#Timer
案例分析
func FailureCase() {
i := 0
go func() {
for {
i = i + 1
time.Sleep(time.Second)
if i > 5 {
break
}
}
}()
for {
exit := false
select {
// 避免使用for
case <-time.Tick(time.Millisecond):
if i > 5 {
exit = true
}
}
if exit {
break
}
}
}
问题:如果FailureCase被频繁调用?
结论:容器CPU使用率峰值翻倍(或者更高)而且居高不下!
[pprof分析](file:///Users/codoon/Documents/工作文档/pprof002.svg)
ANSIC = "Mon Jan _2 15:04:05 2006"
UnixDate = "Mon Jan _2 15:04:05 MST 2006"
RubyDate = "Mon Jan 02 15:04:05 -0700 2006"
RFC822 = "02 Jan 06 15:04 MST"
RFC822Z = "02 Jan 06 15:04 -0700" // RFC822 with numeric zone
RFC850 = "Monday, 02-Jan-06 15:04:05 MST"
RFC1123 = "Mon, 02 Jan 2006 15:04:05 MST"
RFC1123Z = "Mon, 02 Jan 2006 15:04:05 -0700" // RFC1123 with numeric zone
RFC3339 = "2006-01-02T15:04:05Z07:00"
RFC3339Nano = "2006-01-02T15:04:05.999999999Z07:00"
Kitchen = "3:04PM"
// Handy time stamps.
Stamp = "Jan _2 15:04:05"
StampMilli = "Jan _2 15:04:05.000"
StampMicro = "Jan _2 15:04:05.000000"
StampNano = "Jan _2 15:04:05.000000000"
stdDay // "2"
stdUnderDay // "_2"
stdZeroDay // "02"
stdHour = iota + stdNeedClock // "15"
stdHour12 // "3"
stdZeroHour12 // "03"
stdMinute // "4"
stdZeroMinute // "04"
stdSecond // "5"
stdZeroSecond // "05"
stdLongYear = iota + stdNeedDate // "2006"
stdYear // "06"
stdPM = iota + stdNeedClock // "PM"
“2006-01-02 15:04:05”(format.go: nextStdChunk)
系统时钟:
CLOCK_REALTIME(wall clock):当前时间(系统展示的时间,可同步,可修改)
CLOCK_MONOTONIC(monotonic time):单调时间,系统启动后每个计时器中断+1
type Time struct {
// wall and ext encode the wall time seconds, wall time nanoseconds,
// and optional monotonic clock reading in nanoseconds.
//
// From high to low bit position, wall encodes a 1-bit flag (hasMonotonic),
// a 33-bit seconds field, and a 30-bit wall time nanoseconds field.
// The nanoseconds field is in the range [0, 999999999].
// If the hasMonotonic bit is 0, then the 33-bit field must be zero
// and the full signed 64-bit wall seconds since Jan 1 year 1 is stored in ext.
// If the hasMonotonic bit is 1, then the 33-bit field holds a 33-bit
// unsigned wall seconds since Jan 1 year 1885, and ext holds a
// signed 64-bit monotonic clock reading, nanoseconds since process start.
wall uint64
// 从程序启动开始计算
ext int64
// loc specifies the Location that should be used to
// determine the minute, hour, month, day, and year
// that correspond to this Time.
// The nil location means UTC.
// All UTC times are represented with loc==nil, never loc==&utcLoc.
