine_sender,
+                        });
+                    }
+                    drop(sched);
                 }
 
                 // Sleep for a bit longer if the scheduler state hasn't changed in a while.
@@ -142,6 +209,7 @@ impl Runtime {
                 let m = Arc::new(Machine::new(p));
                 to_start.push(m.clone());
                 sched.machines.push(m);
+                assert!(sched.machines.len() <= *MAXPROCS);
             }
         }
 
@@ -326,6 +394,7 @@ impl Machine {
             };
 
             // Unlock the schedule poll the reactor until new I/O events arrive.
+            // println!("polling start");
             sched.polling = true;
             drop(sched);
             rt.reactor.poll(None).unwrap();
@@ -333,6 +402,7 @@ impl Machine {
             // Lock the scheduler again and re-register the machine.
             sched = rt.sched.lock().unwrap();
             sched.polling = false;
+            //println!("polling stop");
             sched.machines.push(m);
 
             runs = 0;
@@ -344,9 +414,14 @@ impl Machine {
 
         // Return the processor to the scheduler and remove the machine.
         if let Some(p) = opt_p {
+            // println!("returning processor to pool");
             let mut sched = rt.sched.lock().unwrap();
             sched.processors.push(p);
+            assert!(sched.processors.len() <= *MAXPROCS);
+            // println!("machines {}", sched.machines.len());
             sched.machines.retain(|elem| !ptr::eq(&**elem, self));
+            // println!("machines retained {}", sched.machines.len());
+            assert!(sched.machines.len() <= *MAXPROCS);
         }
     }
 }