Using sync.Pool to Reduce GC Pressure

Using sync.Pool to Reduce GC Pressure | DevPrep

package main

import (
    "bytes"
    "sync"
)

var bufferPool = sync.Pool{
    New: func() interface{} {
        return new(bytes.Buffer)
    },
}

func processRequest(data []byte) []byte {
    // Get buffer from pool
    buf := bufferPool.Get().(*bytes.Buffer)
    
    // Ensure cleanup
    defer func() {
        buf.Reset()
        bufferPool.Put(buf)
    }()
    
    // Use buffer
    buf.Write(data)
    buf.WriteString(" processed")
    
    // Must copy result before returning buffer to pool
    result := make([]byte, buf.Len())
    copy(result, buf.Bytes())
    
    return result
}

func BenchmarkWithoutPool(b *testing.B) {
    for i := 0; i < b.N; i++ {
        buf := new(bytes.Buffer)
        buf.WriteString("hello world")
        _ = buf.Bytes()
    }
}

func BenchmarkWithPool(b *testing.B) {
    pool := sync.Pool{
        New: func() interface{} {
            return new(bytes.Buffer)
        },
    }
    
    b.ResetTimer()
    for i := 0; i < b.N; i++ {
        buf := pool.Get().(*bytes.Buffer)
        buf.WriteString("hello world")
        _ = buf.Bytes()
        buf.Reset()
        pool.Put(buf)
    }
}

// Results:
// BenchmarkWithoutPool-8   5000000   312 ns/op   112 B/op   2 allocs/op
// BenchmarkWithPool-8     20000000    78 ns/op     0 B/op   0 allocs/op

package jsonutil

import (
    "bytes"
    "encoding/json"
    "sync"
)

var encoderPool = sync.Pool{
    New: func() interface{} {
        return &encoderBuffer{
            buf: new(bytes.Buffer),
        }
    },
}

type encoderBuffer struct {
    buf *bytes.Buffer
    enc *json.Encoder
}

func Marshal(v interface{}) ([]byte, error) {
    eb := encoderPool.Get().(*encoderBuffer)
    defer func() {
        eb.buf.Reset()
        encoderPool.Put(eb)
    }()
    
    if eb.enc == nil {
        eb.enc = json.NewEncoder(eb.buf)
    }
    
    if err := eb.enc.Encode(v); err != nil {
        return nil, err
    }
    
    // Remove trailing newline from Encode
    result := eb.buf.Bytes()
    if len(result) > 0 && result[len(result)-1] == '\n' {
        result = result[:len(result)-1]
    }
    
    // Copy before returning to pool
    out := make([]byte, len(result))
    copy(out, result)
    return out, nil
}

func demonstrateGCInteraction() {
    pool := sync.Pool{
        New: func() interface{} {
            fmt.Println("Creating new object")
            return &bytes.Buffer{}
        },
    }
    
    // Put objects in pool
    for i := 0; i < 10; i++ {
        pool.Put(&bytes.Buffer{})
    }
    
    fmt.Println("Before GC - getting from pool:")
    _ = pool.Get() // Retrieves from pool
    
    // Force GC - pool may be cleared
    runtime.GC()
    
    fmt.Println("After GC - getting from pool:")
    _ = pool.Get() // May call New() if pool was cleared
}

// ❌ WRONG: Using result after Put
buf := pool.Get().(*bytes.Buffer)
buf.WriteString("data")
result := buf.Bytes() // Points to pooled buffer
pool.Put(buf)
return result // DANGER: result may be corrupted

// ✅ CORRECT: Copy before Put
buf := pool.Get().(*bytes.Buffer)
buf.WriteString("data")
result := make([]byte, buf.Len())
copy(result, buf.Bytes())
pool.Put(buf)
return result

Using sync.Pool to Reduce GC Pressure

Problem Statement

What is sync.Pool?

Basic Usage

Benchmark: With vs Without Pool

JSON Encoder Pool (Real-World)

Pool Lifecycle with GC

Common Mistakes

Follow-up Questions

Problem Statement

What is sync.Pool?

Basic Usage

Benchmark: With vs Without Pool

JSON Encoder Pool (Real-World)

Pool Lifecycle with GC

Common Mistakes

Follow-up Questions