loc *Location
}
func Case1() {
// time.Sleep(time.Second / 2)
now := time.Now()
valueNow := reflect.ValueOf(now)
wall := valueNow.FieldByName("wall")
ext := valueNow.FieldByName("ext")
println(wall.Uint(), ext.Int())
}
Output:
--
13776715687332558152 565063
--
13776715794735670552 595682
// 加
func (t Time) Add(d Duration) Time
func (t Time) AddDate(years int, months int, days int) Time
// 减
func (t Time) Sub(u Time) Duration
// “余”(时区问题)
func (t Time) Truncate(d Duration) Time
// 等
func (t Time) Equal(u Time) bool
// 格式化
func (t Time) Format(layout string) string
func Parse(layout, value string) (Time, error)
func ParseInLocation(layout, value string, loc *Location) (Time, error)
// 序列化和反序列化方法
func (t Time) MarshalBinary() ([]byte, error)
func (t Time) MarshalJSON() ([]byte, error)
func (t Time) MarshalText() ([]byte, error)
func (t *Time) UnmarshalBinary(data []byte) error
func (t *Time) UnmarshalJSON(data []byte) error
func (t *Time) UnmarshalText(data []byte) error
func (t Time) String() string
###定时器(timer)
我们经常使用的是time包暴露出来的方法,但是在time包中仅包含一些对timer操作的封装,在runtime/time.go
包含绝大部分的底层实现;
Timer
type Timer struct {
// 时间通道
C <-chan Time
// timer更底层的封装,在runtime/time.go实现
r runtimeTimer
}
// 创建并启动(结束会向C中写入当前时间)
func NewTimer(d Duration) *Timer
// 重置计时(Stop --> 创建)
func (t *Timer) Reset(d Duration) bool
// 停止并从全局时间堆中移除
func (t *Timer) Stop() bool
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
}
golang中最基础的就是Timer,在Timer的基础上封装了After,Tick,Sleep等场景;
After
在给定时间d后触发,只想f函数或者默认函数;例如超时场景
func AfterFunc(d Duration, f func()) *Timer
func After(d Duration) <-chan Time
Tick
在给定时间d的间隔内,循环触发;只支持执行默认函数(写channel);比如限速场景
// Failure中的例子(慎用)
func Tick(d Duration) <-chan Time
// 创建Tick
func NewTicker(d Duration) *Ticker
// 关闭Tick,结束循环
func (t *Ticker) Stop()
func NewTicker(d Duration) *Ticker {
if d <= 0 {
panic(errors.New("non-positive interval for NewTicker"))
}
// Give the channel a 1-element time buffer.
// If the client falls behind while reading, we drop ticks
// on the floor until the client catches up.
c := make(chan Time, 1)
t := &Ticker{
C: c,
r: runtimeTimer{
when: when(d),
// 和NewTimer仅仅相差这一个字段
// 在timerproc会使用这个字段来判断底层timer是否进行reset并重新加入计时循环
period: int64(d),
f: sendTime,
arg: c,
},
}
startTimer(&t.r)
return t
}
这里可以发现在Tick的场景中,由于period被赋值,底层timer会一直生效,所以运行一段时间之后,全局的时间堆回存在大量的timer(timer泄漏),去check和维护这个时间堆,会占用大量的cpu资源;
最佳实践
var ticker = time.NewTicker(100 * time.Millisecond)
// 使用defer在函数退出时关闭timer
defer ticker.Stop()
var counter = 0
for {
select {
case <-serverDone:
return
case <-ticker.C:
counter += 1
}
}
Sleep
func Sleep(d Duration)
具体的实现在runtime/time.go中,使用了比较hack的方式 — go:linkname, 达到跨包访问;
//go:linkname timeSleep time.Sleep
所以简单来说,Sleep做的事情是,将当前goroutine置入waiting状态,再由定时器来唤醒;
// runtimeTimer 与 timer定义完全一致,在golang中可以直接强转
// runtime/time.go
type timer struct {
tb *timersBucket // the bucket the timer lives in
i int // heap index
// 触发时间(now + duration)
when int64
period int64
// 触发动作(默认为sendTime)
f func(interface{}, uintptr) // NOTE: must not be closure
// f的参数(默认为时间通道)
arg interface{}
// timer更新的时候会修改
seq uintptr
}
// 时间堆结构 -- 由单独的timerproc协程维护
type timersBucket struct {
lock mutex
// 当前timerproc执行goroutine
gp *g
// timer goroutine:是否已启动
created bool
// timer goroutine:是否Sleep
sleeping bool
// timer goroutine:是否暂停(len(t) == 0)
rescheduling bool
// timer goroutine:Sleep恢复时间
sleepUntil int64
// timer goroutine:挂起/唤醒 状态量
waitnote note
t []*timer
}
// 实际存储结构,加入填充字段
// runtime/time.go
// timersLen == 64
var timers [timersLen]struct {
timersBucket
// The padding should eliminate false sharing
// between timersBucket values.
