The goroutine scheduler is not preemptive.

go - Why is time.sleep required to run certain goroutines? - Stack Overflow https://stackoverflow.com/questions/15771232/why-is-time-sleep-required-to-run-certain-goroutines

package main

import (
//"time"
"fmt"
)

func say(s string) {
for i := 0; i < 5; i++ {
// time.Sleep(100 * time.Microsecond)
fmt.Println(s)
}
}

func main() {
go say("world")
say("hello")
}

hello
hello
hello
hello
hello

package main

import (
"time"
"fmt"
)

func say(s string) {
for i := 0; i < 5; i++ {
time.Sleep(100 * time.Microsecond)
fmt.Println(s)
}
}

func main() {
go say("world")
say("hello")
}

输出结果不定

hello
world
hello
world
hello
world
world
hello
world
hello

hello
world
hello
world
world
hello
world
hello
hello

world
hello
world
hello
hello
world
hello
world
world
hello