原文地址：
汗，写完这篇就发现 Go 目前的 master 分支上 select 的实现有所修改，比如文中的 hselect 结构体已经消失了。之后还是抽时间分析分析新版。。
select 本身是 Go 提供的一个语法糖，每次你写
select {
}
的时候，实际上是相当于调用了一大堆函数。。只是 Go 的 runtime 内部帮你把这些复杂性屏蔽掉了。但是屏蔽也是有代价的，因为现在为止(1.10)，每次 select 都会执行 newselect 来分配一个 hselect，并且每次执行到 case ... 都相当于是又重新初始化了新的 scase 结构体。所以如果你在 for 循环里执行 select，就相当于是在不停地 new new new。
不过一般情况下也不需要来扣这一点性能。不知道新版是不是针对这些有所优化。等看完新版再来一篇总结。
老版本(<=1.10)的分析如下。
Select
select 的实现在目前 Go 的 master 分支已经有所修改，所以本文只针对 1.10 之前的版本。
基本用法
非阻塞 select:
select {
case <-ch:
default:
}
当 ch 中无数据时，不会阻塞，会直接走 default 分支。这种特性可以用来实现 trylock。
对 nil 进行发送或者接收都会永远阻塞，被包在 select 里也不例外:
var a chan int
select {
case <-ch:
}
println("can not be printed")
for select 组合起来用很常见，不过需要注意，下面的两种场景可能会造成问题:
for {
select {
case d := <-ch:
default:
}
}
这种写法比较危险，如果 ch 中没有数据就会一直死循环 cpu 爆炸。
for {
select {
case d := <-ch:
}
}
你以为这么写就没事了？啊当然，一般情况下还好。但如果 ch 被其它 goroutine close 掉了，那么 d:= <-ch 这种形式就是永远不阻塞，并且会一直返回零值了。如果不想关注这种情况，并且 select 中实际只操作一个 channel，建议写成 for range 形式:
for d := range ch {
// do some happy things with d
}
这样 ch 关闭的时候 for 循环会自动退出。
如果 select 中需要监听多个 channel，并且这些 channel 可能被关闭，那需要老老实实地写双返回值的 channel 取值表达式:
outer:
for {
select {
case d, ok := <-ch1:
if !ok {
break outer
}
case d, ok := <-ch2:
if !ok {
break outer
}
}
}
当然，如果你不确定，可以用下面的 demo 进行验证:
package main
import "time"
func main() {
var ch1 chan int
var ch2 = make(chan int)
close(ch2)
go func() {
for {
select {
case d := <-ch1:
println("ch1", d)
case d := <-ch2:
println("ch2", d)
}
}
}()
time.Sleep(time.Hour)
}
尽管 ch2 已经被关闭，依然会不断地进入 case d:= <-ch2 中。因此在使用 for select 做设计时，请务必考虑当监听的 channel 在外部被正常或意外关闭后会有什么样的后果。
源码分析
数据结构
const (
// scase.kind
// send 或者 recv 发生在一个 nil channel 上，就有可能出现这种情况
caseNil = iota
caseRecv
caseSend
caseDefault
)
// Select statement header.
// Known to compiler.
// Changes here must also be made in src/cmd/internal/gc/select.go's selecttype.
// select 的本体数据结构
type hselect struct {
// 总 case 数
tcase uint16 // total count of scase[]
// 当前已初始化填充的 case
ncase uint16 // currently filled scase[]
// 轮询顺序
pollorder *uint16 // case poll order
// case 的 channel 的加锁顺序
lockorder *uint16 // channel lock order
// case 数组，为了节省一个指针的 8 个字节搞成这样的结构
// 实际上要访问后面的值，还是需要进行指针移动
// 指针移动使用 runtime 内部的 add 函数
scase [1]scase // one per case (in order of appearance)
}
// Select case descriptor.
// Known to compiler.
// Changes here must also be made in src/cmd/internal/gc/select.go's selecttype.
