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Tri-color GC (1)
•
•
• garbage

Tri-color GC (2)
• root scan: root ( )
push stack
• mark phase:
• stack
• stack
•
• stack (
root root )
• sweep phase heap

Tri-color GC (3)
• mark phase & sweep phase
mutator pause time

• A F push stack
Tri-color GC - root scan

• stack A F push stack
Tri-color GC - mark

• B C D stack
Tri-color GC - mark

• mark A J
write barrier (1)

write barrier (4)
• push stack

• < current sweep > current sweep
(1)

GC in Golang (2)
Go’s garbage collector is being designed to
favor low latency and tuning via only a single
knob.
* reference: https://blog.golang.org/go15gc

GC in Golang (3)
• GOGC golﬁng GC
• GOGC=100 heap 100%
GC GC = 200 200%

tradeoff
• tricolor GC pause time
• heap size
sweep
• throughput CPU
GC
• * Spatial
Locality

GODEBUG=gctrace=1
$>GODEBUG=gctrace=1 go run main.go
gc 6 @0.693s 53%: 0.035+334+61 ms clock, 0.10+211/325/0+185 ms cpu, 23->33->33 MB, 45 MB
goal, 4 P
6: GC
0.693s:
53%: CPU GC 53%
0.35: STW sweep termination wall-clock 0.35ms
334: concurrent mark and scan wall-clock 334ms
61: STW mark termination wall-clock 61ms
0.10: STW sweep termination cpu time 0.10ms
221/325/0: assist time, background GC time, and idle GC cpu time 221/325/0 ms
185: STW mark termination cpu time 61ms
23->33->33 MB: GC / gc / live heap size
45MB: heap size
4 p: cpu

test code
package main
import (
"container/list"
"time"
)
func GenList() {
l := list.New()
for i := 0; i < 10000; i++ {
str := "aaaaaaaaaaaaaaaaaaa"
l.PushBack(str)
}
}
func main() {
count := 0
for {
count++
for i := 0; i < 1000; i++ {
go func() {
GenList()
}()
}
time.Sleep(500 * time.Millisecond)
if count == 360 {
count =0
}
}
}

gctrace output
• CPU 32%
GC
• GC pause time
0.18 + 0.98
ms
• heap size
625M

• htop 1313M
Real Memory size

test code
package main
import (
"time"
)
func GenSlice() {
s:= make([]string, 10000)
for i:= 0;i<10000;i++ {
s[i] = str
}
}
func main() {
count := 0
for {
count++
for i := 0; i < 1000; i++ {
go func() {
GenSlice()
}()
}
if count == 360 {
count =0
}
}
}

gctrace output
• CPU 5%
GC
• GC pause time
1.1 + 2.1 ms
• heap size
156M

Real Memory size
• htop 188M

runtime/pprof(1)
package main
import (
"container/list"
"os"
"runtime/pprof"
"sync"
"time"
)
func GenList() {
l := list.New()
for i := 0; i < 10000; i++ {
l.PushBack(str)
}
}
func main() {
var wg sync.WaitGroup
cpuprofile, _ := os.Create("cpu.prof")
memprofile, _ := os.Create("mem.prof")
pprof.StartCPUProfile(cpuprofile)
pprof.WriteHeapProfile(memprofile)
count := 0
times := 10
wg.Add(1000 * times)
for i := 0; i < times; i++ {
count++
for j := 0; j < 1000; j++ {
go func() {
GenList()
wg.Done()
}()
}
}
wg.Wait()
pprof.StopCPUProfile()
cpuprofile.Close()
memprofile.Close()
}

runtime/pprof(2)
$> ./main
$> go tool pprof main cpu.prof
(pprof) png

runtime/pprof(3)
$> go tool pprof main mem.prof
(pprof) png

net/pprof (1)
package main
import (
"container/list"
"log"
"net/http"
_ "net/http/pprof"
"time"
)
func GenList() {
l := list.New()
for i := 0; i < 10000; i++ {
l.PushBack(str)
}
}
func main() {
go func() {
log.Println(http.ListenAndServe("0.0.0.0:7777", nil))
}()
count := 0
for {
count++
for j := 0; j < 1000; j++ {
go func() {
GenList()
}()
}
if count == 360 {
count = 0
}
}
}

net/pprof (2)
http://127.0.0.1:7777/debug/pprof/

Golang GC
• Golang1.6: rescan stack
STW grouting scan
stack
• Golang1.7: List STW
goroutine rescan scan
stack
• Golang1.8: write barrier stack re-scan

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Gc in golang

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