auto merge of #3820 : cgaebel/servo/rebase-lalehs-patch, r=pcwalton

I addressed all but one of the code review comments, which was a request
for documentation on where a number came from (which I'm not qualified to
answer), and rebased this on to latest master.

xref: #3505 cc: @LalehB @larsbergstrom

r? @pcwalton

components/util/workqueue.rs

@@ -16,6 +16,7 @@ use std::rand::weak_rng;
 use std::sync::atomics::{AtomicUint, SeqCst};
 use std::sync::deque::{Abort, BufferPool, Data, Empty, Stealer, Worker};
 use std::task::TaskBuilder;
+use libc::funcs::posix88::unistd::usleep;
 
 /// A unit of work.
 ///
@@ -70,7 +71,9 @@ struct WorkerThread<QueueData, WorkData> {
     rng: XorShiftRng,
 }
 
-static SPIN_COUNT: uint = 1000;
+static SPIN_COUNT: u32 = 128;
+static SPINS_UNTIL_BACKOFF: u32 = 100;
+static BACKOFF_INCREMENT_IN_US: u32 = 5;
 
 impl<QueueData: Send, WorkData: Send> WorkerThread<QueueData, WorkData> {
     /// The main logic. This function starts up the worker and listens for
@@ -84,6 +87,8 @@ impl<QueueData: Send, WorkData: Send> WorkerThread<QueueData, WorkData> {
                 ExitMsg => return,
             };
 
+            let mut back_off_sleep = 0 as u32;
+
             // We're off!
             //
             // FIXME(pcwalton): Can't use labeled break or continue cross-crate due to a Rust bug.
@@ -107,10 +112,18 @@ impl<QueueData: Send, WorkData: Send> WorkerThread<QueueData, WorkData> {
                                 }
                                 Data(work) => {
                                     work_unit = work;
+                                    back_off_sleep = 0 as u32;
                                     break
                                 }
                             }
 
+                            if i > SPINS_UNTIL_BACKOFF {
+                                unsafe {
+                                    usleep(back_off_sleep as u32);
+                                }
+                                back_off_sleep += BACKOFF_INCREMENT_IN_US;
+                            }
+
                             if i == SPIN_COUNT {
                                 match self.port.try_recv() {
                                     Ok(StopMsg) => {