結論
在Go中,map中存放map,上層map執行delete,子層map佔用的內存會釋放,無需手動先釋放子map內存,再在上層map執行刪除。
實驗
在C++中,如果使用了map包含map的數據結構,當要釋放上層map的某一項時,需要手動釋放對應的子map佔用的內存,而在Go中,垃圾回收讓內存管理變得如此簡單。
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
}
結果
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
}
結果
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
參考資料
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