为什么 Goroutines 中的 IO 操作较慢？

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从 goroutine 下载图像后，我正在处理 io。

测试过程中出现问题。

在goroutine中下载图像后，我发现io操作非常慢的情况。

相反，它会在 goroutine 中下载图像 groutine 领域之外的 io 操作速度更快。

我可以知道为什么吗？

下面是测试源码

type imageresult struct {
    targetimagepath string
    success         bool
}

type imagedownloadresult struct {
    targetimagepath string
    response        *http.response
    success         bool
}

func main() {
    downloadurls := []string{
        "https://myhost.com/item/image/path/name_01.png",
        "https://myhost.com/item/image/path/name_02.png",
        "https://myhost.com/item/image/path/name_03.png",
        "https://myhost.com/item/image/path/name_05.png",
        "https://myhost.com/item/image/path/name_07.png",
        "https://myhost.com/item/image/path/name_08.png",
        "https://myhost.com/item/image/path/name_09.png",
        "https://myhost.com/item/image/path/name_10.png",
        "https://myhost.com/item/image/path/name_11.png",
        "https://myhost.com/item/image/path/name_12.png",
        "https://myhost.com/item/image/path/name_13.png",
        "https://myhost.com/item/image/path/name_14.png",
        "https://myhost.com/item/image/path/name_16.png",
    }

    startasyncio := time.now()
    resultchannel := make(chan imageresult)
    for _, downloadurl := range downloadurls {
        go httpfiledownloadandiowithasync(downloadurl, resultchannel)
    }

    for i := 0; i < len(downloadurls); i++ {
        <-resultchannel
    }
    fmt.println("total time async io: ", time.now().sub(startasyncio))

    fmt.println("=======================vs========================")

    startsyncio := time.now()
    resultchannel2 := make(chan imagedownloadresult)
    for _, downloadurl := range downloadurls {
        go httpfiledownloadwithasync(downloadurl, resultchannel2)

    }

    for i := 0; i < len(downloadurls); i++ {
        result := <-resultchannel2
        getbytesfromdownloaddata(result.response, result.targetimagepath)
    }
    fmt.println("total time sync io: ", time.now().sub(startsyncio))
}

func httpfiledownloadandiowithasync(downloadurl string, imageresult chan imageresult) {
    result := imageresult{
        targetimagepath: downloadurl,
    }

    client := http.client{
        checkredirect: func(r *http.request, via []*http.request) error {
            r.url.opaque = r.url.path
            return nil
        },
    }
    // put content on file
    downstart := time.now()
    resp, _ := client.get(downloadurl)
    downend := time.now()
    fmt.println(downloadurl, "file download time : ", downend.sub(downstart))

    defer resp.body.close()

    // file read
    iostart := time.now()
    var buf bytes.buffer
    io.copy(&buf, resp.body)
    ioend := time.now()
    fmt.println(downloadurl, "io time async : ", ioend.sub(iostart))

    result.success = true
    imageresult <- result

}

func httpfiledownloadwithasync(downloadurl string, imageresult chan imagedownloadresult) {
    result := imagedownloadresult{
        targetimagepath: downloadurl,
    }

    client := http.client{
        checkredirect: func(r *http.request, via []*http.request) error {
            r.url.opaque = r.url.path
            return nil
        },
    }
    // put content on file
    downstart := time.now()
    resp, _ := client.get(downloadurl)
    downend := time.now()
    fmt.println(downloadurl, "file download time : ", downend.sub(downstart))

    result.success = true
    result.response = resp
    imageresult <- result

}

func getbytesfromdownloaddata(resp *http.response, downloadurl string) []byte {
    defer func() {
        if err2 := resp.body.close(); err2 != nil {
            fmt.println("fail download by image download close error:", downloadurl)
        }
    }()
    // file read
    starttime := time.now()
    var buf bytes.buffer
    _, err := io.copy(&buf, resp.body)

    if err != nil {
        fmt.println("fail download by read response body:", downloadurl)
        return nil
    }
    fmt.println(downloadurl, "io time sync:", time.now().sub(starttime))
    return buf.bytes()
}

