timer也叫定时器，ticker是反复触发的定时器。实际上 timer和ticker 的代码已经基本不在time包里了，主要都都在golang的runtime包里。
在Go 在1.14版本之后，timer源码实现上有了巨大变更，移除了timersBucket，所有的timer都以最小四叉堆的形式存储 P 中（Golang GPM调度模型中的P）。（最小四叉堆参考之前的最小二叉堆的源码分析，完全一样的逻辑）
type p struct {
id int32
status uint32 // one of pidle/prunning/...
link puintptr
schedtick uint32 // incremented on every scheduler call
syscalltick uint32 // incremented on every system call
sysmontick sysmontick // last tick observed by sysmon
m muintptr // back-link to associated m (nil if idle)
mcache *mcache
pcache pageCache
raceprocctx uintptr
deferpool []*_defer // pool of available defer structs (see panic.go)
deferpoolbuf [32]*_defer
// Cache of goroutine ids, amortizes accesses to runtime·sched.goidgen.
goidcache uint64
goidcacheend uint64
// Queue of runnable goroutines. Accessed without lock.
runqhead uint32
runqtail uint32
runq [256]guintptr
// runnext, if non-nil, is a runnable G that was ready'd by
// the current G and should be run next instead of what's in
// runq if there's time remaining in the running G's time
// slice. It will inherit the time left in the current time
// slice. If a set of goroutines is locked in a
// communicate-and-wait pattern, this schedules that set as a
// unit and eliminates the (potentially large) scheduling
// latency that otherwise arises from adding the ready'd
// goroutines to the end of the run queue.
//
// Note that while other P's may atomically CAS this to zero,
// only the owner P can CAS it to a valid G.
runnext guintptr
// Available G's (status == Gdead)
gFree struct {
gList
n int32
}
sudogcache []*sudog
sudogbuf [128]*sudog
// Cache of mspan objects from the heap.
mspancache struct {
// We need an explicit length here because this field is used
// in allocation codepaths where write barriers are not allowed,
// and eliminating the write barrier/keeping it eliminated from
// slice updates is tricky, moreso than just managing the length
// ourselves.
len int
buf [128]*mspan
}
tracebuf traceBufPtr
// traceSweep indicates the sweep events should be traced.
// This is used to defer the sweep start event until a span
// has actually been swept.
traceSweep bool
// traceSwept and traceReclaimed track the number of bytes
// swept and reclaimed by sweeping in the current sweep loop.
traceSwept, traceReclaimed uintptr
palloc persistentAlloc // per-P to avoid mutex
_ uint32 // Alignment for atomic fields below
// The when field of the first entry on the timer heap.
// This is updated using atomic functions.
// This is 0 if the timer heap is empty.
timer0When uint64
// The earliest known nextwhen field of a timer with
// timerModifiedEarlier status. Because the timer may have been
// modified again, there need not be any timer with this value.
// This is updated using atomic functions.
// This is 0 if there are no timerModifiedEarlier timers.
timerModifiedEarliest uint64
// Per-P GC state
gcAssistTime int64 // Nanoseconds in assistAlloc
gcFractionalMarkTime int64 // Nanoseconds in fractional mark worker (atomic)
// limiterEvent tracks events for the GC CPU limiter.
limiterEvent limiterEvent
// gcMarkWorkerMode is the mode for the next mark worker to run in.
// That is, this is used to communicate with the worker goroutine
// selected for immediate execution by
// gcController.findRunnableGCWorker. When scheduling other goroutines,
// this field must be set to gcMarkWorkerNotWorker.
gcMarkWorkerMode gcMarkWorkerMode
// gcMarkWorkerStartTime is the nanotime() at which the most recent
// mark worker started.
gcMarkWorkerStartTime int64
// gcw is this P's GC work buffer cache. The work buffer is
// filled by write barriers, drained by mutator assists, and
// disposed on certain GC state transitions.
gcw gcWork
// wbBuf is this P's GC write barrier buffer.
//
// TODO: Consider caching this in the running G.
wbBuf wbBuf
runSafePointFn uint32 // if 1, run sched.safePointFn at next safe point
// statsSeq is a counter indicating whether this P is currently
// writing any stats. Its value is even when not, odd when it is.
statsSeq uint32
// Lock for timers. We normally access the timers while running
// on this P, but the scheduler can also do it from a different P.
timersLock mutex
// Actions to take at some time. This is used to implement the
// standard library's time package.
