Syscalls and scheduler
- Go course
- Expert
- Lesson 5/34
- 4 min
Learn how M is managed during a syscall.
Syscalls and handoff
A blocking system call (e.g. a file read) makes the M wait in the kernel. Go doesn't waste the P meanwhile: if the syscall takes long, sysmon takes the P away from that M and gives it to another (new if needed) M. This is called handoff. When the syscall returns, the old M tries to get a P; if it can't, it puts the G on the global queue and sleeps.
G1 read() → M1 blocked
P0 moves to M2 → G2, G3 keep runningMany Ms
Too many concurrent blocking syscalls or cgo calls raise the number of Ms. The default limit is 10,000; beyond it the program crashes. runtime.LockOSThread() pins a goroutine to its own M; it is needed because some C libraries and graphics APIs keep thread-bound state.
Practice Thread lock
Inside a goroutine call runtime.LockOSThread() and defer runtime.UnlockOSThread(), then send "kilitli" over a channel. main reads from the channel and prints it.
Starter code
package main
import (
"fmt"
"runtime"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"fmt"
"runtime"
)
func main() {
ch := make(chan string)
go func() {
runtime.LockOSThread()
defer runtime.UnlockOSThread()
ch <- "kilitli"
}()
fmt.Println(<-ch)
}Expected output
kilitli
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Why doesn't network I/O block an M?
- The GC manages them
- Go waits on network operations in a poller with epoll/kqueue Correct answer
- Network operations are always instant
- They run in a separate process
Explanation: Sockets are opened in non-blocking mode; with no data the G waits, and the netpoller makes it runnable again when ready.
2 Pin to the thread
runtime.____()
defer runtime.UnlockOSThread()runtime.LockOSThread()
defer runtime.UnlockOSThread()Explanation: LockOSThread makes the current goroutine run only on this OS thread.
3 Kind of wait
| Term | Meaning |
|---|---|
File read() |
M blocks, P is handed off |
Soket okuma |
The netpoller waits |
cgo call |
Occupies the M |
LockOSThread |
Binds G to M |