下面是日志。

https://myhost.com/item/image/path/name_13.png File Download Time :  197.058394ms
https://myhost.com/item/image/path/name_12.png File Download Time :  399.633804ms
https://myhost.com/item/image/path/name_08.png File Download Time :  587.309339ms
https://myhost.com/item/image/path/name_08.png IO Time Async :  314.482233ms
https://myhost.com/item/image/path/name_03.png File Download Time :  901.985524ms
https://myhost.com/item/image/path/name_05.png File Download Time :  1.132634351s
https://myhost.com/item/image/path/name_02.png File Download Time :  1.132661015s
https://myhost.com/item/image/path/name_14.png File Download Time :  1.132605289s
https://myhost.com/item/image/path/name_09.png File Download Time :  1.132608987s
https://myhost.com/item/image/path/name_16.png File Download Time :  1.133075291s
https://myhost.com/item/image/path/name_01.png File Download Time :  1.132837045s
https://myhost.com/item/image/path/name_11.png File Download Time :  1.133100234s
https://myhost.com/item/image/path/name_10.png File Download Time :  1.132982295s
https://myhost.com/item/image/path/name_07.png File Download Time :  1.133150493s
https://myhost.com/item/image/path/name_12.png IO Time Async :  1.240533838s
https://myhost.com/item/image/path/name_09.png IO Time Async :  849.335303ms
https://myhost.com/item/image/path/name_03.png IO Time Async :  1.080254194s
https://myhost.com/item/image/path/name_02.png IO Time Async :  849.395964ms
https://myhost.com/item/image/path/name_13.png IO Time Async :  1.784857595s
https://myhost.com/item/image/path/name_14.png IO Time Async :  849.642554ms
https://myhost.com/item/image/path/name_16.png IO Time Async :  849.494898ms
https://myhost.com/item/image/path/name_01.png IO Time Async :  850.297187ms
https://myhost.com/item/image/path/name_10.png IO Time Async :  864.482359ms
https://myhost.com/item/image/path/name_11.png IO Time Async :  864.524354ms
https://myhost.com/item/image/path/name_07.png IO Time Async :  874.676604ms
https://myhost.com/item/image/path/name_05.png IO Time Async :  875.22765ms
Total Time Async IO:  2.008162313s
=======================VS========================
https://myhost.com/item/image/path/name_09.png File Download Time :  72.476375ms
https://myhost.com/item/image/path/name_05.png File Download Time :  73.351299ms
https://myhost.com/item/image/path/name_07.png File Download Time :  92.839309ms
https://myhost.com/item/image/path/name_10.png File Download Time :  105.41514ms
https://myhost.com/item/image/path/name_08.png File Download Time :  136.861107ms
https://myhost.com/item/image/path/name_01.png File Download Time :  137.531384ms
https://myhost.com/item/image/path/name_16.png File Download Time :  204.833342ms
https://myhost.com/item/image/path/name_11.png File Download Time :  225.73164ms
https://myhost.com/item/image/path/name_03.png File Download Time :  238.569755ms
https://myhost.com/item/image/path/name_09.png IO Time Sync: 251.986344ms
https://myhost.com/item/image/path/name_14.png File Download Time :  473.071003ms
https://myhost.com/item/image/path/name_02.png File Download Time :  523.402477ms
https://myhost.com/item/image/path/name_13.png File Download Time :  523.389256ms
https://myhost.com/item/image/path/name_12.png File Download Time :  523.412647ms
https://myhost.com/item/image/path/name_05.png IO Time Sync: 549.364233ms
https://myhost.com/item/image/path/name_07.png IO Time Sync: 890.004µs
https://myhost.com/item/image/path/name_10.png IO Time Sync: 545.761µs
https://myhost.com/item/image/path/name_08.png IO Time Sync: 229.321µs
https://myhost.com/item/image/path/name_01.png IO Time Sync: 601.996µs
https://myhost.com/item/image/path/name_16.png IO Time Sync: 12.912227ms
https://myhost.com/item/image/path/name_11.png IO Time Sync: 148.432703ms
https://myhost.com/item/image/path/name_03.png IO Time Sync: 336.862µs
https://myhost.com/item/image/path/name_14.png IO Time Sync: 239.328µs
https://myhost.com/item/image/path/name_02.png IO Time Sync: 483.976µs
https://myhost.com/item/image/path/name_13.png IO Time Sync: 215.655µs
https://myhost.com/item/image/path/name_12.png IO Time Sync: 265.376µs
Total Time Sync IO:  1.039298797s

正确答案

Go 代码总是在 goroutine 中执行。

Goroutine 是平等的，不区分（除非运行 main() 函数的 main() goroutine 结束时，整个应用程序终止）。

Goroutines 被调度/复用到操作系统线程上，线程在物理或虚拟 CPU 核心上运行。因此，一个 Goroutine 是否比另一个 Goroutine 运行得更慢/更快取决于执行其指令的 CPU 核心的加载方式（系统方面）。最终执行一个 Goroutine 指令的 CPU 核心可能会比另一个 Goroutine 指令被更多地利用，从而导致感知到的 Goroutine 性能更差，但这并不是因为 Go 的运行时和 Goroutine 调度。

请注意，可以使用 runtime.LockOSThread() 将 Goroutine 锁定到操作系统线程，这意味着 Goroutine 将“拥有”该线程（不会将其他 Goroutine 调度到该线程上），但您不会在应用程序中使用它。

因此，您从其他 goroutines 遇到的下载速度较慢的情况与 Go 无关，它可能与您的操作系统和 CPU 负载或您调用的外部服务（HTTP 服务器）有关。

另请注意，再次运行相同的代码可能会花费更少的时间，初始化的代码可能会被重用，并且连接到同一主机也可能会明显更快：DNS 查找会被缓存，甚至 TCP 连接也可能会被缓存/池化。您调用的服务器也可能缓存某些数据，因此获取相同的 URL 也可能会明显更快（在后续调用中从同一服务器获取不同的资源也可能会更快，某些检查（例如身份验证/授权）可能会被缓存）。

查看相关：Order of the code and performance

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