golang: goroute with select doesn't stop unless I added a fmt.Print()

Question

I tried the Go Tour exercise #71

If it is run like go run 71_hang.go ok, it works fine.

However, if you use go run 71_hang.go nogood

Answer 1

You have 100% CPU load because almost all times the default case will be executed, resulting effectively in an infinite loop because it's executed over and over again. In this situation the Go scheduler does not hand control to another goroutine, by design. So any other goroutine will never have the opportunity to set crawling != 0 and you have your infinite loop.

In my opinion you should remove the default case and instead create another channel if you want to play with the select statement.

Otherwise the runtime package helps you to go the dirty way:

• runtime.GOMAXPROCS(2) will work (or export GOMAXPROCS=2), this way you will have more than one OS thread of execution
• call runtime.Gosched() inside Crawl from time to time. Eventhough CPU load is 100%, this will explicitely pass control to another Goroutine.

Edit: Yes, and the reason why fmt.Printf makes a difference: because it explicitely passes control to some syscall stuff... ;)