slice 預先分配內存
slicefunc appendOne(num int) []int {
var res []int
for i := 0; i < num; i++ {
res = append(res, i)
}
return res
}
func appendMany(num int) []int {
res := make([]int, 0, num)
for i := 0; i < num; i++ {
res = append(res, i)
}
return res
}
複製代碼
appendOneappendManyfunc BenchmarkAppendOne(b *testing.B) {
num := 10000
for i := 0; i < b.N; i++ {
_ = appendOne(num)
}
}
func BenchmarkAppendMany(b *testing.B) {
num := 10000
for i := 0; i < b.N; i++ {
_ = appendMany(num)
}
}
複製代碼
運行測試app
$ go test -bench=. -benchmem goos: darwin goarch: amd64 pkg: com.learn/gormLearn/gc_gc BenchmarkAppendOne-4 23163 50675 ns/op 386296 B/op 20 allocs/op BenchmarkAppendMany-4 96781 12241 ns/op 81920 B/op 1 allocs/op PASS 複製代碼
AppendMany81920 B12241 nsAppendOneslicemapfunc makeMap(num int){
m := make(map[int]int,num)
for i:=0;i<len(num);i++{
m[i]=i
}
}
複製代碼
這能夠減小內存拷貝的開銷，也能夠減小rehash開銷。oop
map中保存值，而不是指針，使用分段map
看下面的列子，map分別保存指針，值。性能
func timeGC() time.Duration {
start := time.Now()
runtime.GC()
return time.Since(start)
}
func mapPointer(num int) {
m := make(map[int]*int, num)
for i := 0; i < num; i++ {
m[i] = &i
}
runtime.GC()
fmt.Printf("With %T, GC took %s\n", m, timeGC())
_ = m[0]
}
func mapValue(num int) {
m := make(map[int]int, num)
for i := 0; i < num; i++ {
m[i] = i
}
runtime.GC()
fmt.Printf("With %T, GC took %s\n", m, timeGC())
_ = m[0]
}
func mapPointerShard(num int) {
shards := make([]map[int]*int, 100)
for i := range shards {
shards[i] = make(map[int]*int)
}
for i := 0; i < num; i++ {
shards[i%100][i] = &i
}
runtime.GC()
fmt.Printf("With map shards (%T), GC took %s\n", shards, timeGC())
_ = shards[0][0]
}
func mapValueShard(num int) {
shards := make([]map[int]int, 100)
for i := range shards {
shards[i] = make(map[int]int)
}
for i := 0; i < num; i++ {
shards[i%100][i] = i
}
runtime.GC()
fmt.Printf("With map shards (%T), GC took %s\n", shards, timeGC())
_ = shards[0][0]
}
const N = 5e7 // 5000w
func BenchmarkMapPointer(b *testing.B) {
mapPointer(N)
}
func BenchmarkMapValue(b *testing.B) {
mapValue(N)
}
func BenchmarkMapPointerShard(b *testing.B) {
mapPointerShard(N)
}
func BenchmarkMapValueShard(b *testing.B) {
mapValueShard(N)
}
複製代碼
運行測試
$ go test -bench=^BenchmarkMapPointer$ -benchmem With map[int]*int, GC took 545.139836ms goos: darwin goarch: amd64 pkg: com.learn/gormLearn/gc_gc BenchmarkMapPointer-4 1 9532798100 ns/op 1387850488 B/op 724960 allocs/op $ go test -bench=^BenchmarkMapPointerShard$ -benchmem With map shards ([]map[int]*int), GC took 688.39764ms goos: darwin goarch: amd64 pkg: com.learn/gormLearn/gc_gc BenchmarkMapPointerShard-4 1 20670458639 ns/op 4286763416 B/op 1901279 allocs/op $ go test -bench=^BenchmarkMapValueShard$ -benchmem With map shards ([]map[int]int), GC took 1.965519ms goos: darwin goarch: amd64 pkg: com.learn/gormLearn/gc_gc BenchmarkMapValueShard-4 1 16190847776 ns/op 4385268936 B/op 1918445 allocs/op $ go test -bench=^BenchmarkMapValue$ -benchmem With map[int]int, GC took 22.993926ms goos: darwin goarch: amd64 pkg: com.learn/gormLearn/gc_gc BenchmarkMapValue-4 1 8253025035 ns/op 1444338752 B/op 724512 allocs/op 複製代碼
