fix(rt): bring back dynamic machines

Even if we do not make use of the progress blocking, we do need to make use of the dynamic restarting of machines as far as I understand.

Keeps the perf, while removing the regression from #747

src/rt/runtime.rs

@@ -1,6 +1,7 @@
 use std::cell::Cell;
 use std::io;
 use std::iter;
+use std::ptr;
 use std::sync::atomic::{self, Ordering};
 use std::sync::{Arc, Mutex};
 use std::thread;
@@ -26,6 +27,12 @@ thread_local! {
 struct Scheduler {
     /// Set to `true` while a machine is polling the reactor.
     polling: bool,
+
+    /// Idle processors.
+    processors: Vec<Processor>,
+
+    /// Running machines.
+    machines: Vec<Arc<Machine>>,
 }
 
 /// An async runtime.
@@ -39,9 +46,6 @@ pub struct Runtime {
     /// Handles to local queues for stealing work.
     stealers: Vec<Stealer<Runnable>>,
 
-    /// Machines to start
-    machines: Vec<Arc<Machine>>,
-
     /// The scheduler state.
     sched: Mutex<Scheduler>,
 }
@@ -51,23 +55,17 @@ impl Runtime {
     pub fn new() -> Runtime {
         let cpus = num_cpus::get().max(1);
         let processors: Vec<_> = (0..cpus).map(|_| Processor::new()).collect();
-
-        let machines: Vec<_> = processors
-            .into_iter()
-            .map(|p| Arc::new(Machine::new(p)))
-            .collect();
-
-        let stealers = machines
-            .iter()
-            .map(|m| m.processor.lock().worker.stealer())
-            .collect();
+        let stealers = processors.iter().map(|p| p.worker.stealer()).collect();
 
         Runtime {
             reactor: Reactor::new().unwrap(),
             injector: Injector::new(),
             stealers,
-            machines,
-            sched: Mutex::new(Scheduler { polling: false }),
+            sched: Mutex::new(Scheduler {
+                processors,
+                machines: Vec::new(),
+                polling: false,
+            }),
         }
     }
 
@@ -99,21 +97,57 @@ impl Runtime {
     /// Runs the runtime on the current thread.
     pub fn run(&self) {
         scope(|s| {
-            for m in &self.machines {
-                s.builder()
-                    .name("async-std/machine".to_string())
-                    .spawn(move |_| {
-                        abort_on_panic(|| {
-                            let _ = MACHINE.with(|machine| machine.set(m.clone()));
-                            m.run(self);
+            let mut idle = 0;
+            let mut delay = 0;
+
+            loop {
+                // Get a list of new machines to start, if any need to be started.
+                for m in self.make_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);
+                            })
                         })
-                    })
-                    .expect("cannot start a machine thread");
+                        .expect("cannot start a machine thread");
+                }
+
+                // Sleep for a bit longer if the scheduler state hasn't changed in a while.
+                if idle > 10 {
+                    delay = (delay * 2).min(10_000);
+                } else {
+                    idle += 1;
+                    delay = 1000;
+                }
+
+                thread::sleep(Duration::from_micros(delay));
             }
         })
         .unwrap();
     }
 
+    /// Returns a list of machines that need to be started.
+    fn make_machines(&self) -> Vec<Arc<Machine>> {
+        let mut sched = self.sched.lock().unwrap();
+        let mut to_start = Vec::new();
+
+        // If no machine has been polling the reactor in a while, that means the runtime is
+        // overloaded with work and we need to start another machine.
+        if !sched.polling {
+            if let Some(p) = sched.processors.pop() {
+                let m = Arc::new(Machine::new(p));
+                to_start.push(m.clone());
+                sched.machines.push(m);
+            }
+        }
+
+        to_start
+    }
+
     /// Unparks a thread polling the reactor.
     fn notify(&self) {
         atomic::fence(Ordering::SeqCst);
@@ -139,20 +173,26 @@ impl Runtime {
 /// A thread running a processor.
 struct Machine {
     /// Holds the processor until it gets stolen.
-    processor: Spinlock<Processor>,
+    processor: Spinlock<Option<Processor>>,
 }
 
 impl Machine {
     /// Creates a new machine running a processor.
     fn new(p: Processor) -> Machine {
         Machine {
-            processor: Spinlock::new(p),
+            processor: Spinlock::new(Some(p)),
         }
     }
 
