在 Go 1.3.1 中获取变量内存使用情况

Question

为什么这段代码找不到内存使用（m2-m1）和（m4-m3）？

为什么 (m3-m2) 和 (m5-m4) 需要分配额外的内存？

type T struct {

    B uint8

    S string

    I int

}

func memUsage(mOld, mNew *runtime.MemStats) {

    fmt.Println("Alloc:", mNew.Alloc-mOld.Alloc,

        "HeapAlloc:", mNew.HeapAlloc-mOld.HeapAlloc,

        "TotalAlloc:", mNew.TotalAlloc-mOld.TotalAlloc)

}

func main() {

    var m1, m2, m3, m4, m5, m6 runtime.MemStats

    runtime.ReadMemStats(&m1)

    t := T{}

    runtime.ReadMemStats(&m2)

    fmt.Println(t)

    memUsage(&m1, &m2)

    runtime.ReadMemStats(&m3)

    t2 := "abc"

    runtime.ReadMemStats(&m4)

    fmt.Println(t2)

    memUsage(&m3, &m4)

    runtime.ReadMemStats(&m5)

    t3 := map[int]string{1: "x"}

    runtime.ReadMemStats(&m6)

    fmt.Println(t3)

    memUsage(&m5, &m6)

    memUsage(&m2, &m3)

    memUsage(&m4, &m5)

}

输出

{0  0}

Alloc: 0 HeapAlloc: 0 TotalAlloc: 0

abc

Alloc: 0 HeapAlloc: 0 TotalAlloc: 0

map[1:x]

Alloc: 256 HeapAlloc: 256 TotalAlloc: 256

Alloc: 432 HeapAlloc: 432 TotalAlloc: 432

Alloc: 64 HeapAlloc: 64 TotalAlloc: 64