示例代码：

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package main

import (
    "context"
    "fmt"
    "sync"
    "time"

    "golang.org/x/sync/semaphore"
)

func doSomething(u string) {
    fmt.Println(u)
    time.Sleep(2 * time.Second)
}

const (
    Limit  = 3 // 最大并发执行次数
    Weight = 1
)

func main() {
    urls := []string{
        "http://www.example.com",
        "http://www.example.net",
        "http://www.example.net/foo",
        "http://www.example.net/bar",
        "http://www.example.net/baz",
    }
    s := semaphore.NewWeighted(Limit)
    var w sync.WaitGroup
    for _, u := range urls {
        w.Add(1)
        s.Acquire(context.Background(), Weight)
        go func(u string) {
            doSomething(u)
            s.Release(Weight)
            w.Done()
        }(u)
    }
    w.Wait()
}