// select 中每一个 case 的数据结构定义
type scase struct {
// 数据元素
elem unsafe.Pointer // data element
// channel 本体
c *hchan // chan
// 这个是指 ip 寄存器么？
pc uintptr // return pc (for race detector / msan)
// 应该是 reflect.Kind
kind uint16
// _,ok:= <- ch 里的 ok 吧
receivedp *bool // pointer to received bool, if any
// TODO 这个是干啥的
releasetime int64
}
var (
chansendpc = funcPC(chansend)
chanrecvpc = funcPC(chanrecv)
)
func selectsize(size uintptr) uintptr {
selsize := unsafe.Sizeof(hselect{}) +
(size-1)*unsafe.Sizeof(hselect{}.scase[0]) +
size*unsafe.Sizeof(*hselect{}.lockorder) +
size*unsafe.Sizeof(*hselect{}.pollorder)
return round(selsize, sys.Int64Align)
}
select 初始化
// 创建一个 select 结构体
// 每次写出：
// select { => 在这行会调用 newselect
// 可以使用 go tool compile -S 来查看
// ...
// }
func newselect(sel *hselect, selsize int64, size int32) {
if selsize != int64(selectsize(uintptr(size))) {
print("runtime: bad select size ", selsize, ", want ", selectsize(uintptr(size)), "\n")
throw("bad select size")
}
sel.tcase = uint16(size)
// 初始已填充的 case 数自然是 0
sel.ncase = 0
// hselect 这个结构体的是长下面这样：
// header |scase[0]|scase[1]|scase[2]|lockorder[0]|lockorder[1]|lockorder[2]|pollorder[0]|pollorder[1]|pollorder[2]
// 各个 channel 加锁的顺序
sel.lockorder = (*uint16)(add(unsafe.Pointer(&sel.scase), uintptr(size)*unsafe.Sizeof(hselect{}.scase[0])))
// case 轮询的顺序初始化
sel.pollorder = (*uint16)(add(unsafe.Pointer(sel.lockorder), uintptr(size)*unsafe.Sizeof(*hselect{}.lockorder)))
}
select send
// select {
// case ch<-1: ==> 这时候就会调用 selectsend
// }
func selectsend(sel *hselect, c *hchan, elem unsafe.Pointer) {
pc := getcallerpc()
i := sel.ncase
if i >= sel.tcase {
throw("selectsend: too many cases")
}
sel.ncase = i + 1
if c == nil {
return
}
// 初始化对应的 scase 结构
cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))
cas.pc = pc
cas.c = c
cas.kind = caseSend
cas.elem = elem
}
// compiler implements
//
// select {
// case c <- v:
// ... foo
// default:
// ... bar
// }
//
// as
//
// if selectnbsend(c, v) {
// ... foo
// } else {
// ... bar
// }
//
func selectnbsend(c *hchan, elem unsafe.Pointer) (selected bool) {
return chansend(c, elem, false, getcallerpc())
}
select receive
// select {
// case <-ch: ==> 这时候就会调用 selectrecv
// case ,ok <- ch: 也可以这样写
// 在 ch 被关闭时，这个 case 每次都可能被轮询到
// }
func selectrecv(sel *hselect, c *hchan, elem unsafe.Pointer, received *bool) {
pc := getcallerpc()
i := sel.ncase
if i >= sel.tcase {
throw("selectrecv: too many cases")
}
sel.ncase = i + 1
if c == nil {
return
}
// 初始化对应的 scase 结构
cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))
cas.pc = pc
cas.c = c
cas.kind = caseRecv
cas.elem = elem
// _, ok => 这里的 ok
cas.receivedp = received
}
// compiler implements
//
// select {
// case v = <-c:
// ... foo
// default:
// ... bar
// }
//
// as
//
// if selectnbrecv(&v, c) {
// ... foo
// } else {
// ... bar
// }
//
func selectnbrecv(elem unsafe.Pointer, c *hchan) (selected bool) {
selected, _ = chanrecv(c, elem, false)
return
}
// compiler implements
//
// select {
// case v, ok = <-c:
// ... foo
// default:
// ... bar
// }
//
// as
//
// if c != nil && selectnbrecv2(&v, &ok, c) {
// ... foo
// } else {
// ... bar
// }
//
func selectnbrecv2(elem unsafe.Pointer, received *bool, c *hchan) (selected bool) {
// TODO(khr): just return 2 values from this function, now that it is in Go.
