Question

我需要读取从另一个goroutine设置的结构字段，afaik直接这样做，即使确定会有没有并发访问权限（写入在读取之前完成，通过{{1发出信号） }}）可能导致陈旧数据

将指针发送到结构（在第一个goroutine中创建，在第二个中修改，由第三个读取）将解决可能的过期问题，考虑到我可以保证没有并发访问？

我想避免复制，因为结构很大并且包含巨大的Bytes.Buffer填充在第二个goroutine中，我需要从第3个读取

有一个锁定选项，但考虑到我知道不会有并发访问，这似乎是一种矫枉过正

Answer 1

为了防止在保留阅读能力的同时修改结构，您通常会嵌入sync.RWMutex。这不是免税。您可以在传输过程中简单lock your struct for writes并在方便的时候解锁它。

package main

import (
    "fmt"
    "sync"
    "time"
)

// Big simulates your big struct
type Big struct {
    sync.RWMutex
    value string
}

// pump uses a groutine to take the slice of pointers to Big,
// locks the underlying structs and sends the pointers to
// the locked instances of Big downstream
func pump(bigs []*Big) chan *Big {

    // We make the channel buffered for this example
    // for illustration purposes
    c := make(chan *Big, 3)

    go func() {
        for _, big := range bigs {
            // We lock the struct before sending it to the channel
            // so it can not be changed via pointer while in transit
            big.Lock()
            c <- big
        }
        close(c)
    }()

    return c
}

// sink reads pointers to the locked instances of Big
// reads them and unlocks them
func sink(c chan *Big) {

    for big := range c {
        fmt.Println(big.value)
        time.Sleep(1 * time.Second)
        big.Unlock()

    }
}

// modify tries to achieve locks to the instances and modify them
func modify(bigs []*Big) {
    for _, big := range bigs {

        big.Lock()
        big.value = "modified"
        big.Unlock()
    }
}

func main() {

    bigs := []*Big{&Big{value: "Foo"}, &Big{value: "Bar"}, &Big{value: "Baz"}}
    c := pump(bigs)

    // For the sake of this example, we wait until all entries are
    // send into the channel and hence are locked
    time.Sleep(1 * time.Second)

    // Now we try to modify concurrently before we even start to read
    // the struct of which the pointers were sent into the channel
    go modify(bigs)
    sink(c)

    // We use sleep here to keep waiting for modify() to finish simple.
    // Usually, you'd use a sync.waitGroup
    time.Sleep(1 * time.Second)

    for _, big := range bigs {
        fmt.Println(big.value)
    }

}

Run on playground

Answer 2

这有很多答案，这取决于你的数据结构和程序逻辑。

见：How to lock/synchronize access to a variable in Go during concurrent goroutines?
并且：How to use RWMutex in Golang?

1-使用Stateful Goroutines和频道
2-使用sync.Mutex
3-使用同步/原子
4-使用WaitGroup
5-使用程序逻辑（Semaphore）
...

1：Stateful Goroutines和频道：
我模拟了非常相似的样本（想象你想从一个SSD读取并写入另一个速度不同的SSD）：
在这个示例代码中，一个goroutine（名为write）做一些工作准备数据并填充大结构，另一个goroutine（名为read）从big struct读取数据然后做一些工作，而manger goroutine，保证不会同时访问相同的数据。与渠道完成的三个goroutines之间的沟通。在您的情况下，您可以使用指针来显示频道数据，或者像这个样本一样使用全局结构输出将是这样的：
mean = 36.6920166015625 stdev = 6.068973186592054

我希望这可以帮助你理解。
工作示例代码：

package main

import (
    "fmt"
    "math"
    "math/rand"
    "runtime"
    "sync"
    "time"
)

type BigStruct struct {
    big     []uint16
    rpos    int
    wpos    int
    full    bool
    empty   bool
    stopped bool
}

func main() {
    wg.Add(1)
    go write()
    go read()
    go manage()
    runtime.Gosched()
    stopCh <- <-time.After(5 * time.Second)
    wg.Wait()
    mean := Mean(hist)
    stdev := stdDev(hist, mean)
    fmt.Println("mean=", mean, "stdev=", stdev)
}

