缓存池 bytebufferpool 库实现原理

// https://github.com/valyala/bytebufferpool/blob/18533face0/bytebuffer_example_test.go
package bytebufferpool_test

import (
	"fmt"

	"github.com/valyala/bytebufferpool"
)

func ExampleByteBuffer() {
	// 从缓存池取 Get()
	bb := bytebufferpool.Get()

	// 用法
	bb.WriteString("first linen")
	bb.Write([]byte("second linen"))
	bb.B = append(bb.B, "third linen"...)

	fmt.Printf("bytebuffer contents=%q", bb.B)

	// 使用完毕，放回缓存池 Put()
	// It is safe to release byte buffer now, since it is no longer used.
	bytebufferpool.Put(bb)
}

const (
	// 定位数据索引位置，使用位操作性能比较高效
	minBitSize = 6 // 2**6=64 is a CPU cache line size
	// 数组索引个数 0~19
	steps      = 20

	// 最小缓存对象 和 最大缓存对象大小
	minSize = 1 << minBitSize
	maxSize = 1 << (minBitSize + steps - 1)

	// 校准阈值, 这里指的调用次数
	calibrateCallsThreshold = 42000
	// 百分比，校准数据基数
	maxPercentile           = 0.95
)

// ByteBuffer provides byte buffer, which can be used for minimizing
// memory allocations.
//
// ByteBuffer may be used with functions appending data to the given []byte
// slice. See example code for details.
//
// Use Get for obtaining an empty byte buffer.
type ByteBuffer struct {

	// B is a byte buffer to use in append-like workloads.
	// See example code for details.
	B []byte
}

// Pool represents byte buffer pool.
//
// Distinct pools may be used for distinct types of byte buffers.
// Properly determined byte buffer types with their own pools may help reducing
// memory waste.
type Pool struct {
	calls       [steps]uint64
	calibrating uint64

	defaultSize uint64
	maxSize     uint64

	pool sync.Pool
}

var defaultPool Pool

// Get returns an empty byte buffer from the pool.
func Get() *ByteBuffer { return defaultPool.Get() }

// Put returns byte buffer to the pool.
func Put(b *ByteBuffer) { defaultPool.Put(b) }

// Get returns new byte buffer with zero length.
//
// The byte buffer may be returned to the pool via Put after the use
// in order to minimize GC overhead.
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)),
	}
}

// Put releases byte buffer obtained via Get to the pool.
//
// The buffer mustn't be accessed after returning to the pool.
func (p *Pool) Put(b *ByteBuffer) {
	// 对象在数据中的位置
	idx := index(len(b.B))

	// 校准条件
	if atomic.AddUint64(&p.calls[idx], 1) > calibrateCallsThreshold {
		p.calibrate()
	}

	// 是否需要放入pool池中，还是直接丢弃交给GC
	maxSize := int(atomic.LoadUint64(&p.maxSize))
	if maxSize == 0 || cap(b.B) <= maxSize {
		b.Reset()
		p.pool.Put(b)
	}
}

func (p *Pool) calibrate() {
	if !atomic.CompareAndSwapUint64(&p.calibrating, 0, 1) {
		return
	}

	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,
		})
	}
	sort.Sort(a)

	defaultSize := a[0].size
	maxSize := defaultSize

	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
		}
	}

	atomic.StoreUint64(&p.defaultSize, defaultSize)
	atomic.StoreUint64(&p.maxSize, maxSize)

	atomic.StoreUint64(&p.calibrating, 0)
}