When a goroutine blocks on I/O how does the scheduler identify that it has stopped blocking?

Question

From what I've read here, the golang scheduler will automatically determine if a goroutine is blocking on I/O, and will automatically switch to processing others goroutines on a thread that isn't blocked.

What I'm wondering is how the scheduler then figures out that that goroutine has stopped blocking on I/O.

Does it just do some kind of polling every so often to check if it's still blocking? Is there some kind of background thread running that checks the status of all goroutines?

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For example, if you were to do an HTTP GET request inside a goroutine that took 5s to get a response, it would block while waiting for the response, and the scheduler would switch to processing another goroutine. Now given that, when the server returns a response, how does the scheduler understand that the response has arrived, and it's time to go back to the goroutine that made the GET so that it can process the result of the GET?

Answer 1

All I/O must be done through syscalls, and the way syscalls are implemented in Go, they are always called through code that is controlled by the runtime. This means that when you call a syscall, instead of just calling it directly (thus giving up control of the thread to the kernel), the runtime is notified of the syscall you want to make, and it does it on the goroutine's behalf. This allows it to, for example, do a non-blocking syscall instead of a blocking one (essentially telling the kernel, "please do this thing, but instead of blocking until it's done, return immediately, and let me know later once the result is ready"). This allows it to continue doing other work in the meantime.