Why Do Go Goroutine Behaviors Differ Between the Playground and Local Execution?

Discordant Behaviors Between Go Playground and Local Machine Execution

Background

In an attempt to clarify misunderstandings regarding goroutines, a user turned to the Go Playground and executed the following code:

<code class="go">package main

import (
    "fmt"
)

func other(done chan bool) {
    done <- true
    go func() {
        for {
            fmt.Println("Here")
        }
    }()
}

func main() {
    fmt.Println("Hello, playground")
    done := make(chan bool)
    go other(done)
    <-done
    fmt.Println("Finished.")
}</code>

Observed Outcomes

Go Playground:

• Encountered an error: "Process took too long."
• Implied that the goroutine within other runs perpetually.

Local Execution:

• Produced output almost instantaneously:

  Hello, playground.
  Finished.

• Indicated that the goroutine within other terminates upon completion of the main goroutine.

Explanation

Go Playground:

• Default GOMAXPROCS is set to 1.
• Only one goroutine executes at a time, preventing scheduler switching when goroutines are non-blocking.
• The main goroutine blocks on waiting for a message from the done channel.
• The goroutine within other executes indefinitely, leading to the timeout.

Local Execution:

• GOMAXPROCS is likely set to the number of CPU cores, typically defaulting to a value greater than 1.
• The scheduler switches between goroutines, allowing the main goroutine to progress even with a non-blocking goroutine running concurrently.
• Once main() exits, the program terminates without waiting for the indefinitely running goroutine to complete.

Non-Deterministic Behavior

Note that the Go Playground currently uses a cached version of the output, so subsequent runs may not accurately reflect the actual execution.

Conclusion

Understanding the impact of GOMAXPROCS on goroutine execution is crucial for designing appropriate concurrency models. The default settings on the Go Playground may not always mimic the behavior of a local machine, highlighting the importance of testing under different configurations.

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