Golang在sync里面提供了对象池Pool。一般大家都叫这个为对象池。众所周知，go是自动垃圾回收的，这大大减少了程序编程负担。但gc是一把双刃剑。比如对刚刚上手的程序员是友好的，但是后期随着项目变得越来越巨大，维护的内存问题也会逐渐暴露出来。

sync.Pool是一个可以存或取的临时对象池。对外提供New、Get、Put等API。本文对sync.Pool进行分析。

Pool设计的目的是什么？

Pool 用来保存和复用临时对象，以减少内存分配，降低CG压力。

代码实现

关于代码的实现，最好的办法就是看源代码（src/pkg/sync/pool.go）。

数据结构定义如下：

Pool结构体：

type Pool struct {
	noCopy noCopy

	local     unsafe.Pointer // local fixed-size per-P pool, actual type is [P]poolLocal
	localSize uintptr        // size of the local array

	victim     unsafe.Pointer // local from previous cycle
	victimSize uintptr        // size of victims array

	// New optionally specifies a function to generate
	// a value when Get would otherwise return nil.
	// It may not be changed concurrently with calls to Get.
	New func() interface{}
}

对各个成员含义解析：

noCopy： 防止sync.Pool被复制
local： poolLocal数组的指针
localSize： poolLocal数组大小
victim ：poolvictim数组的指针
victimSize： poolvictim数组大小
New ：函数指针申请具体的对象，便于用户定制各种类型的对象

poolLocalInternal 结构体：

// Local per-P Pool appendix.
type poolLocalInternal struct {
	private interface{} // Can be used only by the respective P.
	shared  poolChain   // Local P can pushHead/popHead; any P can popTail.
}

对各个成员含义解析：
private ：只能由相应的P使用
shared ：本地P可以推送/popHead;任何P都可以poolChain

poolLocal 结构体：

type poolLocal struct {
	poolLocalInternal

	// Prevents false sharing on widespread platforms with
	// 128 mod (cache line size) = 0 .
	
	// 将 poolLocal 补齐至两个缓存行的倍数，防止 false sharing,
	// 每个缓存行具有 64 bytes，即 512 bit
	// 目前我们的处理器一般拥有 32 * 1024 / 64 = 512 条缓存行
	pad [128 - unsafe.Sizeof(poolLocalInternal{})%128]byte
}

一个poolLocal与一个P绑定，也就是说一个P持有一个poolLocal。每个 poolLocal 的大小均为缓存行的偶数倍。

Pool对外暴露的主要有三个接口

New func() interface{}

func (p *Pool) Put(x interface{})

func (p *Pool) Get() interface{}

Put

Put的过程就是将临时对象放进 Pool 里面。优先把元素放在 private 池中；如果 private 不为空，则放在 shared 池中。源码如下：

// Put adds x to the pool.
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()
	}
	l, _ := p.pin()
	if l.private == nil {
		l.private = x
		x = nil
	}
	if x != nil {
		l.shared.pushHead(x)
	}
	runtime_procUnpin()
	if race.Enabled {
		race.Enable()
	}
}

Get
会先从 per-P 的 poolLocal slice 中选取一个 poolLocal。源码如下：

// If Get would otherwise return nil and p.New is non-nil, Get returns
// the result of calling p.New.
func (p *Pool) Get() interface{} {
	if race.Enabled {
		race.Disable()
	}
	l, pid := p.pin()
	// 先从private取
	x := l.private
	l.private = nil
	if x == nil {
		// Try to pop the head of the local shard. We prefer
		// the head over the tail for temporal locality of
		// reuse.
		x, _ = l.shared.popHead()
		if x == nil {
		// 如果取不到，则获取新的缓存对象
			x = p.getSlow(pid)
		}
	}
	runtime_procUnpin()
	if race.Enabled {
		race.Enable()
		if x != nil {
			race.Acquire(poolRaceAddr(x))
		}
	}
	// 如果 getSlow 还是获取不到，则 New 一个
	if x == nil && p.New != nil {
		x = p.New()
	}
	return x
}

func (p *Pool) getSlow(pid int) interface{} {
	// See the comment in pin regarding ordering of the loads.
	size := atomic.LoadUintptr(&p.localSize) // load-acquire
	locals := p.local                        // load-consume
	// Try to steal one element from other procs.
	for i := 0; i < int(size); i++ {
		l := indexLocal(locals, (pid+i+1)%int(size))
		if x, _ := l.shared.popTail(); x != nil {
			return x
		}
	}

