我有一个 go 程序如下。它启动 NumberOfCPUs-1 goroutines 并且在每个 goroutine 内部只更新全局变量 x。输出为 x = 0。

func main() {
    var x int
    threads := runtime.GOMAXPROCS(0)-1
    for i := 0; i < threads; i++ {
        go func() {
            for {
                x++
            }
        }()
    }
    time.Sleep(time.Second)
    fmt.Println("x =", x)
}

如果我稍微改变一下程序，像这样:

func main() {
    var x int
    threads := runtime.GOMAXPROCS(0)
    for i := 0; i < threads; i++ {
        go func() {
            for {
                x++
                time.Sleep(0)
            }
        }()
    }
    time.Sleep(time.Second)
    fmt.Println("x =", x)
}

x 将是一些随机的大值。

我认为这可能与 goroutine 调度器有关。在第一种情况下，goroutines 的数量小于 cpu cores 的数量，因此 main func 可以与所有现有的 goroutines 一起执行。因为在每个 goroutine 内部没有系统调用、I/O 或 channel 通信发生，goroutine 调度程序将无法工作。而且因为 goroutines 没有被中断，更新的 x 没有机会被写回。

虽然在第二种情况下，goroutines 的数量等于 cpu 核心的数量，为了让 main func 有机会运行，我放了一个 time.Sleep(0) 更新 x 之后。我认为每次 goroutine scheduler 切换 goroutines 时，更新的 x 都会被写回到它原来的内存位置。

谁能证实我的想法？有什么遗漏吗？

谢谢。

最佳答案

您有多个 goroutines 共享同一个变量 x 且读写不同步。你有数据竞争。因此，x 的结果未定义。使用选项 -race 运行竞争检测器。参见 Introducing the Go Race Detector .

package main

import (
    "fmt"
    "runtime"
    "time"
)

func main() {
    var x int
    threads := runtime.GOMAXPROCS(0) - 1
    for i := 0; i < threads; i++ {
        go func() {
            for {
                x++
            }
        }()
    }
    time.Sleep(time.Second)
    fmt.Println("x =", x)
}

输出:

$ go run -race race1.go
==================
WARNING: DATA RACE
Read at 0x00c420084010 by goroutine 7:
  main.main.func1()
      /home/peter/gopath/src/race1.go:15 +0x3b

Previous write at 0x00c420084010 by goroutine 6:
  main.main.func1()
      /home/peter/gopath/src/race1.go:15 +0x54

Goroutine 7 (running) created at:
  main.main()
      /home/peter/gopath/src/race1.go:13 +0xb6

Goroutine 6 (running) created at:
  main.main()
      /home/peter/gopath/src/race1.go:13 +0xb6
==================
x = 24717968
Found 1 data race(s)
exit status 66

package main

import (
    "fmt"
    "runtime"
    "time"
)

func main() {
    var x int
    threads := runtime.GOMAXPROCS(0)
    for i := 0; i < threads; i++ {
        go func() {
            for {
                x++
                time.Sleep(0)
            }
        }()
    }
    time.Sleep(time.Second)
    fmt.Println("x =", x)
}

输出:

$ go run -race race2.go
==================
WARNING: DATA RACE
Read at 0x00c4200140d0 by goroutine 7:
  main.main.func1()
      /home/peter/gopath/src/race2.go:15 +0x3b

Previous write at 0x00c4200140d0 by goroutine 6:
  main.main.func1()
      /home/peter/gopath/src/race2.go:15 +0x54

Goroutine 7 (running) created at:
  main.main()
      /home/peter/gopath/src/race2.go:13 +0xb3

Goroutine 6 (running) created at:
  main.main()
      /home/peter/gopath/src/race2.go:13 +0xb3
==================
==================
WARNING: DATA RACE
Read at 0x00c4200140d0 by goroutine 8:
  main.main.func1()
      /home/peter/gopath/src/race2.go:15 +0x3b

Previous write at 0x00c4200140d0 by goroutine 6:
  main.main.func1()
      /home/peter/gopath/src/race2.go:15 +0x54

Goroutine 8 (running) created at:
  main.main()
      /home/peter/gopath/src/race2.go:13 +0xb3

Goroutine 6 (running) created at:
  main.main()
      /home/peter/gopath/src/race2.go:13 +0xb3
==================
x = 14739962
Found 2 data race(s)
exit status 66

关于go - 在 goroutine 中更新全局变量的不同行为，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/45607048/