sync.Pool实现原理
sync.Pool实现原理
对象的创建和销毁会消耗一定的系统资源（内存，gc等），过多的创建销毁对象会带来内存不稳定与更长的gc停顿，因为go的gc不存在分代，因而更加不擅长处理这种问题。因而go早早就推出Pool包用于缓解这种情况。Pool用于核心的功能就是Put和Get。当我们需要一个对象的时候通过Get获取一个，创建的对象也可以Put放进池子里，通过这种方式可以反复利用现有对象，这样gc就不用高频的促发内存gc了。
结构
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
// 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{}
}创建时候指定New方法用于创建默认对象，local，localSize会在随后用到的时候生成. local是一个poolLocalInternal的切片指针。
type poolLocalInternal struct {
private interface{} // Can be used only by the respective P.
shared []interface{} // Can be used by any P.
Mutex // Protects shared.
}当不同的p调用Pool时,每个p都会在local上分配这样一个poolLocal，索引值就是p的id。 private存放的对象只能由创建的p读写，shared则会在多个p之间共享。
PUT
// 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
}
runtime_procUnpin()
if x != nil {
l.Lock()
l.shared = append(l.shared, x)
l.Unlock()
}
if race.Enabled {
race.Enable()
}
}Put先要通过pin函数获取当前Pool对应的pid位置上的localPool，然后检查private是否存在，存在则设置到private上，如果不存在就追加到shared尾部。
func (p *Pool) pin() *poolLocal {
pid := runtime_procPin()
// In pinSlow we store to localSize and then to local, 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 { // 这句话的意思是如果当前pool的localPool切片尚未创建，尚未创建这句话肯定是false的
return indexLocal(l, pid)
}
return p.pinSlow()
}pin函数先通过自旋加锁(可以避免p自身发生并发)，在检查本地local切片的size，size大于当前pid则使用pid去本地local切片上索引到localpool对象，否则就要走pinSlow对象创建本地localPool切片了.
func (p *Pool) pinSlow() *poolLocal {
// 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)
}
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]
}pinShow先要取消自旋锁，因为后面的lock内部也会尝试自旋锁，下面可能会操作allpool因而这里需要使用互斥锁allPoolsMu，然后又加上自旋锁，（这里注释说不会发生poolCleanup，但是查看代码gcstart只是查看了当前m的lock状态，然而避免不了其他m触发的gc，尚存疑），这里会再次尝试之前的操作，因为可能在unpin，pin之间有并发产生了poolocal，确认本地local切片是空的才会生成一个新的pool。后面是创建Pool上的localPool切片，runtime.GOMAXPROCS这里的作用是返回p的数量，用于确定pool的localpool的数量.
GET
func (p *Pool) Get() interface{} {
if race.Enabled {
race.Disable()
}
l := p.pin()
x := l.private
l.private = nil
runtime_procUnpin()
if x == nil {
l.Lock()
last := len(l.shared) - 1
if last >= 0 {
x = l.shared[last]
l.shared = l.shared[:last]
}
l.Unlock()
if x == nil {
x = p.getSlow()
}
}
if race.Enabled {
race.Enable()
if x != nil {
race.Acquire(poolRaceAddr(x))
}
}
if x == nil && p.New != nil {
x = p.New()
}
return x
}GET 先调用pin获取本地local，这个具体流程和上面一样了，如果当前private存在返回private上面的对象，如果不存在就从shared查找，存在返回尾部对象，反之就要从其他的p的localPool里面偷了。
func (p *Pool) getSlow() (x interface{}) {
// See the comment in pin regarding ordering of the loads.
size := atomic.LoadUintptr(&p.localSize) // load-acquire
local := p.local // load-consume
// Try to steal one element from other procs.
pid := runtime_procPin()
runtime_procUnpin()
for i := 0; i < int(size); i++ {
l := indexLocal(local, (pid+i+1)%int(size))
l.Lock()
last := len(l.shared) - 1
if last >= 0 {
x = l.shared[last]
l.shared = l.shared[:last]
l.Unlock()
break
}
l.Unlock()
}
return x
}首先就要获取当前size，用于轮询p的local，这里的查询顺序不是从0开始，而是是从当前p的位置往后查一圈。查到依次检查每个p的shared上是否存在对象，如果存在就获取末尾的值。 如果所有p的poollocal都是空的，那么初始化的New函数就起作用了，调用这个New函数创建一个新的对象出来。
清理
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{}
}pool对象的清理是在每次gc之前清理，通过runtime_registerPoolCleanup函数注册一个上面的poolCleanup对象，内部会把这个函数设置到clearpool函数上面，然后每次gc之前会调用clearPool来取消所有pool的引用，重置所有的Pool。代码很简单就是轮询一边设置nil，然后取消所有poollocal，pool引用。方法简单粗暴。由于clearPool是在STW中调用的，如果Pool存在大量对象会拉长STW的时间，在已经有提案来修复这个问题了(CL 166961.)[https://go-review.googlesource.com/c/go/+/166961/]
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