Understanding goroutines

Question

I\'m trying to understand concurrency in Go. In particular, I wrote this thread-unsafe program:

package main

import          


        

Answer 1

It's an interaction of two things. One, by default, Go only uses a single core, and two, Go must schedule goroutines cooperatively. Your function inc_x doesn't yield and so it monopolizes the single core being used. Relieving either of these conditions will lead to the output you expect.

Saying "core" is a bit of a gloss. Go may actually use multiple cores behind the scenes, but it uses a variable called GOMAXPROCS to determine the number of threads to schedule your goroutines which are performing non-system tasks. As explained in the FAQ and Effective Go the default is 1, but it may be set higher with an environment variable or a runtime function. This will likely give the output you expect, but only if your processor has multiple cores.

Independently of cores and GOMAXPROCS, you can give the goroutine scheduler in the runtime a chance to do it's job. The scheduler cannot preempt a running goroutine but must wait for it to come back to the runtime and request some service, such as IO, time.Sleep(), or runtime.Gosched(). Adding anything like this in inc_x produces expected output. The goroutine running main() is already requesting a service with fmt.Println, so with the two goroutines now periodically yielding to the runtime, it can do some sort of fair scheduling.