go对象池化组件bytebufferpool使用详解

func main() {
   bf := bytebufferpool.Get()
   bf.WriteString("Hello")
   bf.WriteString(" World!!")
   fmt.Println(bf.String())
}

type Pool struct {
   calls       [steps]uint64
   calibrating uint64
   defaultSize uint64
   maxSize     uint64
   pool sync.Pool
}

const (
   minBitSize = 6 // 2**6=64 is a CPU cache line size
   steps      = 20
   minSize = 1 << minBitSize
   maxSize = 1 << (minBitSize + steps - 1)
   calibrateCallsThreshold = 42000
   maxPercentile           = 0.95
)

func (p *Pool) Get() *ByteBuffer {
   v := p.pool.Get()
   if v != nil {
      return v.(*ByteBuffer)
   }
   return &ByteBuffer{
      B: make([]byte, 0, atomic.LoadUint64(&p.defaultSize)),
   }
}

func (p *Pool) Put(b *ByteBuffer) {
   idx := index(len(b.B))
   if atomic.AddUint64(&p.calls[idx], 1) > calibrateCallsThreshold {
      p.calibrate()
   }
   maxSize := int(atomic.LoadUint64(&p.maxSize))
   if maxSize == 0 || cap(b.B) <= maxSize {
      b.Reset()
      p.pool.Put(b)
   }
}

func index(n int) int {
   n--
   n >>= minBitSize
   idx := 0
   for n > 0 {
      n >>= 1
      idx++
   }
   if idx >= steps {
      idx = steps - 1
   }
   return idx
}

func (p *Pool) calibrate() {
   // 如果正在重新计算，则返回，控制多并发
   if !atomic.CompareAndSwapUint64(&p.calibrating, 0, 1) {
      return
   }
   // 计算每一段区间中的元素个数 & 元素总个数
   a := make(callSizes, 0, steps)
   var callsSum uint64
   for i := uint64(0); i < steps; i++ {
      calls := atomic.SwapUint64(&p.calls[i], 0)
      callsSum += calls
      a = append(a, callSize{
         calls: calls,
         size:  minSize << i,
      })
   }
   // 按照对象元素的个数从大到小排序
   sort.Sort(a)
   // defaultSize 为内部切片的默认大小，减少扩容次数
   // maxSize 限制放入pool中的最大元素大小
   defaultSize := a[0].size
   maxSize := defaultSize
   // 将前95%元素中的最大size给maxSize
   maxSum := uint64(float64(callsSum) * maxPercentile)
   callsSum = 0
   for i := 0; i < steps; i++ {
      if callsSum > maxSum {
         break
      }
      callsSum += a[i].calls
      size := a[i].size
      if size > maxSize {
         maxSize = size
      }
   }
   // 对defaultSize和maxSize进行赋值
   atomic.StoreUint64(&p.defaultSize, defaultSize)
   atomic.StoreUint64(&p.maxSize, maxSize)
   atomic.StoreUint64(&p.calibrating, 0)
}