Mutex

• 互斥鎖的代價比原子操作大, 可通過原子檢查狀態提高性能

  type Once struct{
    m Mutx
    done uint32
  }
  func (o *Once) Do(f func()){
    if atomic.LoadUint32(&o.done)==1{
        return
    }
  }
  

• Mutex is used guards access to shared resoures.
• It is developers convention to call Lock() to access shared memory and call Unlock() when done.
• The critical section represents the bottleneck between the goroutines.

When to use channels and when to use mutex

Mutex

• Used for protect shared resources.
• sync.Mutex - Provide exclusive access to a shared resource.

Exercises

https://go.dev/play/p/mqjVI-1aZRd

package main

import (
    "fmt"
    "runtime"
    "sync"
)

func main() {

    runtime.GOMAXPROCS(4)

    var balance int
    var wg sync.WaitGroup
    var mu sync.Mutex

    deposit := func(amount int) {
        mu.Lock()
        balance += amount
        mu.Unlock()
    }

    withdrawal := func(amount int) {
        mu.Lock()
        defer mu.Unlock()
        balance -= amount
    }

    // we are making 100 times deposits of $1
    // and 100 times withdrawal of $1, concurrently.
    // run the program and check result.

    wg.Add(100)
    for i := 0; i < 100; i++ {
        go func() {
            defer wg.Done()
            deposit(1)
        }()
    }

    wg.Add(100)
    for i := 0; i < 100; i++ {
        go func() {
            defer wg.Done()
            withdrawal(1)
        }()
    }

    wg.Wait()
    fmt.Println(balance)
}