//
pad [cpu.CacheLinePadSize - unsafe.Sizeof(timersBucket{})%cpu.CacheLinePadSize]byte
}
false sharing: CPU的缓存系统通常是以缓存行(cacheline,一般为64字节)来读取数据的,如果有两个进程的数据同时落到了一个cacheline, 一个进程的数据被修改了,整个cacheline需要重新加载,在高并发的场景中,这种相互之间的影响是不可忽略的;
而对于,定时器这样可能会频繁更新的数据结构,单独存在于一个或者多个cacheline是很有必要的;
时间堆结构(timersBucket)
全局timers
的长度为64,每个timer为独立的timersBucket
结构,每个timersBucket
独立维护一个timer
堆(以数组结构存储);
在每个timersBucket中,timer满足"四叉小顶堆"数据结构,元素按广度优先
的顺序存储在数组中;以及有如下特性:
1.父节点index = (当前节点index - 1)/ 4
2.每次调整之后,index为0的节点对应的timer为优先级最高的(when最小);
3.小顶堆的特性:层数越大的timer,when越大(index越大的timer,when一般越大),方便查找when值最小的timer;
调整算法(siftupTimer & siftdownTimer)
调整涉及到两种场景:新增和删除;
新增
— timerBucket分配
func addtimer(t *timer) {
tb := t.assignBucket()
lock(&tb.lock)
ok := tb.addtimerLocked(t)
unlock(&tb.lock)
if !ok {
badTimer()
}
}
func (t *timer) assignBucket() *timersBucket {
// 和M相关的
id := uint8(getg().m.p.ptr().id) % timersLen
t.tb = &timers[id].timersBucket
return t.tb
}
func (tb *timersBucket) addtimerLocked(t *timer) bool {
// 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(tb.t)
tb.t = append(tb.t, t)
if !siftupTimer(tb.t, t.i) {
return false
}
if t.i == 0 {
// 以下状态由timerproc更新
// siftup moved to top: new earliest deadline.
if tb.sleeping && tb.sleepUntil > t.when {
tb.sleeping = false
notewakeup(&tb.waitnote)
}
if tb.rescheduling {
tb.rescheduling = false
goready(tb.gp, 0)
}
if !tb.created {
// 初次使用timerBucket,触发对应timerproc
tb.created = true
go timerproc(tb)
}
}
return true
}
— siftupTimer
func siftupTimer(t []*timer, i int) bool {
if i >= len(t) {
return false
}
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
i = p
}
if tmp != t[i] {
t[i] = tmp
t[i].i = i
}
return true
}
删除
— 删除元素
将last元素调整到已删除的index位置,调整数组的长度,以达到修改删除元素的目的;
func (tb *timersBucket) deltimerLocked(t *timer) (removed, ok bool) {
// 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(tb.t) - 1
if i < 0 || i > last || tb.t[i] != t {
return false, true
}
if i != last {
tb.t[i] = tb.t[last]
tb.t[i].i = i
}
tb.t[last] = nil
tb.t = tb.t[:last]
ok = true
// i对应的是原堆中最后一个元素
if i != last {
if !siftupTimer(tb.t, i) {
ok = false
}
if !siftdownTimer(tb.t, i) {
ok = false
}
}
return true, ok
}
— siftupTimer & siftdownTimer
为什么需要siftupTimer?
i对应的是原堆的最后一个元素,因该是属于when最大的一个层次的timers;但是存在的情况是,删除的元素和last在同一层,交换值后不满足父子关系;
siftdownTimer
func siftdownTimer(t []*timer, i int) bool {
n := len(t)
if i >= n {
return false
}
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
i = c
}
if tmp != t[i] {
t[i] = tmp
t[i].i = i
}
return true
}
时间检查(timerproc)
每一个timerBucket会有一个单独的timer goroutine来维护,所以并不是每一个timer对应一个goroutine;
waiting:无timer,timer goroutine置入waiting状态,等待重新调度;
sleeping(挂起):有timer等待触发但不是现在,timer goroutine进入短暂挂起;
// 时间堆结构 -- 由单独的timerproc协程维护
type timersBucket struct {
lock mutex
// 当前timerproc执行goroutine
gp *g
// timer goroutine:是否已启动
created bool
// timer goroutine:是否挂起
sleeping bool
// timer goroutine:是否需要重新调度(goready)
rescheduling bool
// timer goroutine:挂起结束时间
sleepUntil int64
// timer goroutine:挂起/唤醒 状态量(默认note_cleared)
waitnote note
t []*timer
}
noteclear/ notetsleepg / notewakeup 底层原理和gopark/goready一致,只是notetsleepg支持定时唤醒;
参考:
https://github.com/cch123/golang-notes/blob/master/timer.md
Future: