Go單線程多goroutine訪問(wèn)一個(gè)map會(huì)遇到并發(fā)讀寫panic么？

一個(gè) Go 大佬群中嚴(yán)肅的討論了一個(gè)問(wèn)題：Go 程序單線程多 goroutine 訪問(wèn)一個(gè) map 會(huì)遇到并發(fā)讀寫 panic 么？

答案是肯定的，因?yàn)槌霈F(xiàn)了這個(gè)問(wèn)題所以大家才在群中討論。

為什么呢？因?yàn)閱尉€程意味著并行單元只有一個(gè)(多線程也可能并行單元只有一個(gè))，但是多 goroutine 意味著并發(fā)單元有多個(gè)，如果并發(fā)單元同時(shí)執(zhí)行，即使是單線程，可能就會(huì)產(chǎn)生數(shù)據(jù)競(jìng)爭(zhēng)的問(wèn)題，除非這些 goroutine 是順序執(zhí)行的。

舉一個(gè)例子哈：

func TestCounter() {
    runtime.GOMAXPROCS(1)
 var counter int
 var wg sync.WaitGroup
 wg.Add(10)
 for i := 0; i < 10; i++ {
  i := i
  go func() {
   fmt.Printf("start task#%d, counter: %d\n", i, counter)
   for j := 0; j < 10_0000; j++ {
    counter++
   }
   fmt.Printf("end task#%d, counter: %d\n", i, counter)
   wg.Done()
  }()
 }
 wg.Wait()
 fmt.Println(counter)
}

這段測(cè)試代碼是啟動(dòng) 10 個(gè) goroutine 對(duì)計(jì)數(shù)器加一，每個(gè) goroutine 負(fù)責(zé)加 10 萬(wàn)次。在我的 MBP m1 筆記本上，每次的結(jié)果都是 100 萬(wàn)，符合期望。如果你運(yùn)行這段代碼，會(huì)發(fā)現(xiàn) goroutine 其實(shí)是一個(gè)一個(gè)串行執(zhí)行的(9->0->1->2->3->4->5->6->7->8，當(dāng)然可能在你的機(jī)器上不是這樣的)，如果是串行執(zhí)行，不會(huì)有并發(fā)問(wèn)題：

start task#9, counter: 0
end task#9, counter: 100000
start task#0, counter: 100000
end task#0, counter: 200000
start task#1, counter: 200000
end task#1, counter: 300000
start task#2, counter: 300000
end task#2, counter: 400000
start task#3, counter: 400000
end task#3, counter: 500000
start task#4, counter: 500000
end task#4, counter: 600000
start task#5, counter: 600000
end task#5, counter: 700000
start task#6, counter: 700000
end task#6, counter: 800000
start task#7, counter: 800000
end task#7, counter: 900000
start task#8, counter: 900000
end task#8, counter: 1000000
1000000

插入runtime.Gosched()

為了制造點(diǎn)緊張氣氛，我將代碼改寫成下面這樣子，將counter++三條指令明顯寫成三條語(yǔ)句，并在中間插入runtime.Gosched(),故意給其它 goroutine 的執(zhí)行制造機(jī)會(huì)：

func TestCounter2() {
    runtime.GOMAXPROCS(1)
 var counter int
 var wg sync.WaitGroup
 wg.Add(10)
 for i := 0; i < 10; i++ {
  i := i
  go func() {
   fmt.Printf("start task#%d, counter: %d\n", i, counter)
   for j := 0; j < 10_0000; j++ {
    temp := counter
    runtime.Gosched()
    temp = temp + 1
    counter = temp
   }
   fmt.Printf("end task#%d, counter: %d\n", i, counter)
   wg.Done()
  }()
 }
 wg.Wait()
 fmt.Println(counter)
}

運(yùn)行這段代碼，你就會(huì)明顯看到數(shù)據(jù)不一致的效果，即使是單個(gè)線程運(yùn)行 goroutine,也出現(xiàn)了數(shù)據(jù)競(jìng)爭(zhēng)的問(wèn)題：

start task#9, counter: 0
start task#0, counter: 0
start task#1, counter: 0
start task#2, counter: 0
start task#3, counter: 0
start task#4, counter: 0
start task#5, counter: 0
start task#6, counter: 0
start task#7, counter: 0
start task#8, counter: 0
end task#9, counter: 100000
end task#1, counter: 100000
end task#3, counter: 100000
end task#2, counter: 100000
end task#5, counter: 100000
end task#0, counter: 100000
end task#4, counter: 100000
end task#6, counter: 100000
end task#7, counter: 100000
end task#8, counter: 100000
100000

這個(gè)結(jié)果非常離譜，期望 100 萬(wàn)，最后只有 10 萬(wàn)。

訪問(wèn)同一個(gè) map對(duì)象也有可能出現(xiàn)并發(fā)bug

因?yàn)閱蝹€(gè)線程運(yùn)行多個(gè) goroutine 會(huì)有數(shù)據(jù)競(jìng)爭(zhēng)的問(wèn)題，所以訪問(wèn)同一個(gè) map 對(duì)象也有可能出現(xiàn)并發(fā) bug,比如下面的代碼，10 個(gè) goroutine 并發(fā)的寫同一個(gè) map:

func TestMap() {
 var m = make(map[int]int)
 var wg sync.WaitGroup
 wg.Add(10)
 for i := 0; i < 10; i++ {
  i := i
  go func() {
   fmt.Printf("start map task#%d, m: %v\n", i, len(m))
   for j := 0; j < 10_0000; j++ {
    m[j] = i*10_0000 + j
   }
   fmt.Printf("end map task#%d, m: %v\n", i, len(m))
   wg.Done()
  }()
 }
 wg.Wait()
}

大概率會(huì)出現(xiàn) panic:

start map task#9, m: 0
start map task#0, m: 49152
fatal error: concurrent map writes
goroutine 41 [running]:
main.TestMap.func1()
 /Users/chaoyuepan/study/single_thread/main.go:72 +0xcc
created by main.TestMap in goroutine 1
 /Users/chaoyuepan/study/single_thread/main.go:69 +0x4c
goroutine 1 [semacquire]:
sync.runtime_Semacquire(0x140000021a0?)
 /usr/local/go/src/runtime/sema.go:62 +0x2c
sync.(*WaitGroup).Wait(0x1400000e1d0)
 /usr/local/go/src/sync/waitgroup.go:116 +0x74
main.TestMap()
 /Users/chaoyuepan/study/single_thread/main.go:79 +0xb8
main.main()
 /Users/chaoyuepan/study/single_thread/main.go:15 +0x2c

以上就是Go語(yǔ)言單線程運(yùn)行的并發(fā)問(wèn)題解析的詳細(xì)內(nèi)容，更多關(guān)于Go單線程運(yùn)行并發(fā)問(wèn)題的資料請(qǐng)關(guān)注腳本之家其它相關(guān)文章！

最新評(píng)論