Yp Xie, The Go scheduler is cooperative: https://en.wikipedia.org/wiki/Cooperative_multitasking
You are using one processor: runtime.GOMAXPROCS(1). Force the main goroutine to yield: runtime.Gosched(). For example, https://play.golang.org/p/7HnogkMOOo 0: main 1: main 0: not main 2: main 1: not main 3: main 2: not main 4: main 3: not main 4: not main Peter On Sunday, August 20, 2017 at 9:35:50 AM UTC, Yp Xie wrote: > > PS: https://play.golang.org/p/ax_bsnEJ1Q > > I added more goroutine to see what the result would be: > > package main > > import ( > "runtime" > "fmt" > "sync" > ) > > func main() { > runtime.GOMAXPROCS(1) > wg := sync.WaitGroup{} > > for i := 0; i < 5; i++ { > wg.Add(1) > fmt.Printf("%d: main\n", i) > go func (i int) { > fmt.Printf("%d: not main\n", i) > wg.Done() > } (i) > } > > //time.Sleep(1 * time.Nanosecond) > wg.Wait() > } > > https://play.golang.org/p/19bgbXJ54- > > > > The result is like this: > > 0: main > 1: main > 2: main > 3: main > 4: main > 4: not main > 0: not main > 1: not main > 2: not main > 3: not main > > The fifth goroutine got executed first(except for the main goroutine). > > And if I put the main goroutine to sleep for some time by uncomment the > sleep line, the result is: > > > 0: main > 1: main > 2: main > 3: main > 4: main > 0: not main > 1: not main > 2: not main > 3: not main > 4: not main > > > I am really confused with the results. > > > 在 2017年8月20日星期日 UTC+8下午12:57:43，Yp Xie写道： >> >> Hi guys, >> >> I am learning golang with the fantastic book "Go in Action". And I have >> some problems with understanding when to switch a goroutine to be run in >> chapter 6. >> >> package main >> >> 1 >> 2 import ( >> 3 "fmt" >> 4 "runtime" >> 5 "sync" >> 6 ) >> 7 >> 8 func main() { >> 9 runtime.GOMAXPROCS(1) >> 10 >> 11 var wg sync.WaitGroup >> 12 wg.Add(2) >> 13 >> // first goroutine >> 14 go func() { >> 15 defer wg.Done() >> 16 for c := 0; c < 3; c++ { >> 17 for char := 'a'; char < 'a'+26; char++ { >> 18 fmt.Printf("%c", char) >> 19 } >> 20 } >> 21 fmt.Println("") >> 22 }() >> 23 >> // second goroutine >> 24 go func() { >> 25 defer wg.Done() >> 26 for c := 0; c < 3; c++ { >> 27 for char := 'A'; char < 'A'+26; char++ { >> 28 fmt.Printf("%c", char) >> 29 } >> 30 } >> 31 fmt.Println("") >> 32 }() >> 33 >> 34 wg.Wait() >> } >> >> >> Why the result is printing uppercase letters first? >> >> I mean the two goroutines are passed in a waiting queue in order of the >> time they were generated, and they were meant to be executed in the same >> order. >> >> So why the second goroutine got executed first? >> >> Or is the fmt.Println method is IO-related? >> >> Please help me understand this situation. >> >> Thanks! >> >> >> -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
