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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来源：CSDN
作者：Minger~
链接：https://blog.csdn.net/chen1415886044/article/details/104599640