std::rt: Reduce MessageQueue contention

It's not a huge win but it does reduce the amount of time spent
contesting the message queue when the schedulers are under load

src/libstd/rt/message_queue.rs

@@ -16,32 +16,66 @@
 use vec::OwnedVector;
 use cell::Cell;
 use option::*;
-use unstable::sync::Exclusive;
+use unstable::sync::{UnsafeAtomicRcBox, LittleLock};
 use clone::Clone;
 
 pub struct MessageQueue<T> {
-    priv queue: Exclusive<~[T]>
+    priv state: UnsafeAtomicRcBox<State<T>>
+}
+
+struct State<T> {
+    count: uint,
+    queue: ~[T],
+    lock: LittleLock
 }
 
 impl<T: Send> MessageQueue<T> {
     pub fn new() -> MessageQueue<T> {
         MessageQueue {
-            queue: Exclusive::new(~[])
+            state: UnsafeAtomicRcBox::new(State {
+                count: 0,
+                queue: ~[],
+                lock: LittleLock::new()
+            })
         }
     }
 
     pub fn push(&mut self, value: T) {
         unsafe {
             let value = Cell::new(value);
-            self.queue.with(|q| q.push(value.take()) );
+            let state = self.state.get();
+            do (*state).lock.lock {
+                (*state).count += 1;
+                (*state).queue.push(value.take());
+            }
         }
     }
 
     pub fn pop(&mut self) -> Option<T> {
         unsafe {
-            do self.queue.with |q| {
-                if !q.is_empty() {
-                    Some(q.shift())
+            let state = self.state.get();
+            do (*state).lock.lock {
+                if !(*state).queue.is_empty() {
+                    (*state).count += 1;
+                    Some((*state).queue.shift())
+                } else {
+                    None
+                }
+            }
+        }
+    }
+
+    /// A pop that may sometimes miss enqueued elements, but is much faster
+    /// to give up without doing any synchronization
+    pub fn casual_pop(&mut self) -> Option<T> {
+        unsafe {
+            let state = self.state.get();
+            // NB: Unsynchronized check
+            if (*state).count == 0 { return None; }
+            do (*state).lock.lock {
+                if !(*state).queue.is_empty() {
+                    (*state).count += 1;
+                    Some((*state).queue.shift())
                 } else {
                     None
                 }
@@ -53,7 +87,7 @@ pub fn pop(&mut self) -> Option<T> {
 impl<T: Send> Clone for MessageQueue<T> {
     fn clone(&self) -> MessageQueue<T> {
         MessageQueue {
-            queue: self.queue.clone()
+            state: self.state.clone()
         }
     }
 }

src/libstd/rt/sched.rs

@@ -83,6 +83,14 @@ pub struct Scheduler {
     idle_callback: Option<~PausibleIdleCallback>
 }
 
+/// An indication of how hard to work on a given operation, the difference
+/// mainly being whether memory is synchronized or not
+#[deriving(Eq)]
+enum EffortLevel {
+    DontTryTooHard,
+    GiveItYourBest
+}
+
 impl Scheduler {
 
     // * Initialization Functions
@@ -237,14 +245,21 @@ fn run_sched_once() {
 
         // First we check for scheduler messages, these are higher
         // priority than regular tasks.
-        let sched = match sched.interpret_message_queue() {
+        let sched = match sched.interpret_message_queue(DontTryTooHard) {
             Some(sched) => sched,
             None => return
         };
 
         // This helper will use a randomized work-stealing algorithm
         // to find work.
-        let mut sched = match sched.do_work() {
+        let sched = match sched.do_work() {
+            Some(sched) => sched,
+            None => return
+        };
+
+        // Now, before sleeping we need to find out if there really
+        // were any messages. Give it your best!
+        let mut sched = match sched.interpret_message_queue(GiveItYourBest) {
             Some(sched) => sched,
             None => return
         };
@@ -277,10 +292,18 @@ fn run_sched_once() {
     // returns the still-available scheduler. At this point all
     // message-handling will count as a turn of work, and as a result
     // return None.
-    fn interpret_message_queue(~self) -> Option<~Scheduler> {
+    fn interpret_message_queue(~self, effort: EffortLevel) -> Option<~Scheduler> {
 
         let mut this = self;
-        match this.message_queue.pop() {
+
+        let msg = if effort == DontTryTooHard {
+            // Do a cheap check that may miss messages
+            this.message_queue.casual_pop()
+        } else {
+            this.message_queue.pop()
+        };
+
+        match msg {
             Some(PinnedTask(task)) => {
                 let mut task = task;
                 task.give_home(Sched(this.make_handle()));