Golang对map中元素delete后无法释放内存

首先我们看下面的代码

package main

import (
    "fmt"
    "runtime"
)

func main() {
    v := struct{}{}

    a := make(map[int]struct{})
    printMemStats("After Map Make", a)
    for i := 0; i < 10000; i++ {
        a[i] = v
    }

    runtime.GC()
    printMemStats("After Map Add 100000", a)

    for i := 0; i < 10000-1; i++ {
        delete(a, i)
    }

    runtime.GC()
    printMemStats("After Map Delete 9999", a)

    for i := 0; i < 10000-1; i++ {
        a[i] = v
    }

    runtime.GC()
    printMemStats("After Map Add 9999 again", a)

    a = nil
    runtime.GC()
    printMemStats("After Map Set nil", a)
}

func printMemStats(mag string, mp map[int]struct{}) {
    var m runtime.MemStats
    runtime.ReadMemStats(&m)
    fmt.Printf("[%v]    map address = %p,   memory = %vKB   , GC Times = %v , Map Length = %d\n", mag, &mp, m.Alloc/1024, m.NumGC, len(mp))
}

执行结果

[After Map Make]        map address = 0xc000006028,     memory = 189KB  , GC Times = 0  , Map Length = 0
[After Map Add 100000]  map address = 0xc000006008,     memory = 364KB  , GC Times = 1  , Map Length = 10000
[After Map Delete 9999] map address = 0xc000006008,     memory = 366KB  , GC Times = 2  , Map Length = 1
[After Map Add 9999 again]      map address = 0xc000006008,     memory = 366KB  , GC Times = 3  , Map Length = 10000
[After Map Set nil]     map address = 0xc0000fa010,     memory = 188KB  , GC Times = 4  , Map Length = 0

分析

在golang中的,在我们删除map键值对的时候,并不会真正的删除,而是进行标记。那么随着键值对越来越多,甚至会引起OOM

解决方法

    c := make(map[int]struct{})
    for i := 0; i < 100; i++ {
        c[i] = a[i]
    }
    a = c

这样就可以了

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