Golang sync.Pool详解

太过爱你忘了你带给我的痛 2023-06-15 09:54 195阅读 0赞

Golang中sync.Pool用来提高对象复用几率，减少gc的压力，减少内存分配，它是线程安全的，常用来存储并复用临时对象。

### 原理 ###

为了减小并发中锁的竞争，sync.pool为每个P（对象cpu线程）分配一个子池子poolLocal，每个poolLocal有private对象和shared共享列表对象，private对象只有对应的P可访问，无需加锁， shared共享列表对象可被其它P共享，需要加锁。

### Pool结构体 ###

type Pool struct {
    	noCopy noCopy   //该对象不能被copy使用
    
    	local     unsafe.Pointer // [p]poolLocal,固定长度
    	localSize uintptr        //本地缓冲池poolLocal的数量
    
    	New func() interface{}  //用户自定义的用于生成对象的方法
    }

### Get函数 ###

作用：从Pool中获取一个对象，如果获取不到并且New函数不为空，则通过New创建一个对象并返回。否则返回nil

func (p *Pool) Get() interface{} {
    	if race.Enabled {
    		race.Disable()
    	}
    	//获取当前线程的poolLocal
    	l := p.pin()
    	//如果private对象不为空则直接返回，并将其置为nil
    	x := l.private
    	l.private = nil
    	runtime_procUnpin()
    	if x == nil {
    	    //private不存在则加锁从shared列表中拿
    		l.Lock()
    		last := len(l.shared) - 1
    		if last >= 0 {
    			x = l.shared[last]
    			l.shared = l.shared[:last]
    		}
    		l.Unlock()
    		//如果shared对象列表依然没有的话，则从其它P的poolLocal获取
    		if x == nil {
    			x = p.getSlow()
    		}
    	}
    	if race.Enabled {
    		race.Enable()
    		if x != nil {
    			race.Acquire(poolRaceAddr(x))
    		}
    	}
    	// 如果存在New func回调函数，则执行
    	if x == nil && p.New != nil {
    		x = p.New()
    	}
    	return x
    }

### Put()函数 ###

作用：将对象x放入到Pool中，以便复用该对象

func (p *Pool) Put(x interface{}) {
    	if x == nil {
    		return
    	}
    	if race.Enabled {
    		if fastrand()%4 == 0 {
    			// Randomly drop x on floor.
    			return
    		}
    		race.ReleaseMerge(poolRaceAddr(x))
    		race.Disable()
    	}
    	//如果当前poolLocal的private对象为nil，则直接赋值
    	l := p.pin()
    	if l.private == nil {
    		l.private = x
    		x = nil
    	}
    	runtime_procUnpin()
    	//否则加到当前poolLocal的shared列表中
    	if x != nil {
    		l.Lock()
    		l.shared = append(l.shared, x)
    		l.Unlock()
    	}
    	if race.Enabled {
    		race.Enable()
    	}
    }

### poolCleanup回收对象 ###

sync.Pool会在gc时回收pool中的对象

func init() {
        runtime_registerPoolCleanup(poolCleanup)
    }
    
    func indexLocal(l unsafe.Pointer, i int) *poolLocal {
        lp := unsafe.Pointer(uintptr(l) + uintptr(i)*unsafe.Sizeof(poolLocal{}))
        return (*poolLocal)(lp)
    }
    
    // Implemented in runtime.
    func runtime_registerPoolCleanup(cleanup func())
    func runtime_procPin() int
    func runtime_procUnpin()
    
    func poolCleanup() {
        // This function is called with the world stopped, at the beginning of a garbage collection.
        // It must not allocate and probably should not call any runtime functions.
        // Defensively zero out everything, 2 reasons:
        // 1. To prevent false retention of whole Pools.
        // 2. If GC happens while a goroutine works with l.shared in Put/Get,
        //    it will retain whole Pool. So next cycle memory consumption would be doubled.
        for i, p := range allPools {
            allPools[i] = nil
            for i := 0; i < int(p.localSize); i++ {
                l := indexLocal(p.local, i)
                l.private = nil
                for j := range l.shared {
                    l.shared[j] = nil
                }
                l.shared = nil
            }
            p.local = nil
            p.localSize = 0
        }
        allPools = []*Pool{}
    }

感谢：[http://www.mckee.cn/golang/pkg/syncpool][http_www.mckee.cn_golang_pkg_syncpool]

[https://segmentfault.com/a/1190000019973632?utm\_source=tag-newest][https_segmentfault.com_a_1190000019973632_utm_source_tag-newest]

[http_www.mckee.cn_golang_pkg_syncpool]: http://www.mckee.cn/golang/pkg/syncpool
[https_segmentfault.com_a_1190000019973632_utm_source_tag-newest]: https://segmentfault.com/a/1190000019973632?utm_source=tag-newest