Go Object Pool Usage & Optimization — Theory

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var ppFree = sync.Pool{
	New: func() any { return new(pp) },
}

// newPrinter allocates a new pp struct or grabs a cached one.
func newPrinter() *pp {
	p := ppFree.Get().(*pp)
	p.panicking = false
	p.erroring = false
	p.wrapErrs = false
	p.fmt.init(&p.buf)
	return p
}

// free saves used pp structs in ppFree; avoids an allocation per invocation.
func (p *pp) free() {
	// Proper usage of a sync.Pool requires each entry to have approximately
	// the same memory cost. To obtain this property when the stored type
	// contains a variably-sized buffer, we add a hard limit on the maximum buffer
	// to place back in the pool.
	//
	// See https://golang.org/issue/23199
	if cap(p.buf) > 64<<10 {
		return
	}

	p.buf = p.buf[:0]
	p.arg = nil
	p.value = reflect.Value{}
	p.wrappedErr = nil
	ppFree.Put(p)
}

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type bufferPool struct {
	pool sync.Pool
}

func newBufferPool() *bufferPool {
	return &bufferPool{
		pool: sync.Pool{
			New: func() interface{} {
				return new(bytes.Buffer)
			},
		},
	}
}

func (p *bufferPool) get() *bytes.Buffer {
	return p.pool.Get().(*bytes.Buffer)
}

func (p *bufferPool) put(b *bytes.Buffer) {
	p.pool.Put(b)
}

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// ServeHTTP conforms to the http.Handler interface.
func (engine *Engine) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	c := engine.pool.Get().(*Context)
	c.writermem.reset(w)
	c.Request = req
	c.reset()

	engine.handleHTTPRequest(c)

	engine.pool.Put(c)
}

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var (
	lz4WriterPool = sync.Pool{
		New: func() interface{} {
			return lz4.NewWriter(nil)
		},
	}

	gzipWriterPool = sync.Pool{
		New: func() interface{} {
			return gzip.NewWriter(nil)
		},
	}
	gzipWriterPoolForCompressionLevel1 = sync.Pool{
		New: func() interface{} {
			gz, err := gzip.NewWriterLevel(nil, 1)
			if err != nil {
				panic(err)
			}
			return gz
		},
	}
	// ...
)

func compress(cc CompressionCodec, level int, data []byte) ([]byte, error) {
	switch cc {
	case CompressionNone:
		return data, nil
	case CompressionGZIP:
		var (
			err    error
			buf    bytes.Buffer
			writer *gzip.Writer
		)

		switch level {
		case CompressionLevelDefault:
			writer = gzipWriterPool.Get().(*gzip.Writer)
			defer gzipWriterPool.Put(writer)
			writer.Reset(&buf)
		case 1:
			writer = gzipWriterPoolForCompressionLevel1.Get().(*gzip.Writer)
			defer gzipWriterPoolForCompressionLevel1.Put(writer)
			writer.Reset(&buf)
		// ...
		default:
			writer, err = gzip.NewWriterLevel(&buf, level)
			if err != nil {
				return nil, err
			}
		}
		if _, err := writer.Write(data); err != nil {
			return nil, err
		}
		if err := writer.Close(); err != nil {
			return nil, err
		}
		return buf.Bytes(), nil
	case CompressionSnappy:
		return snappy.Encode(data), nil
	case CompressionLZ4:
		writer := lz4WriterPool.Get().(*lz4.Writer)
		defer lz4WriterPool.Put(writer)

		var buf bytes.Buffer
		writer.Reset(&buf)

		if _, err := writer.Write(data); err != nil {
			return nil, err
		}
		if err := writer.Close(); err != nil {
			return nil, err
		}
		return buf.Bytes(), nil
	case CompressionZSTD:
		return zstdCompress(ZstdEncoderParams{level}, nil, data)
	default:
		return nil, PacketEncodingError{fmt.Sprintf("unsupported compression codec (%d)", cc)}
	}
}

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var _jsonPool = sync.Pool{New: func() interface{} {
	return &jsonEncoder{}
}}

func getJSONEncoder() *jsonEncoder {
	return _jsonPool.Get().(*jsonEncoder)
}

func putJSONEncoder(enc *jsonEncoder) {
	if enc.reflectBuf != nil {
		enc.reflectBuf.Free()
	}
	enc.EncoderConfig = nil
	enc.buf = nil
	enc.spaced = false
	enc.openNamespaces = 0
	enc.reflectBuf = nil
	enc.reflectEnc = nil
	_jsonPool.Put(enc)
}

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// Provides leaky buffer, based on the example in Effective Go.
package shadowsocks

type LeakyBuf struct {
	bufSize  int // size of each buffer
	freeList chan []byte
}

const leakyBufSize = 4108 // data.len(2) + hmacsha1(10) + data(4096)
const maxNBuf = 2048

var leakyBuf = NewLeakyBuf(maxNBuf, leakyBufSize)

// NewLeakyBuf creates a leaky buffer which can hold at most n buffer, each
// with bufSize bytes.
func NewLeakyBuf(n, bufSize int) *LeakyBuf {
	return &LeakyBuf{
		bufSize:  bufSize,
		freeList: make(chan []byte, n),
	}
}

// Get returns a buffer from the leaky buffer or create a new buffer.
func (lb *LeakyBuf) Get() (b []byte) {
	select {
	case b = <-lb.freeList:
	default:
		b = make([]byte, lb.bufSize)
	}
	return
}

// Put add the buffer into the free buffer pool for reuse. Panic if the buffer
// size is not the same with the leaky buffer's. This is intended to expose
// error usage of leaky buffer.
func (lb *LeakyBuf) Put(b []byte) {
	if len(b) != lb.bufSize {
		panic("invalid buffer size that's put into leaky buffer")
	}
	select {
	case lb.freeList <- b:
	default:
	}
	return
}