Go的map中删除子map，内存会自动释放吗？

大彬 ·

结论

在Go中，map中存放map，上层map执行delete，子层map占用的内存会释放，无需手动先释放子map内存，再在上层map执行删除。

实验

在C++中，如果使用了map包含map的数据结构，当要释放上层map的某一项时，需要手动释放对应的子map占用的内存，而在Go中，垃圾回收让内存管理变得如此简单。

package main

import (
    "log"
    "runtime"
)

var lastTotalFreed uint64
var intMap map[int]int
var cnt = 8192

func main() {
    printMemStats()

    initMap()
    runtime.GC()
    printMemStats()

    log.Println(len(intMap))
    for i := 0; i < cnt; i++ {
        delete(intMap, i)
    }
    log.Println(len(intMap))

    runtime.GC()
    printMemStats()

    intMap = nil
    runtime.GC()
    printMemStats()
}

func initMap() {
    intMap = make(map[int]int, cnt)

    for i := 0; i < cnt; i++ {
        intMap[i] = i
    }
}

func printMemStats() {
    var m runtime.MemStats
    runtime.ReadMemStats(&m)
    log.Printf("Alloc = %v TotalAlloc = %v  Just Freed = %v Sys = %v NumGC = %v\n",
        m.Alloc/1024, m.TotalAlloc/1024, ((m.TotalAlloc-m.Alloc)-lastTotalFreed)/1024, m.Sys/1024, m.NumGC)

    lastTotalFreed = m.TotalAlloc - m.Alloc
}

结果

2018/09/29 20:09:25 Alloc = 65 TotalAlloc = 65  Just Freed = 0 Sys = 1700 NumGC = 0
2018/09/29 20:09:25 Alloc = 387 TotalAlloc = 391  Just Freed = 3 Sys = 3076 NumGC = 1
2018/09/29 20:09:25 8192
2018/09/29 20:09:25 0
2018/09/29 20:09:25 Alloc = 387 TotalAlloc = 392  Just Freed = 1 Sys = 3140 NumGC = 2
2018/09/29 20:09:25 Alloc = 74 TotalAlloc = 394  Just Freed = 314 Sys = 3140 NumGC = 3

package main

import (
    "log"
    "runtime"
)

var intMapMap map[int]map[int]int

var cnt = 1024
var lastTotalFreed uint64 // size of last memory has been freed

func main() {
    // 1
    printMemStats()

    // 2
    initMapMap()
    runtime.GC()
    printMemStats()

    // 3
    fillMapMap()
    runtime.GC()
    printMemStats()

    // 4
    log.Println(len(intMapMap))
    for i := 0; i < cnt; i++ {
        delete(intMapMap, i)
    }
    log.Println(len(intMapMap))
    runtime.GC()
    printMemStats()

    // 5
    intMapMap = nil
    runtime.GC()
    printMemStats()
}

func initMapMap() {
    intMapMap = make(map[int]map[int]int, cnt)
    for i := 0; i < cnt; i++ {
        intMapMap[i] = make(map[int]int, cnt)
    }
}

func fillMapMap() {
    for i := 0; i < cnt; i++ {
        for j := 0; j < cnt; j++ {
            intMapMap[i][j] = j
        }
    }
}

func printMemStats() {
    var m runtime.MemStats
    runtime.ReadMemStats(&m)
    log.Printf("Alloc = %v TotalAlloc = %v  Just Freed = %v Sys = %v NumGC = %v\n",
        m.Alloc/1024, m.TotalAlloc/1024, ((m.TotalAlloc-m.Alloc)-lastTotalFreed)/1024, m.Sys/1024, m.NumGC)

    lastTotalFreed = m.TotalAlloc - m.Alloc
}

结果

2018/09/29 20:10:27 Alloc = 64 TotalAlloc = 64  Just Freed = 0 Sys = 1700 NumGC = 0
2018/09/29 20:10:27 Alloc = 41154 TotalAlloc = 41157  Just Freed = 3 Sys = 46026 NumGC = 5
2018/09/29 20:10:27 Alloc = 41241 TotalAlloc = 41293  Just Freed = 48 Sys = 47082 NumGC = 6
2018/09/29 20:10:27 1024
2018/09/29 20:10:27 0
2018/09/29 20:10:27 Alloc = 114 TotalAlloc = 41295  Just Freed = 41128 Sys = 47082 NumGC = 7
2018/09/29 20:10:27 Alloc = 74 TotalAlloc = 41296  Just Freed = 41 Sys = 47082 NumGC = 8