Does time.Sleep Block a Goroutine in Go?

Will time.Sleep Block Goroutine?

Introduction

The time package in Go provides various time-related functions, including time.Sleep, which pauses the execution of a goroutine for a specified duration. This question explores whether time.Sleep truly blocks a goroutine and examines the behavior of the Go scheduler when goroutines are using time.Sleep.

Explanation

According to the provided response, time.Sleep indeed blocks a goroutine. However, it is important to note that the concept of "blocking" in Go is not strictly defined. In this context, "blocking" means that the execution of the next statement cannot proceed immediately because the current statement is still in progress.

Goroutine Scheduling

Goroutines are scheduled onto threads by the Go scheduler. The scheduler follows the "MPG model," where P represents the number of processors, M represents the number of operating system threads, and G represents the number of goroutines. When a goroutine is blocked, the scheduler detaches it from its current M and finds an idle M to schedule it on or creates a new M if necessary.

Thread Count Observation

The observation that the number of threads increases when the value of n is increased is a consequence of the scheduler's behavior. As more goroutines are created, the scheduler creates additional M threads to handle them. However, the fact that only a limited number of additional threads are created is due to the scheduler's ability to determine that not all goroutines require separate threads.

Comparison with Real IO

The provided example using real IO (ioutil.WriteFile) illustrates that when goroutines are performing blocking operations that involve real system resources, such as IO, the number of threads created is significantly higher. This is because the scheduler cannot avoid creating threads to handle these blocking operations.

Conclusion

While time.Sleep does block goroutines, it is the responsibility of the scheduler to manage the mapping of goroutines to threads efficiently. Developers need not worry about the specifics of scheduler behavior and can rely on the Go runtime to handle these issues effectively.

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