	// Try the victim cache. We do this after attempting to steal
	// from all primary caches because we want objects in the
	// victim cache to age out if at all possible.
	size = atomic.LoadUintptr(&p.victimSize)
	if uintptr(pid) >= size {
		return nil
	}
	locals = p.victim
	l := indexLocal(locals, pid)
	if x := l.private; x != nil {
		l.private = nil
		return x
	}
	for i := 0; i < int(size); i++ {
		l := indexLocal(locals, (pid+i)%int(size))
		if x, _ := l.shared.popTail(); x != nil {
			return x
		}
	}

	// Mark the victim cache as empty for future gets don't bother
	// with it.
	atomic.StoreUintptr(&p.victimSize, 0)

	return nil
}

// pin pins the current goroutine to P, disables preemption and
// returns poolLocal pool for the P and the P's id.
// Caller must call runtime_procUnpin() when done with the pool.
func (p *Pool) pin() (*poolLocal, int) {
	pid := runtime_procPin()
	// In pinSlow we store to local and then to localSize, here we load in opposite order.
	// Since we've disabled preemption, GC cannot happen in between.
	// Thus here we must observe local at least as large localSize.
	// We can observe a newer/larger local, it is fine (we must observe its zero-initialized-ness).
	s := atomic.LoadUintptr(&p.localSize) // load-acquire
	l := p.local                          // load-consume
	if uintptr(pid) < s {
		return indexLocal(l, pid), pid
	}
	return p.pinSlow()
}

func (p *Pool) pinSlow() (*poolLocal, int) {
	// Retry under the mutex.
	// Can not lock the mutex while pinned.
	runtime_procUnpin()
	allPoolsMu.Lock()
	defer allPoolsMu.Unlock()
	pid := runtime_procPin()
	// poolCleanup won't be called while we are pinned.
	s := p.localSize
	l := p.local
	if uintptr(pid) < s {
		return indexLocal(l, pid), pid
	}
	if p.local == nil {
		allPools = append(allPools, p)
	}
	// If GOMAXPROCS changes between GCs, we re-allocate the array and lose the old one.
	size := runtime.GOMAXPROCS(0)
	local := make([]poolLocal, size)
	atomic.StorePointer(&p.local, unsafe.Pointer(&local[0])) // store-release
	atomic.StoreUintptr(&p.localSize, uintptr(size))         // store-release
	return &local[pid], pid
}

获取对象有三个来源：

1、优先从 private 中选择对象。
2、若取不到，则从shared池获取。
3、若还是取不到，则使用 New 方法新建

获取对象顺序是先从private池获取对象，如果不成功则从shared池获取，如果继续不成功，则从New方法新建，也就是说从系统的Heap内存获取。

CleanUp实现

注册poolCleanup函数。源码如下：

func init() {
	runtime_registerPoolCleanup(poolCleanup)
}

下面看看Pool的清理函数poolCleanup()是怎么清理Pool的，源码如下：

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.

	// Because the world is stopped, no pool user can be in a
	// pinned section (in effect, this has all Ps pinned).

	// Drop victim caches from all pools.
	for _, p := range oldPools {
		p.victim = nil
		p.victimSize = 0
	}

	// Move primary cache to victim cache.
	for _, p := range allPools {
		p.victim = p.local
		p.victimSize = p.localSize
		p.local = nil
		p.localSize = 0
	}

	// The pools with non-empty primary caches now have non-empty
	// victim caches and no pools have primary caches.
	oldPools, allPools = allPools, nil
}

var (
	allPoolsMu Mutex

	// allPools is the set of pools that have non-empty primary
	// caches. Protected by either 1) allPoolsMu and pinning or 2)
	// STW.
	allPools []*Pool

	// oldPools is the set of pools that may have non-empty victim
	// caches. Protected by STW.
	oldPools []*Pool
)

该函数内不能分配内存且不能调用任何运行时函数。实际上就是将所有的对象置为 nil，等着GC做自动回收。

总结

sync.Pool的Get方法不会对获取到的对象做任何的保证，因为放入的本地子池中的值可能在任何是由被删除，而且不会通知调用者。sync.Pool主要用途是增加临时对象的重用率，减少GC负担。

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