goparkunlock vs notetsleepg in runtime/timers.go

[Alexander Morozov]

Hello!

I'm trying to understand the code in src/runtime/timers.go and found that it uses two functions to "freeze" timers goroutine

goparkunlock and notetsleepg. I wonder what's the difference between goparkunlock and notetsleepg with negative ns . Is it related to "faketime"?

Thanks!

[Austin Clements]

This is not related to faketime. These are just two different synchronization mechanisms used for two different purposes. We can't use gopark for both cases because gopark doesn't understand timeouts (it would be turtles all the way down :). We probably could use notetsleepg for both, but notetsleepg consumes an OS thread for the entire duration of the sleep (so the OS ultimately implements the time keeping). By combining the two mechanisms, timerproc doesn't consume any OS resources if there are no timers.

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[Alexander Morozov]

Thanks for a thorough explanation, Austin!