// Must hold timersLock to access.
timers []*timer
// Number of timers in P's heap.
// Modified using atomic instructions.
numTimers uint32
// Number of timerDeleted timers in P's heap.
// Modified using atomic instructions.
deletedTimers uint32
// Race context used while executing timer functions.
timerRaceCtx uintptr
// maxStackScanDelta accumulates the amount of stack space held by
// live goroutines (i.e. those eligible for stack scanning).
// Flushed to gcController.maxStackScan once maxStackScanSlack
// or -maxStackScanSlack is reached.
maxStackScanDelta int64
// gc-time statistics about current goroutines
// Note that this differs from maxStackScan in that this
// accumulates the actual stack observed to be used at GC time (hi - sp),
// not an instantaneous measure of the total stack size that might need
// to be scanned (hi - lo).
scannedStackSize uint64 // stack size of goroutines scanned by this P
scannedStacks uint64 // number of goroutines scanned by this P
// preempt is set to indicate that this P should be enter the
// scheduler ASAP (regardless of what G is running on it).
preempt bool
// Padding is no longer needed. False sharing is now not a worry because p is large enough
// that its size class is an integer multiple of the cache line size (for any of our architectures).
}超级复杂的p结构体，但本篇仅需要关注 p结构体中的 timers []*timer timer的最小四叉堆，timer0When uint64 timer最小四叉堆的第一个条目的 when 字段。
Timer结构体和其构造函数：
type Timer struct {
C <-chan Time
r runtimeTimer
}
func NewTimer(d Duration) *Timer {
c := make(chan Time, 1)
t := &Timer{
C: c,
r: runtimeTimer{
when: when(d),
f: sendTime,
arg: c,
},
}
startTimer(&t.r)
return t
}回调函数sendTime将当前时间放入管道：
func sendTime(c any, seq uintptr) {
select {
case c.(chan Time) <- Now():
default:
}
}startTimer方法映射的是runtime中的startTimer方法：
// startTimer adds t to the timer heap.
//
//go:linkname startTimer time.startTimer
func startTimer(t *timer) {
if raceenabled {
racerelease(unsafe.Pointer(t))
}
addtimer(t)
}Timer结构体包含一个runtimeTimer结构体，runtimeTimer结构体映射的是runtime包下的timer结构体。
// If this struct changes, adjust ../time/sleep.go:/runtimeTimer.
type timer struct {
// If this timer is on a heap, which P's heap it is on.
// puintptr rather than *p to match uintptr in the versions
// of this struct defined in other packages.
pp puintptr
// Timer wakes up at when, and then at when+period, ... (period > 0 only)
// each time calling f(arg, now) in the timer goroutine, so f must be
// a well-behaved function and not block.
//
// when must be positive on an active timer.
when int64
period int64
f func(any, uintptr)
arg any
seq uintptr
// What to set the when field to in timerModifiedXX status.
nextwhen int64
// The status field holds one of the values below.
status uint32
}addtimer方法做的事情：
- t.when 和 t.period 不可以是负数。
- 将定时器 的状态改成 timerWaiting。
- 调用cleantimers对定时器最小四叉堆做清理工作。
- 调用doaddtimer将定时器timer加入到最小四叉堆中。
- 调用 wakeNetPoller 唤醒 netpoller 中休眠的线程。
// addtimer adds a timer to the current P.
// This should only be called with a newly created timer.
// That avoids the risk of changing the when field of a timer in some P's heap,
// which could cause the heap to become unsorted.
func addtimer(t *timer) {
// when must be positive. A negative value will cause runtimer to
// overflow during its delta calculation and never expire other runtime
// timers. Zero will cause checkTimers to fail to notice the timer.
if t.when <= 0 {
throw("timer when must be positive")
}
if t.period < 0 {
throw("timer period must be non-negative")
}
if t.status != timerNoStatus {
throw("addtimer called with initialized timer")
}
t.status = timerWaiting
when := t.when
// Disable preemption while using pp to avoid changing another P's heap.
