Work stealing
- Go course
- Expert
- Lesson 3/34
- 4 min
Learn the work-stealing mechanism of idle processors.
Work stealing
Each P has a local queue with 256 slots; there is also a global queue shared by all Ps. When a P runs out of work it checks, in order: its own queue → the global queue → the network poller → stealing half of another P's queue.
P0: [G1 G2 G3 G4] P1: []
P1 steals → P0: [G1 G2] P1: [G3 G4]This balances load across cores without a central lock.
runnext and locality
A newly created goroutine is placed in the creating P's runnext slot and becomes the next to run. In producer–consumer patterns the two goroutines run back-to-back on the same P, keeping caches warm. If the local queue overflows, half of it moves to the global queue.
Practice Parallel workers
Start 8 goroutines with a sync.WaitGroup; each sums 1 to 1000 and writes the result into a slice at its own index. Wait for all of them and print the first value in the slice.
Starter code
package main
import (
"fmt"
"sync"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"fmt"
"sync"
)
func main() {
sonuc := make([]int, 8)
var wg sync.WaitGroup
for i := 0; i < 8; i++ {
wg.Add(1)
go func(i int) {
defer wg.Done()
t := 0
for j := 1; j <= 1000; j++ {
t += j
}
sonuc[i] = t
}(i)
}
wg.Wait()
fmt.Println(sonuc[0])
}Expected output
500500
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Why is the global queue sometimes checked before the local one?
- Because it's faster
- Because the local queue can't be empty
- Because the GC requires it
- So goroutines in the global queue don't starve Correct answer
Explanation: Every 61 scheduling ticks the scheduler checks the global queue; otherwise busy local queues could keep global work waiting forever.
2 Amount stolen
calinan := len(kurban.kuyruk) / ____calinan := len(kurban.kuyruk) / 2Explanation: Stealing half quickly balances work between two Ps.
3 Queue and feature
| Term | Meaning |
|---|---|
Yerel kuyruk |
Per P, 256 slots |
Global kuyruk |
Shared, locked |
runnext |
The next G to run |
Work stealing |
Taking work from another P |