问为什么在“热身”之后解封JSON要快得多？

EN

package main

import (
    "time"
    "fmt"
    "encoding/json"
)

var b []byte

type Dog struct {
    Age int `json:"Age"`
}

func unmarshalDog() {
    dogCopy := Dog{}

    start := time.Now().UnixNano()
    json.Unmarshal(b, &dogCopy)
    end := time.Now().UnixNano()
    fmt.Printf("Time to marshal/unmarshal: %d\n", end-start)
}

func main() {
    // Marshal an object into a byte array which we will repeatedly unmarshal from
    d := Dog {
        Age: 5,
    }

    var err error
    b, err = json.Marshal(d)
    if err != nil {
        panic(err)
    }

    for i := 0; i < 20; i++ {
        unmarshalDog()
    }

    // Allow the goroutines to finish before terminating execution.
    time.Sleep(3 * time.Second)
}

$ go run testJSONPerformance.go
Time to marshal/unmarshal: 34127
Time to marshal/unmarshal: 1465
Time to marshal/unmarshal: 979
Time to marshal/unmarshal: 892
Time to marshal/unmarshal: 849
Time to marshal/unmarshal: 814
Time to marshal/unmarshal: 822
Time to marshal/unmarshal: 822
Time to marshal/unmarshal: 815
Time to marshal/unmarshal: 829
Time to marshal/unmarshal: 822
Time to marshal/unmarshal: 819

Time to marshal/unmarshal: 36540
Time to marshal/unmarshal: 4652
Time to marshal/unmarshal: 56959
Time to marshal/unmarshal: 3887
Time to marshal/unmarshal: 57068
Time to marshal/unmarshal: 3519
Time to marshal/unmarshal: 37160