selected, *received = chanrecv(c, elem, false)
return
}
select default
// select {
// default: ==> 这时候就会调用 selectdefault
// }
func selectdefault(sel *hselect) {
pc := getcallerpc()
i := sel.ncase
if i >= sel.tcase {
throw("selectdefault: too many cases")
}
sel.ncase = i + 1
// 初始化对应的 scase 结构
cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))
cas.pc = pc
cas.c = nil
cas.kind = caseDefault
}
select 执行
加解锁:
// 对所有 case 对应的 channel 加锁
// 需要按照 lockorder 数组中的元素索引来搞
// 否则可能有循环等待的死锁
func sellock(scases []scase, lockorder []uint16) {
var c *hchan
for _, o := range lockorder {
c0 := scases[o].c
if c0 != nil && c0 != c {
c = c0
lock(&c.lock)
}
}
}
// 解锁，比较简单
func selunlock(scases []scase, lockorder []uint16) {
// We must be very careful here to not touch sel after we have unlocked
// the last lock, because sel can be freed right after the last unlock.
// Consider the following situation.
// First M calls runtime·park() in runtime·selectgo() passing the sel.
// Once runtime·park() has unlocked the last lock, another M makes
// the G that calls select runnable again and schedules it for execution.
// When the G runs on another M, it locks all the locks and frees sel.
// Now if the first M touches sel, it will access freed memory.
for i := len(scases) - 1; i >= 0; i-- {
c := scases[lockorder[i]].c
if c == nil {
break
}
if i > 0 && c == scases[lockorder[i-1]].c {
continue // will unlock it on the next iteration
}
unlock(&c.lock)
}
}
func selparkcommit(gp *g, _ unsafe.Pointer) bool {
// This must not access gp's stack (see gopark). In
// particular, it must not access the *hselect. That's okay,
// because by the time this is called, gp.waiting has all
// channels in lock order.
var lastc *hchan
for sg := gp.waiting; sg != nil; sg = sg.waitlink {
if sg.c != lastc && lastc != nil {
// As soon as we unlock the channel, fields in
// any sudog with that channel may change,
// including c and waitlink. Since multiple
// sudogs may have the same channel, we unlock
// only after we've passed the last instance
// of a channel.
unlock(&lastc.lock)
}
lastc = sg.c
}
if lastc != nil {
unlock(&lastc.lock)
}
return true
}
// 没有 case 直接 block，这种情况下让该 goroutine 挂起进休眠
func block() {
gopark(nil, nil, "select (no cases)", traceEvGoStop, 1) // forever
}
select 执行:
// selectgo implements the select statement.
//
// *sel is on the current goroutine's stack (regardless of any
// escaping in selectgo).
//
// selectgo returns the index of the chosen scase, which matches the
// ordinal position of its respective select{recv,send,default} call.
// selectgo 是在初始化完成之后执行 select 逻辑的函数
// 返回值是要执行的 scase 的 index
func selectgo(sel *hselect) int {
if sel.ncase != sel.tcase {
throw("selectgo: case count mismatch")
}
// len 和 cap 就是 scase 的总数
// 这时候 ncase 和 tcase 已经是相等的了
scaseslice := slice{unsafe.Pointer(&sel.scase), int(sel.ncase), int(sel.ncase)}
scases := *(*[]scase)(unsafe.Pointer(&scaseslice))
var t0 int64
if blockprofilerate > 0 {
t0 = cputicks()
for i := 0; i < int(sel.ncase); i++ {
scases[i].releasetime = -1
}
}
// The compiler rewrites selects that statically have
// only 0 or 1 cases plus default into simpler constructs.
// The only way we can end up with such small sel.ncase
// values here is for a larger select in which most channels
// have been nilled out. The general code handles those
// cases correctly, and they are rare enough not to bother
// optimizing (and needing to test).
// generate permuted order
pollslice := slice{unsafe.Pointer(sel.pollorder), int(sel.ncase), int(sel.ncase)}
pollorder := *(*[]uint16)(unsafe.Pointer(&pollslice))
// 这个洗牌有点挫。。。。
for i := 1; i < int(sel.ncase); i++ {
j := fastrandn(uint32(i + 1))
// 这时候 pollorder[i] 其实就是 i
// 这么写少了一次交换
pollorder[i] = pollorder[j]
pollorder[j] = uint16(i)
}
// sort the cases by Hchan address to get the locking order.