mp := acquirem()
pp := getg().m.p.ptr()
lock(&pp.timersLock)
cleantimers(pp)
doaddtimer(pp, t)
unlock(&pp.timersLock)
wakeNetPoller(when)
releasem(mp)
}cleantimers方法获取heap上的第一个item，检查其状态。
若是timerDeleted状态，将其置为timerRemoving状态，调用dodeltimer0方法pop heap首个item，并置为timerRemoved状态。
若是timerModifiedEarlier, timerModifiedLater状态，将其置为timerRemoving状态，改变when字段为nextwhen字段的值，调用dodeltimer0方法pop heap首个item再将其push到heap，并置为timerWaiting状态。
上面几种状态都调用到的dodeltimer0方法，就是从heap上pop出首个元素（或者说删除更准确），完成do首个定时器time0的操作。具体是将切片的首个元素赋值成末尾元素从而实现time0的删除，然后调用siftdownTimer方法对time0元素在heap最小四叉堆做下沉操作。调用updateTimer0When方法更新上面复杂的p结构体的timer0When字段的操作。heap的长度减1。
func cleantimers(pp *p) {
gp := getg()
for {
if len(pp.timers) == 0 {
return
}
// This loop can theoretically run for a while, and because
// it is holding timersLock it cannot be preempted.
// If someone is trying to preempt us, just return.
// We can clean the timers later.
if gp.preemptStop {
return
}
t := pp.timers[0]
if t.pp.ptr() != pp {
throw("cleantimers: bad p")
}
switch s := atomic.Load(&t.status); s {
case timerDeleted:
if !atomic.Cas(&t.status, s, timerRemoving) {
continue
}
dodeltimer0(pp)
if !atomic.Cas(&t.status, timerRemoving, timerRemoved) {
badTimer()
}
atomic.Xadd(&pp.deletedTimers, -1)
case timerModifiedEarlier, timerModifiedLater:
if !atomic.Cas(&t.status, s, timerMoving) {
continue
}
// Now we can change the when field.
t.when = t.nextwhen
// Move t to the right position.
dodeltimer0(pp)
doaddtimer(pp, t)
if !atomic.Cas(&t.status, timerMoving, timerWaiting) {
badTimer()
}
default:
// Head of timers does not need adjustment.
return
}
}
}
func dodeltimer0(pp *p) {
if t := pp.timers[0]; t.pp.ptr() != pp {
throw("dodeltimer0: wrong P")
} else {
t.pp = 0
}
last := len(pp.timers) - 1
if last > 0 {
pp.timers[0] = pp.timers[last]
}
pp.timers[last] = nil
pp.timers = pp.timers[:last]
if last > 0 {
siftdownTimer(pp.timers, 0)
}
updateTimer0When(pp)
n := atomic.Xadd(&pp.numTimers, -1)
if n == 0 {
// If there are no timers, then clearly none are modified.
atomic.Store64(&pp.timerModifiedEarliest, 0)
}
}
func updateTimer0When(pp *p) {
if len(pp.timers) == 0 {
atomic.Store64(&pp.timer0When, 0)
} else {
atomic.Store64(&pp.timer0When, uint64(pp.timers[0].when))
}
}doaddtimer方法将入参的timer t push到golang GMP调度的P对应的最小四叉堆上。具体设置timer的pp字段指定p，将timer先放进heap的末尾，再调用siftupTimer方法对末尾元素做上浮操作。
// doaddtimer adds t to the current P's heap.
// The caller must have locked the timers for pp.
func doaddtimer(pp *p, t *timer) {
// Timers rely on the network poller, so make sure the poller
// has started.
if netpollInited == 0 {
netpollGenericInit()
}
if t.pp != 0 {
throw("doaddtimer: P already set in timer")
}
t.pp.set(pp)
i := len(pp.timers)
pp.timers = append(pp.timers, t)
siftupTimer(pp.timers, i)
if t == pp.timers[0] {
atomic.Store64(&pp.timer0When, uint64(t.when))
}
atomic.Xadd(&pp.numTimers, 1)
}siftupTimer方法和siftdownTimer方法分别是对最小四叉堆的上浮操作和下沉操作，参考之前的golang标准库heap源码的分析，逻辑是一样的。
func siftupTimer(t []*timer, i int) int {
if i >= len(t) {
badTimer()
}
when := t[i].when
if when <= 0 {
badTimer()
}
tmp := t[i]
for i > 0 {
p := (i - 1) / 4 // parent
if when >= t[p].when {
break
}
t[i] = t[p]
i = p
}
if tmp != t[i] {
t[i] = tmp
}
return i
}
func siftdownTimer(t []*timer, i int) {
n := len(t)
if i >= n {
badTimer()
}
when := t[i].when
if when <= 0 {
badTimer()
}
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]
i = c
}
if tmp != t[i] {
t[i] = tmp
}
}timer 的触发 涉及golang的GMP，golang的调度，待以后再分析。netpoll 也待以后分析。
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