Work stealing

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.

Exercise 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.

Show example 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])
}

Test yourself

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
Show answer

So goroutines in the global queue don't starve — Every 61 scheduling ticks the scheduler checks the global queue; otherwise busy local queues could keep global work waiting forever.

Amount stolen

calinan := len(kurban.kuyruk) / ____

Show answer

calinan := len(kurban.kuyruk) / 2

Queue and feature

Yerel kuyruk

Per P, 256 slots
Global kuyruk

Shared, locked
runnext

The next G to run
Work stealing

Taking work from another P

Next lesson: Preemption

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