go/src/pkg/runtime/time.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Time-related runtime and pieces of package time.

package runtime

import "unsafe"

// Package time knows the layout of this structure.
// If this struct changes, adjust ../time/sleep.go:/runtimeTimer and netpoll.goc:/timer.
// For GOOS=nacl, package syscall knows the layout of this structure.
// If this struct changes, adjust ../syscall/net_nacl.go:/runtimeTimer.
type timer struct {
	i int // heap index

	// Timer wakes up at when, and then at when+period, ... (period > 0 only)
	// each time calling f(now, arg) in the timer goroutine, so f must be
	// a well-behaved function and not block.
	when   int64
	period int64
	f      func(interface{})
	arg    interface{}
}

var timers struct {
	lock         lock
	gp           *g
	created      bool
	sleeping     bool
	rescheduling bool
	waitnote     note
	t            []*timer
}

// Package time APIs.
// Godoc uses the comments in package time, not these.

// time.now is implemented in assembly.

// Sleep puts the current goroutine to sleep for at least ns nanoseconds.
func timeSleep(ns int64) {
	if ns <= 0 {
		return
	}

	t := new(timer)
	t.when = gonanotime() + ns
	t.f = goroutineReady
	t.arg = getg()
	golock(&timers.lock)
	addtimerLocked(t)
	goparkunlock(&timers.lock, "sleep")
}

// startTimer adds t to the timer heap.
func startTimer(t *timer) {
	if raceenabled {
		racerelease(unsafe.Pointer(t))
	}
	addtimer(t)
}

// stopTimer removes t from the timer heap if it is there.
// It returns true if t was removed, false if t wasn't even there.
func stopTimer(t *timer) bool {
	return deltimer(t)
}

// Go runtime.

// Ready the goroutine arg.
func goroutineReady(arg interface{}) {
	goready(arg.(*g))
}

func addtimer(t *timer) {
	golock(&timers.lock)
	addtimerLocked(t)
	gounlock(&timers.lock)
}

// Add a timer to the heap and start or kick the timer proc.
// If the new timer is earlier than any of the others.
// Timers are locked.
func addtimerLocked(t *timer) {
	// when must never be negative; otherwise timerproc will overflow
	// during its delta calculation and never expire other runtime·timers.
	if t.when < 0 {
		t.when = 1<<63 - 1
	}
	t.i = len(timers.t)
	timers.t = append(timers.t, t)
	siftupTimer(t.i)
	if t.i == 0 {
		// siftup moved to top: new earliest deadline.
		if timers.sleeping {
			timers.sleeping = false
			gonotewakeup(&timers.waitnote)
		}
		if timers.rescheduling {
			timers.rescheduling = false
			goready(timers.gp)
		}
	}
	if !timers.created {
		timers.created = true
		go timerproc()
	}
}

// Delete timer t from the heap.
// Do not need to update the timerproc: if it wakes up early, no big deal.
func deltimer(t *timer) bool {
	// Dereference t so that any panic happens before the lock is held.
	// Discard result, because t might be moving in the heap.
	_ = t.i

	golock(&timers.lock)
	// t may not be registered anymore and may have
	// a bogus i (typically 0, if generated by Go).
	// Verify it before proceeding.
	i := t.i
	last := len(timers.t) - 1
	if i < 0 || i > last || timers.t[i] != t {
		gounlock(&timers.lock)
		return false
	}
	if i != last {
		timers.t[i] = timers.t[last]
		timers.t[i].i = i
	}
	timers.t[last] = nil
	timers.t = timers.t[:last]
	if i != last {
		siftupTimer(i)
		siftdownTimer(i)
	}
	gounlock(&timers.lock)
	return true
}

// Timerproc runs the time-driven events.
// It sleeps until the next event in the timers heap.
// If addtimer inserts a new earlier event, addtimer1 wakes timerproc early.
func timerproc() {
	timers.gp = getg()
	timers.gp.issystem = 1
	for {
		golock(&timers.lock)
		timers.sleeping = false
		now := gonanotime()
		delta := int64(-1)
		for {
			if len(timers.t) == 0 {
				delta = -1
				break
			}
			t := timers.t[0]
			delta = t.when - now
			if delta > 0 {
				break
			}
			if t.period > 0 {
				// leave in heap but adjust next time to fire
				t.when += t.period * (1 + -delta/t.period)
				siftdownTimer(0)
			} else {
				// remove from heap
				last := len(timers.t) - 1
				if last > 0 {
					timers.t[0] = timers.t[last]
					timers.t[0].i = 0
				}
				timers.t[last] = nil
				timers.t = timers.t[:last]
				if last > 0 {
					siftdownTimer(0)
				}
				t.i = -1 // mark as removed
			}
			f := t.f
			arg := t.arg
			gounlock(&timers.lock)
			if raceenabled {
				raceacquire(unsafe.Pointer(t))
			}
			f(arg)
			golock(&timers.lock)
		}
		if delta < 0 {
			// No timers left - put goroutine to sleep.
			timers.rescheduling = true
			goparkunlock(&timers.lock, "timer goroutine (idle)")
			continue
		}
		// At least one timer pending.  Sleep until then.
		timers.sleeping = true
		gonoteclear(&timers.waitnote)
		gounlock(&timers.lock)
		gonotetsleepg(&timers.waitnote, delta)
	}
}

// Heap maintenance algorithms.

func siftupTimer(i int) {
	t := timers.t
	when := t[i].when
	tmp := t[i]
	for i > 0 {
		p := (i - 1) / 4 // parent
		if when >= t[p].when {
			break
		}
		t[i] = t[p]
		t[i].i = i
		t[p] = tmp
		t[p].i = p
		i = p
	}
}

func siftdownTimer(i int) {
	t := timers.t
	n := len(t)
	when := t[i].when
	tmp := t[i]
	for {
		c := i*4 + 1 // left child
		c3 := c + 2  // mid child
		if c >= n {
			break
		}
		w := t[c].when
		if c+1 < n && t[c+1].when < w {
			w = t[c+1].when
			c++
		}
		if c3 < n {
			w3 := t[c3].when
			if c3+1 < n && t[c3+1].when < w3 {
				w3 = t[c3+1].when
				c3++
			}
			if w3 < w {
				w = w3
				c = c3
			}
		}
		if w >= when {
			break
		}
		t[i] = t[c]
		t[i].i = i
		t[c] = tmp
		t[c].i = c
		i = c
	}
}