An experimental implemenation of Ticker using two goroutines for all tickers.

Feel free to suggest other approaches.

R=rsc
CC=cw, golang-dev
https://golang.org/cl/193070

src/pkg/time/tick.go

@@ -4,100 +4,159 @@
 
 package time
 
-// TODO(rsc): This implementation of Tick is a
-// simple placeholder.  Eventually, there will need to be
-// a single central time server no matter how many tickers
-// are active.
-//
-// Also, if timeouts become part of the select statement,
-// perhaps the Ticker is just:
-//
-//	func Ticker(ns int64, c chan int64) {
-//		for {
-//			select { timeout ns: }
-//			nsec, err := Nanoseconds();
-//			c <- nsec;
-//		}
-
+import (
+	"once"
+)
 
 // A Ticker holds a synchronous channel that delivers `ticks' of a clock
 // at intervals.
 type Ticker struct {
 	C        <-chan int64 // The channel on which the ticks are delivered.
-	done     chan bool
+	c        chan<- int64 // The same channel, but the end we use.
 	ns       int64
 	shutdown bool
+	nextTick int64
+	next     *Ticker
 }
 
 // Stop turns off a ticker.  After Stop, no more ticks will be sent.
-func (t *Ticker) Stop() {
-	t.shutdown = true
-	go t.drain()
+func (t *Ticker) Stop() { t.shutdown = true }
+
+// Tick is a convenience wrapper for NewTicker providing access to the ticking
+// channel only.  Useful for clients that have no need to shut down the ticker.
+func Tick(ns int64) <-chan int64 {
+	if ns <= 0 {
+		return nil
+	}
+	return NewTicker(ns).C
 }
 
-func (t *Ticker) drain() {
-	for {
-		select {
-		case <-t.C:
-		case <-t.done:
-			return
+type alarmer struct {
+	wakeUp   chan bool // wakeup signals sent/received here
+	wakeMeAt chan int64
+	wakeTime int64
+}
+
+// Set alarm to go off at time ns, if not already set earlier.
+func (a *alarmer) set(ns int64) {
+	// If there's no wakeLoop or the next tick we expect is too late, start a new wakeLoop
+	if a.wakeMeAt == nil || a.wakeTime > ns {
+		// Stop previous wakeLoop.
+		if a.wakeMeAt != nil {
+			a.wakeMeAt <- -1
 		}
+		a.wakeMeAt = make(chan int64, 10)
+		go wakeLoop(a.wakeMeAt, a.wakeUp)
+		a.wakeMeAt <- ns
 	}
 }
 
-func (t *Ticker) ticker(c chan<- int64) {
-	now := Nanoseconds()
-	when := now
-	for !t.shutdown {
-		when += t.ns // next alarm
+// Channel to notify tickerLoop of new Tickers being created.
+var newTicker chan *Ticker
 
-		// if c <- now took too long, skip ahead
-		if when < now {
-			// one big step
-			when += (now - when) / t.ns * t.ns
-		}
-		for when <= now {
-			// little steps until when > now
-			when += t.ns
-		}
+func startTickerLoop() {
+	newTicker = make(chan *Ticker)
+	go tickerLoop()
+}
 
-		for !t.shutdown && when > now {
-			// limit individual sleeps so that stopped
-			// long-term tickers don't pile up.
-			const maxSleep = 1e9
-			if when-now > maxSleep {
-				Sleep(maxSleep)
-			} else {
-				Sleep(when - now)
-			}
-			now = Nanoseconds()
+// wakeLoop delivers ticks at scheduled times, sleeping until the right moment.
+// If another, earlier Ticker is created while it sleeps, tickerLoop() will start a new
+// wakeLoop but they will share the wakeUp channel and signal that this one
+// is done by giving it a negative time request.
+func wakeLoop(wakeMeAt chan int64, wakeUp chan bool) {
+	for {
+		wakeAt := <-wakeMeAt
+		if wakeAt < 0 { // tickerLoop has started another wakeLoop
+			return
 		}
-		if t.shutdown {
-			break
+		now := Nanoseconds()
+		if wakeAt > now {
+			Sleep(wakeAt - now)
+			now = Nanoseconds()
 		}
-		c <- now
+		wakeUp <- true
 	}
-	t.done <- true
 }
 
-// Tick is a convenience wrapper for NewTicker providing access to the ticking
-// channel only.  Useful for clients that have no need to shut down the ticker.
-func Tick(ns int64) <-chan int64 {
-	if ns <= 0 {
-		return nil
+// A single tickerLoop serves all ticks to Tickers.  It waits for two events:
+// either the creation of a new Ticker or a tick from the alarm,
+// signalling a time to wake up one or more Tickers.
+func tickerLoop() {
+	// Represents the next alarm to be delivered.
+	var alarm alarmer
+	// All wakeLoops deliver wakeups to this channel.
+	alarm.wakeUp = make(chan bool, 10)
+	var now, prevTime, wakeTime int64
+	var tickers *Ticker
+	for {
+		select {
+		case t := <-newTicker:
+			// Add Ticker to list
+			t.next = tickers
+			tickers = t
+			// Arrange for a new alarm if this one precedes the existing one.
+			alarm.set(t.nextTick)
+		case <-alarm.wakeUp:
+			now = Nanoseconds()
+			// Ignore an old time due to a dying wakeLoop
+			if now < prevTime {
+				continue
+			}
+			wakeTime = now + 1e15 // very long in the future
+			var prev *Ticker = nil
+			// Scan list of tickers, delivering updates to those
+			// that need it and determining the next wake time.
+			// TODO(r): list should be sorted in time order.
+			for t := tickers; t != nil; t = t.next {
+				if t.shutdown {
+					// Ticker is done; remove it from list.
+					if prev == nil {
+						tickers = t.next
+					} else {
+						prev.next = t.next
+					}
+					continue
+				}
+				if tickers.nextTick <= now {
+					if len(t.c) == 0 {
+						// Only send if there's room.  We must not block.
+						// The channel is allocated with a one-element
+						// buffer, which is sufficient: if he hasn't picked
+						// up the last tick, no point in sending more.
+						t.c <- now
+					}
+					t.nextTick += t.ns
+					if t.nextTick <= now {
+						// Still behind; advance in one big step.
+						t.nextTick += (now - t.nextTick + t.ns) / t.ns * t.ns
+					}
+					if t.nextTick > now && t.nextTick < wakeTime {
+						wakeTime = t.nextTick
+					}
+				}
+				prev = t
+			}
+			if tickers != nil {
+				// Please send wakeup at earliest required time.
+				// If there are no tickers, don't bother.
+				alarm.wakeMeAt <- wakeTime
+			}
+		}
+		prevTime = now
 	}
-	return NewTicker(ns).C
 }
 
-// Ticker returns a new Ticker containing a synchronous channel that will
+// Ticker returns a new Ticker containing a channel that will
 // send the time, in nanoseconds, every ns nanoseconds.  It adjusts the
 // intervals to make up for pauses in delivery of the ticks.
 func NewTicker(ns int64) *Ticker {
 	if ns <= 0 {
 		return nil
 	}
-	c := make(chan int64)
-	t := &Ticker{c, make(chan bool), ns, false}
-	go t.ticker(c)
+	c := make(chan int64, 1) //  See comment on send in tickerLoop
+	t := &Ticker{c, c, ns, false, Nanoseconds() + ns, nil}
+	once.Do(startTickerLoop)
+	// must be run in background so global Tickers can be created
+	go func() { newTicker <- t }()
 	return t
 }