What are the three background goroutines in a Go program?

Question

Go seems to always have at least 4 goroutines running at any given time. What are the other three that are not the main goroutine?

http://play.golang.org/p/MQBiLmHXBK

package main

import (
    "fmt"
    "runtime"
)

func main() {
    fmt.Println(runtime.NumGoroutine()) //4
}

Answer 1

Those aren't threads, they're goroutines, and the number may vary based on current implementation (i.e. in go1.2 it would have printed 2).

Right now, it looks like you have 1 for main, and 3 for runtime/gc.

import "fmt"

func main() {
    fmt.Println(func() string { panic(nil); return "" }())
}

This shows

goroutine 16 [running]:
runtime.panic(0x0, 0x0)
    /usr/local/go/src/pkg/runtime/panic.c:279 +0xf5
main.func·001(0x3ea4f, 0xdc4b0)
    /test/threads.go:6 +0x28
main.main()
    /test/threads.go:6 +0x1e

goroutine 17 [runnable]:
runtime.MHeap_Scavenger()
    /usr/local/go/src/pkg/runtime/mheap.c:507
runtime.goexit()
    /usr/local/go/src/pkg/runtime/proc.c:1445

goroutine 18 [runnable]:
bgsweep()
    /usr/local/go/src/pkg/runtime/mgc0.c:1976
runtime.goexit()
    /usr/local/go/src/pkg/runtime/proc.c:1445

goroutine 19 [runnable]:
runfinq()
    /usr/local/go/src/pkg/runtime/mgc0.c:2606
runtime.goexit()
    /usr/local/go/src/pkg/runtime/proc.c:1445

if you remove fmt, and use the bootstrapping print function you only get 2 goroutines.

import "runtime"

func main() {
    print(runtime.NumGoroutine(), "\n")
}

// prints 2

If you ever want to know exactly what goroutines are running, print a stack trace, call panic, or kill the process with SIGQUIT (which prints a stack trace and exits). If you run the absolute minimum program you can get a stack trace from you can see the 2 goroutines:

package main

func main() {
    panic(nil)
}

Goroutines are very inexpensive, and many things will start and stop more goroutines, so trying to track their lower bound isn't very useful. Notice how even though there's only 2 goroutines, (main/runtime.panic, and runtime.MHeap_Scavenger), the count is already up to 17.

panic: nil

goroutine 16 [running]:
runtime.panic(0x0, 0x0)
    /usr/local/go/src/pkg/runtime/panic.c:279 +0xf5
main.main()
    /test/threads.go:4 +0x28

goroutine 17 [runnable]:
runtime.MHeap_Scavenger()
    /usr/local/go/src/pkg/runtime/mheap.c:507
runtime.goexit()
    /usr/local/go/src/pkg/runtime/proc.c:1445
exit status 2