const N = 1024 * 1024 * 1024

var wg sync.WaitGroup
var stopCh chan time.Time = make(chan time.Time)

var hist []int = make([]int, 65536)

var s *BigStruct = &BigStruct{empty: true,
    big: make([]uint16, N), //2GB
}

var rc chan uint16 = make(chan uint16)
var wc chan uint16 = make(chan uint16)

func next(pos int) int {
    pos++
    if pos >= N {
        pos = 0
    }
    return pos
}

func manage() {
    dataReady := false
    var data uint16
    for {
        if !dataReady && !s.empty {
            dataReady = true
            data = s.big[s.rpos]
            s.rpos++
            if s.rpos >= N {
                s.rpos = 0
            }
            s.empty = s.rpos == s.wpos
            s.full = next(s.wpos) == s.rpos
        }
        if dataReady {
            select {
            case rc <- data:
                dataReady = false
            default:
                runtime.Gosched()
            }
        }
        if !s.full {
            select {
            case d := <-wc:
                s.big[s.wpos] = d
                s.wpos++
                if s.wpos >= N {
                    s.wpos = 0
                }
                s.empty = s.rpos == s.wpos
                s.full = next(s.wpos) == s.rpos
            default:
                runtime.Gosched()
            }
        }
        if s.stopped {
            if s.empty {
                wg.Done()
                return
            }
        }

    }
}

func read() {
    for {
        d := <-rc
        hist[d]++
    }
}

func write() {
    for {
        wc <- uint16(rand.Intn(65536))
        select {
        case <-stopCh:
            s.stopped = true
            return
        default:
            runtime.Gosched()
        }
    }
}

func stdDev(data []int, mean float64) float64 {
    sum := 0.0
    for _, d := range data {
        sum += math.Pow(float64(d)-mean, 2)
    }
    variance := sum / float64(len(data)-1)
    return math.Sqrt(variance)
}
func Mean(data []int) float64 {
    sum := 0.0
    for _, d := range data {
        sum += float64(d)
    }
    return sum / float64(len(data))
}

5：某些用例的另一种方式（更快）：
这里是另一种使用共享数据结构进行读取作业/写入作业/处理作业的方法，它在第一篇文章中被分开，现在这里做同样的3个作业没有通道且没有互斥。

工作样本：

package main

import (
    "fmt"
    "math"
    "math/rand"
    "time"
)

type BigStruct struct {
    big     []uint16
    rpos    int
    wpos    int
    full    bool
    empty   bool
    stopped bool
}

func manage() {
    for {
        if !s.empty {
            hist[s.big[s.rpos]]++ //sample read job with any time len
            nextPtr(&s.rpos)
        }
        if !s.full && !s.stopped {
            s.big[s.wpos] = uint16(rand.Intn(65536)) //sample wrire job with any time len
            nextPtr(&s.wpos)
        }
        if s.stopped {
            if s.empty {
                return
            }
        } else {
            s.stopped = time.Since(t0) >= 5*time.Second
        }
    }
}

func main() {
    t0 = time.Now()
    manage()
    mean := Mean(hist)
    stdev := StdDev(hist, mean)
    fmt.Println("mean=", mean, "stdev=", stdev)
    d0 := time.Since(t0)
    fmt.Println(d0) //5.8523347s
}

var t0 time.Time

const N = 100 * 1024 * 1024

var hist []int = make([]int, 65536)

var s *BigStruct = &BigStruct{empty: true,
    big: make([]uint16, N), //2GB
}

func next(pos int) int {
    pos++
    if pos >= N {
        pos = 0
    }
    return pos
}
func nextPtr(pos *int) {
    *pos++
    if *pos >= N {
        *pos = 0
    }

    s.empty = s.rpos == s.wpos
    s.full = next(s.wpos) == s.rpos
}

func StdDev(data []int, mean float64) float64 {
    sum := 0.0
    for _, d := range data {
        sum += math.Pow(float64(d)-mean, 2)
    }
    variance := sum / float64(len(data)-1)
    return math.Sqrt(variance)
}
func Mean(data []int) float64 {
    sum := 0.0
    for _, d := range data {
        sum += float64(d)
    }
    return sum / float64(len(data))
}

Golang在goroutines

2 个答案: