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# 10.1.1 环境变量 GOMAXPROCS

这个 `GOMAXPROCS` 环境变量（和Go 函数）允许您限制操作系统线程数，它能同时执行用户级 Go 代码。Go 1.5 版本开始，`GOMAXPROCS` 的默认值应该是您 Unix 机器的内核数。

如果您决定分配给 `GOMAXPROCS` 的值小于您的 Unix 机器的内核数的话，可能会影响您的程序的性能。然而，使用比可用内核大的 `GOMAXPROCS` 值也一定不会使您的程序运行的快。

您可用通过编程的方式来发现 `GOMAXPROCS` 环境变量的值，即相关代码。在下面的 `maxprocs.go` 程序中找到：

```go
package main

import (
    "fmt"
    "runtime"
)

func getGOMAXPROCS() int {
    return runtime.GOMAXPROCS(0)
}

func main() {
    fmt.Printf("GOMAXPROCS:%d\n", getGOMAXPROCS())
}
```

在一台 Intel i7 处理器的机器上执行 `maxprocs.go` 产生如下输出：

```
$go run maxprocs.go
GOMAXPROCS: 8
```

然而，您可以在程序执行前，通过改变 `GOMAXPROCS` 环境变量的值来修改上面的输出。在 `bash(1)` Unix shell 里执行下面的命令：

```
$go version
go version go1.9.4 darwin/amd64
$export GOMAXPROCS=800; go run maxprocs.go
GOMAXPROCS: 800
$export GOMAXPROCS=4; go run maxprocs.go
GOMAXPROCS: 4
```


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