     /// Schedules a task onto the machine.
     fn schedule(&self, rt: &Runtime, task: Runnable) {
-        self.processor.lock().schedule(rt, task);
+        match self.processor.lock().as_mut() {
+            None => {
+                rt.injector.push(task);
+                rt.notify();
+            }
+            Some(p) => p.schedule(rt, task),
+        }
     }
 
     /// Finds the next runnable task.
@@ -160,14 +200,16 @@ impl Machine {
         let mut retry = false;
 
         // First try finding a task in the local queue or in the global queue.
-        if let Some(task) = self.processor.lock().pop_task() {
-            return Steal::Success(task);
-        }
+        if let Some(p) = self.processor.lock().as_mut() {
+            if let Some(task) = p.pop_task() {
+                return Steal::Success(task);
+            }
 
-        match self.processor.lock().steal_from_global(rt) {
-            Steal::Empty => {}
-            Steal::Retry => retry = true,
-            Steal::Success(task) => return Steal::Success(task),
+            match p.steal_from_global(rt) {
+                Steal::Empty => {}
+                Steal::Retry => retry = true,
+                Steal::Success(task) => return Steal::Success(task),
+            }
         }
 
         // Try polling the reactor, but don't block on it.
@@ -175,16 +217,18 @@ impl Machine {
 
         // Try finding a task in the local queue, which might hold tasks woken by the reactor. If
         // the local queue is still empty, try stealing from other processors.
-        if progress {
-            if let Some(task) = self.processor.lock().pop_task() {
-                return Steal::Success(task);
+        if let Some(p) = self.processor.lock().as_mut() {
+            if progress {
+                if let Some(task) = p.pop_task() {
+                    return Steal::Success(task);
+                }
             }
-        }
 
-        match self.processor.lock().steal_from_others(rt) {
-            Steal::Empty => {}
-            Steal::Retry => retry = true,
-            Steal::Success(task) => return Steal::Success(task),
+            match p.steal_from_others(rt) {
+                Steal::Empty => {}
+                Steal::Retry => retry = true,
+                Steal::Success(task) => return Steal::Success(task),
+            }
         }
 
         if retry { Steal::Retry } else { Steal::Empty }
@@ -208,7 +252,9 @@ impl Machine {
             // Check if `task::yield_now()` was invoked and flush the slot if so.
             YIELD_NOW.with(|flag| {
                 if flag.replace(false) {
-                    self.processor.lock().flush_slot(rt);
+                    if let Some(p) = self.processor.lock().as_mut() {
+                        p.flush_slot(rt);
+                    }
                 }
             });
 
@@ -219,12 +265,13 @@ impl Machine {
                 runs = 0;
                 rt.quick_poll().unwrap();
 
-                let mut p = self.processor.lock();
-                if let Steal::Success(task) = p.steal_from_global(rt) {
-                    p.schedule(rt, task);
-                }
+                if let Some(p) = self.processor.lock().as_mut() {
+                    if let Steal::Success(task) = p.steal_from_global(rt) {
+                        p.schedule(rt, task);
+                    }
 
-                p.flush_slot(rt);
+                    p.flush_slot(rt);
+                }
             }
 
             // Try to find a runnable task.
@@ -245,7 +292,9 @@ impl Machine {
 
             // Put the current thread to sleep a few times.
             if fails <= YIELDS + SLEEPS {
+                let opt_p = self.processor.lock().take();
                 thread::sleep(Duration::from_micros(10));
+                *self.processor.lock() = opt_p;
                 continue;
             }
 
@@ -266,6 +315,16 @@ impl Machine {
                 break;
             }
 
+            // Take out the machine associated with the current thread.
+            let m = match sched
+                .machines
+                .iter()
+                .position(|elem| ptr::eq(&**elem, self))
+            {
+                None => break, // The processor was stolen.
+                Some(pos) => sched.machines.swap_remove(pos),
+            };
+
             // Unlock the schedule poll the reactor until new I/O events arrive.
             sched.polling = true;
             drop(sched);
@@ -274,10 +333,21 @@ impl Machine {
             // Lock the scheduler again and re-register the machine.
             sched = rt.sched.lock().unwrap();
             sched.polling = false;
+            sched.machines.push(m);
 
             runs = 0;
             fails = 0;
         }
+
+        // When shutting down the thread, take the processor out if still available.
+        let opt_p = self.processor.lock().take();
+
+        // Return the processor to the scheduler and remove the machine.
+        if let Some(p) = opt_p {
+            let mut sched = rt.sched.lock().unwrap();
+            sched.processors.push(p);
+            sched.machines.retain(|elem| !ptr::eq(&**elem, self));
+        }
     }
 }