runtime.Gosched

Learn to voluntarily yield a goroutine with Gosched.

runtime.Gosched

runtime.Gosched() yields the current goroutine and puts it back on the queue; the P runs another goroutine, and then its turn comes again. The goroutine doesn't block, it just yields.

runtime.GOMAXPROCS(1)
go fmt.Println("goroutine")
runtime.Gosched()      // give the other one a chance
fmt.Println("main")

Since preemption arrived in Go 1.14 it's rarely needed in real code; you see it in tests and tight-loop experiments.

Don't use it for synchronization

The assumption "I called Gosched, so the goroutine must have run" is wrong; the scheduler doesn't guarantee it, and behavior changes on multi-core machines. To know that work has finished use real synchronization tools such as channels, sync.WaitGroup or sync.Once.

Exercise Wait for the flag

Declare var hazir atomic.Bool. A goroutine sets the flag to true. main calls runtime.Gosched() in a loop until the flag is true, then prints "hazır".

Show example solution

package main

import (
    "fmt"
    "runtime"
    "sync/atomic"
)

func main() {
    var hazir atomic.Bool
    go hazir.Store(true)
    for !hazir.Load() {
        runtime.Gosched()
    }
    fmt.Println("hazır")
}

Test yourself

What is the key difference between Gosched and time.Sleep(0)?
- They are identical
- Sleep(0) panics
- Gosched ends the program
- Gosched returns to the queue right away; Sleep goes through the timer
Show answer

Gosched returns to the queue right away; Sleep goes through the timer — Gosched only yields; it is not a synchronization tool. For ordering guarantees use channels or a WaitGroup.

Yield

for !hazir.Load() {
    runtime.____()
}

Show answer

for !hazir.Load() {
    runtime.Gosched()
}

Tool and guarantee

Gosched

Only yields
Channel

Ordering guarantee
WaitGroup

Waits for completion
atomic.Bool

Safe flag

Next lesson: runtime.Stack

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