GODEBUG=gctrace=1$ GODEBUG=gctrace=1 go test -bench=^BenchmarkMapPointer$ -benchmem ... gc 3 @0.130s 19%: 0.006+424+0.013 ms clock, 0.027+0.18/424/848+0.055 ms cpu, 1224->1224->1224 MB, 1225 MB goal, 4 P gc 4 @9.410s 2%: 0.005+543+0.002 ms clock, 0.022+0/543/1628+0.011 ms cpu, 1325->1325->1323 MB, 2448 MB goal, 4 P (forced) gc 5 @9.957s 3%: 0.003+547+0.003 ms clock, 0.013+0/547/1631+0.013 ms cpu, 1323->1323->1323 MB, 2647 MB goal, 4 P (forced) With map[int]*int, GC took 550.40821ms 複製代碼
gctrace0.013+0/547/1631+0.013 ms cpu0.0130/547/1631mutator assist54716310.013$ GODEBUG=gctrace=1 go test -bench=^BenchmarkMapValue$ -benchmem ... gc 3 @0.018s 0%: 0.005+0.14+0.015 ms clock, 0.021+0.054/0.020/0.19+0.060 ms cpu, 1224->1224->1224 MB, 1225 MB goal, 4 P gc 4 @8.334s 0%: 0.006+21+0.003 ms clock, 0.027+0/6.4/21+0.013 ms cpu, 1379->1379->1334 MB, 2448 MB goal, 4 P (forced) gc 5 @8.358s 0%: 0.003+19+0.003 ms clock, 0.014+0/5.0/20+0.015 ms cpu, 1334->1334->1334 MB, 2668 MB goal, 4 P (forced) 複製代碼
mapstring與[]byte的轉換
stringstring[]bytefunc Example() {
s := "Hello,world"
b := []byte(s)
}
複製代碼
string[]bytefunc String2Bytes(s string) []byte {
stringHeader := (*reflect.StringHeader)(unsafe.Pointer(&s))
bh := reflect.SliceHeader{
Data: stringHeader.Data,
Len: stringHeader.Len,
Cap: stringHeader.Len,
}
return *(*[]byte)(unsafe.Pointer(&bh))
}
func Bytes2String(b []byte) string {
sliceHeader
sh := reflect.StringHeader{
Data: sliceHeader.Data,
Len: sliceHeader.Len,
}
return *(*string)(unsafe.Pointer(&sh))
}
複製代碼
函數返回值使用值，不使用指針
對佔用空間少，頻繁分配的函數，若是函數返回指針，會帶來內存逃逸，使得原來能夠分配在棧（stack）上的內存，須要分配在堆（heap）上。在棧上進行小對象拷貝的性能很好，比分配對象在堆上要好得多。 看下面的例子，2個函數分別返回值，和指針。
type S struct {
a, b, c int64
d, e, f string
g, h, i float64
}
func byCopy() S {
return S{
a: 1, b: 1, c: 1,
e: "lyp", f: "lyp",
g: 1.0, h: 1.0, i: 1.0,
}
}
func byPointer() *S {
return &S{
a: 1, b: 1, c: 1,
e: "lyp", f: "lyp",
g: 1.0, h: 1.0, i: 1.0,
}
}
複製代碼
benchmark函數
func BenchmarkMemoryStack(b *testing.B) {
var s S
f, err := os.Create("stack.out")
if err != nil {
panic(err)
}
defer f.Close()
err = trace.Start(f)
if err != nil {
panic(err)
}
for i := 0; i < b.N; i++ {
s = byCopy()
}
trace.Stop()
b.StopTimer()
_ = fmt.Sprintf("%v", s.a)
}
func BenchmarkMemoryHeap(b *testing.B) {
var s *S
f, err := os.Create("heap.out")
if err != nil {
panic(err)
}
defer f.Close()
err = trace.Start(f)
if err != nil {
panic(err)
}
for i := 0; i < b.N; i++ {
s = byPointer()
}
trace.Stop()
b.StopTimer()
_ = fmt.Sprintf("%v", s.a)
}
複製代碼
運行
go test ./... -bench=BenchmarkMemoryHeap -benchmem -run=^$ -count=10 goos: darwin goarch: amd64 pkg: com.learn/gormLearn/gc_gc BenchmarkMemoryHeap-4 19625536 53.0 ns/op 96 B/op 1 allocs/op go test ./... -bench=BenchmarkMemoryStack -benchmem -run=^$ -count=10 goos: darwin goarch: amd64 pkg: com.learn/gormLearn/gc_gc BenchmarkMemoryStack-4 163253341 7.22 ns/op 0 B/op 0 allocs/op 複製代碼
7.22 ns/op53.0 ns/op使用struct{}優化
struct{}func assign(num int) {
m := make(map[int]bool, num)
for i := 0; i < num; i++ {
m[i] = true
}
}
func assignStruct(num int) {
m := make(map[int]struct{}, num)
for i := 0; i < num; i++ {
m[i] = struct{}{}
}
}
複製代碼
struct{}GC分析的工具
- go tool pprof
- go tool trace
- go build -gcflags=」-m」
- GODEBUG=」gctrace=1」
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