Concurrent GC
- Go course
- Expert
- Lesson 9/34
- 4 min
Learn how GC runs concurrently with the application.
Concurrent GC
Go's GC marks concurrently with the program. The world stops only for two brief moments (STW): when marking turns on and when it finishes. During marking about 25% of CPU goes to the GC; goroutines that allocate a lot are also forced to help through "mark assist".
GODEBUG=gctrace=1 ./app
# gc 12 @3.1s 2%: 0.03+1.2+0.02 ms clock, ...
# STW conc. STWThe write barrier
If the program writes to a pointer field during marking, a short piece of code inserted by the compiler runs and paints the target object grey. Go uses a hybrid barrier (Yuasa + Dijkstra); thanks to it goroutine stacks don't need rescanning at the end of marking, keeping the second STW very short.
Practice Pause time
Call runtime.GC(), then read the last pause with runtime.ReadMemStats: m.PauseNs[(m.NumGC+255)%256]. Print whether the pause was shorter than one second (< 1e9).
Starter code
package main
import (
"fmt"
"runtime"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"fmt"
"runtime"
)
func main() {
runtime.GC()
var m runtime.MemStats
runtime.ReadMemStats(&m)
son := m.PauseNs[(m.NumGC+255)%256]
fmt.Println(son < 1e9)
}Expected output
true
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 When is the write barrier active?
- Always
- Only during STW
- Never
- Only during the mark phase Correct answer
Explanation: The barrier watches pointer writes while marking runs; when the GC isn't running it is off and costs nothing.
2 GC trace
GODEBUG=____=1 ./appGODEBUG=gctrace=1 ./appExplanation: gctrace=1 writes timings and heap sizes for every GC cycle to stderr.
3 GC phase
| Term | Meaning |
|---|---|
STW 1 |
Turn marking on |
Concurrent marking |
While the program runs |
STW 2 |
Finish marking |
Sweeping |
Lazy, during allocation |