// simple heap sort, to guarantee n log n time and constant stack footprint.
// 按 hchan 的地址来进行排序，以生成加锁顺序
// 用堆排序来保证 nLog(n) 的时间复杂度
lockslice := slice{unsafe.Pointer(sel.lockorder), int(sel.ncase), int(sel.ncase)}
lockorder := *(*[]uint16)(unsafe.Pointer(&lockslice))
for i := 0; i < int(sel.ncase); i++ {
// 初始化 lockorder 数组
j := i
// Start with the pollorder to permute cases on the same channel.
c := scases[pollorder[i]].c
for j > 0 && scases[lockorder[(j-1)/2]].c.sortkey() < c.sortkey() {
k := (j - 1) / 2
lockorder[j] = lockorder[k]
j = k
}
lockorder[j] = pollorder[i]
}
for i := int(sel.ncase) - 1; i >= 0; i-- {
// 堆排序
o := lockorder[i]
c := scases[o].c
lockorder[i] = lockorder[0]
j := 0
for {
k := j*2 + 1
if k >= i {
break
}
if k+1 < i && scases[lockorder[k]].c.sortkey() < scases[lockorder[k+1]].c.sortkey() {
k++
}
if c.sortkey() < scases[lockorder[k]].c.sortkey() {
lockorder[j] = lockorder[k]
j = k
continue
}
break
}
lockorder[j] = o
}
/*
for i := 0; i+1 < int(sel.ncase); i++ {
if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() {
print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
throw("select: broken sort")
}
}
*/
// lock all the channels involved in the select
// 对涉及到的所有 channel 都加锁
sellock(scases, lockorder)
var (
gp *g
sg *sudog
c *hchan
k *scase
sglist *sudog
sgnext *sudog
qp unsafe.Pointer
nextp **sudog
)
loop:
// pass 1 - look for something already waiting
var dfli int
var dfl *scase
var casi int
var cas *scase
// 虽然看着是一个 for 循环
// 但实际上有 case ready 的时候
// 直接就用 goto 跳出了
for i := 0; i < int(sel.ncase); i++ {
// 按 pollorder 的顺序进行遍历
casi = int(pollorder[i])
cas = &scases[casi]
c = cas.c
switch cas.kind {
case caseNil:
// 这个 case 要怎么触发？
continue
case caseRecv:
// <- ch 的情况
// 根据有没有等待的 goroutine 队列执行不同的操作
sg = c.sendq.dequeue()
if sg != nil {
goto recv
}
if c.qcount > 0 {
goto bufrecv
}
if c.closed != 0 {
goto rclose
}
case caseSend:
// ch <- 1 的情况，也是一些基本的 channel 操作
if c.closed != 0 {
goto sclose
}
sg = c.recvq.dequeue()
if sg != nil {
goto send
}
if c.qcount < c.dataqsiz {
goto bufsend
}
case caseDefault:
dfli = casi
dfl = cas
}
}
// 这里是在前面进了 caseDefault 才会走到
// 感觉这代码写的也挺挫的
if dfl != nil {
// 之间是进了 default，才会进这个 if
selunlock(scases, lockorder)
casi = dfli
cas = dfl
goto retc
}
// pass 2 - enqueue on all chans
// 没有任何一个 case 满足，且没有 default
// 这种情况下需要把当前的 goroutine 入队所有 channel 的等待队列里
gp = getg()
if gp.waiting != nil {
throw("gp.waiting != nil")
}
nextp = &gp.waiting
// 按照加锁的顺序把 gorutine 入每一个 channel 的等待队列
for _, casei := range lockorder {
casi = int(casei)
cas = &scases[casi]
// 空 channel
if cas.kind == caseNil {
continue
}
c = cas.c
sg := acquireSudog()
sg.g = gp
sg.isSelect = true
// No stack splits between assigning elem and enqueuing
// sg on gp.waiting where copystack can find it.
sg.elem = cas.elem
sg.releasetime = 0
if t0 != 0 {
sg.releasetime = -1
}
sg.c = c
// Construct waiting list in lock order.
