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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
}
476
A goroutine is a function that executes simultaneously with other goroutines in a program and are lightweight threads managed by Go. A goroutine takes about 2kB of stack space to initialize.
In the case of goroutines, since stack size can grow dynamically, you can spawn 1000 goroutines without a problem. As a goroutine starts with 8KB (2KB since Go 1.4) of stack space, most of them generally don't grow bigger than that.
A goroutine is a function that runs independently of the function that started it. Sometimes Go developers explain a goroutine as a function that runs as if it were on its own thread. Channel is a pipeline for sending and receiving data. Think of it as a socket that runs inside your program.
A goroutine is a lightweight execution thread in the Go programming language and a function that executes concurrently with the rest of the program. Goroutines are incredibly cheap when compared to traditional threads as the overhead of creating a goroutine is very low.
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
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