How does sync.Pool reduce GC pressure in high-throughput Go services?

sync.Pool is a concurrent pool of reusable temporary objects. By returning objects to the pool after use instead of discarding them, allocation and GC pressure are significantly reduced — critical for high-throughput services like HTTP servers and JSON encoders.

// Pool of byte buffers reused per request
var bufPool = sync.Pool{
    New: func() any {
        buf := make([]byte, 0, 4096)
        return &buf
    },
}

func handleRequest(w http.ResponseWriter, r *http.Request) {
    bufPtr := bufPool.Get().(*[]byte) // may be new or recycled
    buf := (*bufPtr)[:0]              // reset length, keep capacity

    // Use buf as scratch space...
    buf = append(buf, buildResponse(r)...)
    w.Write(buf)

    // Return to pool — ALWAYS reset state first!
    *bufPtr = buf
    bufPool.Put(bufPtr)
}

// KEY RULES:
// 1. GC MAY drain the pool at any time — pool items are NOT permanent
// 2. Get() returns a new object (via New) or a recycled one — both OK
// 3. ALWAYS reset state before Put() — recycled objects carry old data
// 4. Never use a pooled object after Put() — it belongs to the pool
// 5. All objects in the pool must be the same type

// Real usage:
// - encoding/json uses sync.Pool for encoder/decoder state
// - net/http uses it for request/response buffers
// - fmt uses it for print state (pp struct)

// Benchmarks show 40%+ reduction in allocations under sustained load

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