*nextp = sg
nextp = &sg.waitlink
switch cas.kind {
case caseRecv:
// recv 的情况进 recvq
c.recvq.enqueue(sg)
case caseSend:
// send 的情况进 sendq
c.sendq.enqueue(sg)
}
}
// wait for someone to wake us up
gp.param = nil
// 当前 goroutine 进入休眠，等待被唤醒
gopark(selparkcommit, nil, "select", traceEvGoBlockSelect, 1)
sellock(scases, lockorder)
gp.selectDone = 0
sg = (*sudog)(gp.param)
gp.param = nil
// pass 3 - dequeue from unsuccessful chans
// otherwise they stack up on quiet channels
// record the successful case, if any.
// We singly-linked up the SudoGs in lock order.
// 被唤醒后执行下面的代码
casi = -1
cas = nil
sglist = gp.waiting
// Clear all elem before unlinking from gp.waiting.
for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
sg1.isSelect = false
sg1.elem = nil
sg1.c = nil
}
gp.waiting = nil
for _, casei := range lockorder {
k = &scases[casei]
if k.kind == caseNil {
continue
}
if sglist.releasetime > 0 {
k.releasetime = sglist.releasetime
}
if sg == sglist {
// sg has already been dequeued by the G that woke us up.
casi = int(casei)
cas = k
} else {
c = k.c
if k.kind == caseSend {
c.sendq.dequeueSudoG(sglist)
} else {
c.recvq.dequeueSudoG(sglist)
}
}
sgnext = sglist.waitlink
sglist.waitlink = nil
releaseSudog(sglist)
sglist = sgnext
}
if cas == nil {
// We can wake up with gp.param == nil (so cas == nil)
// when a channel involved in the select has been closed.
// It is easiest to loop and re-run the operation;
// we'll see that it's now closed.
// Maybe some day we can signal the close explicitly,
// but we'd have to distinguish close-on-reader from close-on-writer.
// It's easiest not to duplicate the code and just recheck above.
// We know that something closed, and things never un-close,
// so we won't block again.
goto loop
}
c = cas.c
if cas.kind == caseRecv && cas.receivedp != nil {
*cas.receivedp = true
}
if msanenabled {
if cas.kind == caseRecv && cas.elem != nil {
msanwrite(cas.elem, c.elemtype.size)
} else if cas.kind == caseSend {
msanread(cas.elem, c.elemtype.size)
}
}
selunlock(scases, lockorder)
goto retc
bufrecv:
// can receive from buffer
if msanenabled && cas.elem != nil {
msanwrite(cas.elem, c.elemtype.size)
}
if cas.receivedp != nil {
*cas.receivedp = true
}
qp = chanbuf(c, c.recvx)
if cas.elem != nil {
typedmemmove(c.elemtype, cas.elem, qp)
}
typedmemclr(c.elemtype, qp)
c.recvx++
if c.recvx == c.dataqsiz {
c.recvx = 0
}
c.qcount--
selunlock(scases, lockorder)
goto retc
bufsend:
// can send to buffer
if msanenabled {
msanread(cas.elem, c.elemtype.size)
}
typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem)
c.sendx++
if c.sendx == c.dataqsiz {
c.sendx = 0
}
c.qcount++
selunlock(scases, lockorder)
goto retc
recv:
// can receive from sleeping sender (sg)
recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
if cas.receivedp != nil {
*cas.receivedp = true
}
goto retc
rclose:
// read at end of closed channel
selunlock(scases, lockorder)
if cas.receivedp != nil {
*cas.receivedp = false
}
if cas.elem != nil {
typedmemclr(c.elemtype, cas.elem)
}
goto retc
send:
// can send to a sleeping receiver (sg)
if msanenabled {
msanread(cas.elem, c.elemtype.size)
}
send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
goto retc
retc:
if cas.releasetime > 0 {
blockevent(cas.releasetime-t0, 1)
}
return casi
sclose:
// send on closed channel
// 向关闭 channel 发送数据
// 直接 panic
selunlock(scases, lockorder)
panic(plainError("send on closed channel"))
}
