goroutine挂起程序中的循环

Question

Why does the following program hang when the time.Sleep(time.Nanosecond) is commented out?

package main

import "fmt"
import "time"
import "sync/atomic"

func main() {
    var ops uint64 = 0
    for i := 0; i < 50; i++ {
        go func() {
            for {
                atomic.AddUint64(&ops, 1)
                time.Sleep(time.Nanosecond)
            }
        }()
    }

    time.Sleep(time.Millisecond)
    opsFinal := atomic.LoadUint64(&ops)
    fmt.Println("ops:", opsFinal)
}

Second question, why does running this program in the sandbox result in "process took too long"?

Answer 1

It's because goroutine is cooperative (not fully preemptive) task, and the context switch only happen when there's some IO, system call, time.Sleep(), or calling large function which have to extend the stack.

Reference:

• Go Routine Blocking the Other One
• Preemption since Go 1.2

Your atomic.AddUint64(&ops, 1) is small function which does not have to extend stack. so the context switch never occurs.

Since main thread is also a goroutine, it does not acquire context switch, and sleeps forever.

There's an open issue to make golang preemptive in tight loop, but not yet solved.

More useful references:

• A pitfall of golang scheduler
• golang scheduler is not preemptive - it's cooperative?