Go 使用互斥锁(sync.Mutex)实现线程安全的内存本地缓存(LocalCache)

Posted 2022-05-12 东海陈光剑

本地缓存实现

var cache = struct  // 声明 struct 字面量 cahce (匿名结构体)
    sync.Mutex                   // 互斥锁, 内嵌 Struct
    caches     map[string]string // kv 内存存储

    caches: make(map[string]string), // 初始化 kv 存储


// GetCache safe get memory cache
func GetCache(key string) string 
    cache.Lock()           // 锁住
    v := cache.caches[key] // 获取缓存值
    defer cache.Unlock()   // 释放锁
    return v


// PutCache safe put memory cache
func PutCache(key string, val string) 
    cache.Lock()            // 锁住
    cache.caches[key] = val // 设置缓存值
    defer cache.Unlock()    // 释放锁


func main() 
    for i := 0; i < 10; i++ 
        PutCache(fmt.Sprintf("key%d", i), fmt.Sprintf("value%d", i))
    
    var v = GetCache("key1")
    fmt.Println(v) // value1

sync.Mutex 说明

核心实现: CompareAndSwap

// Package sync provides basic synchronization primitives such as mutual
// exclusion locks. Other than the Once and WaitGroup types, most are intended
// for use by low-level library routines. Higher-level synchronization is
// better done via channels and communication.
//
// Values containing the types defined in this package should not be copied.
package sync

import (
    "internal/race"
    "sync/atomic"
    "unsafe"
)

func throw(string) // provided by runtime

// A Mutex is a mutual exclusion lock.
// The zero value for a Mutex is an unlocked mutex.
//
// A Mutex must not be copied after first use.
type Mutex struct 
    state int32
    sema  uint32


// A Locker represents an object that can be locked and unlocked.
type Locker interface 
    Lock()
    Unlock()


const (
    mutexLocked = 1 << iota // mutex is locked
    mutexWoken
    mutexStarving
    mutexWaiterShift = iota

    // Mutex fairness.
    //
    // Mutex can be in 2 modes of operations: normal and starvation.
    // In normal mode waiters are queued in FIFO order, but a woken up waiter
    // does not own the mutex and competes with new arriving goroutines over
    // the ownership. New arriving goroutines have an advantage -- they are
    // already running on CPU and there can be lots of them, so a woken up
    // waiter has good chances of losing. In such case it is queued at front
    // of the wait queue. If a waiter fails to acquire the mutex for more than 1ms,
    // it switches mutex to the starvation mode.
    //
    // In starvation mode ownership of the mutex is directly handed off from
    // the unlocking goroutine to the waiter at the front of the queue.
    // New arriving goroutines don't try to acquire the mutex even if it appears
    // to be unlocked, and don't try to spin. Instead they queue themselves at
    // the tail of the wait queue.
    //
    // If a waiter receives ownership of the mutex and sees that either
    // (1) it is the last waiter in the queue, or (2) it waited for less than 1 ms,
    // it switches mutex back to normal operation mode.
    //
    // Normal mode has considerably better performance as a goroutine can acquire
    // a mutex several times in a row even if there are blocked waiters.
    // Starvation mode is important to prevent pathological cases of tail latency.
    starvationThresholdNs = 1e6
)

// Lock locks m.
// If the lock is already in use, the calling goroutine
// blocks until the mutex is available.
func (m *Mutex) Lock() 
    // Fast path: grab unlocked mutex.
    if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) 
        if race.Enabled 
            race.Acquire(unsafe.Pointer(m))
        
        return
    
    // Slow path (outlined so that the fast path can be inlined)
    m.lockSlow()




// Unlock unlocks m.
// It is a run-time error if m is not locked on entry to Unlock.
//
// A locked Mutex is not associated with a particular goroutine.
// It is allowed for one goroutine to lock a Mutex and then
// arrange for another goroutine to unlock it.
func (m *Mutex) Unlock() 
    if race.Enabled 
        _ = m.state
        race.Release(unsafe.Pointer(m))
    

    // Fast path: drop lock bit.
    new := atomic.AddInt32(&m.state, -mutexLocked)
    if new != 0 
        // Outlined slow path to allow inlining the fast path.
        // To hide unlockSlow during tracing we skip one extra frame when tracing GoUnblock.
        m.unlockSlow(new)
    


...

内嵌 struct

通常我们使用"有名"的 struct 嵌套如下:

package main
 
import "fmt"
 
type People struct
    name string
    age int8

 
type Employee struct
    profile *People
    salary float32

func main() 
   p := &People
           name: "Mr.Lee",
           age: 32,
       
   e := &Employee
       profile: p,
       salary: 600000,
   
    fmt.Println("Hello world!", e.profile.name)

go语言中虽然没有继承,但是可以结构内嵌,达到类似继承的效果:

type Info struct 
    sex int
    name string
    age int
    address string

 
type User struct
    like string
    Info

 
type Admin struct 
    unlike string
    Info

 
user:= User
user.sex=0
user.address="广州市"
user.like="游戏"
f.Println(user)
    
    
admin:= AdminInfo:Infosex:1//还可以这样声明一些属性值,因为Info是结构体,匿名,所以需要这样声明
admin.address="广州市"
admin.unlike="游戏"
f.Println(admin)

如果嵌入结构的字段和外部结构的字段相同,那么, 为了避免命名冲突, 想要修改嵌入结构的字段值需要加上外部结构中声明的嵌入结构名称.

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