The document discusses Go's lightweight scheduler. It introduces Go routines and explains that Go's scheduler runs within the application rather than relying on the OS scheduler. It describes the basic structures used including M (threads), P (contexts), and G (Go routines). It then explains how the scheduler is initialized, how new Go routines are created and scheduled, and how scheduling occurs in response to system calls or other triggers. Source code locations are provided and diagrams illustrate the relationships between threads, contexts, and Go routines.

1. Introduction to Go
Scheduler

2. Introduction o Go
• Very easy to use lots of light weight processes
(Go routines) in the same time.
• Use the “go” keyword.

3. Why Not Use System
Scheduler
• Processes in an application don’t need too many
context.
• It’s difficult for OS to handle too many threads or
processes.
• System scheduler is too overhead.

4. What Will Include
• Basic structures
• The init of the scheduler
• The init of a Go routine
• The schedule that happens in the system call
• How to change current running Go routine

5. Source Code
• src/pkg/runtime/proc.c
• src/pkg/runtime/runtime.h
• src/pkg/runtime/asm_386.s

6. Basic Structures
• M: OS threads.
• P: Context to run Go routines.
• G: Go routine.

7. Basic Structures
M0 M1 M2
P G
P
G0 G
G0
G
G
G
G

8. The Init of Scheduler
M0
G0

9. M0
G0

10. runtime·schedinit
M0
P P(idle)
G0

11. runtime·main
main·main
runtime.newproc
runtime·mstart
M0
P P(idle)
G0
G
main
(run the
scheduler to
execute G)

12. Init of Another Go Routine
• newproc
• newproc1
M0
P P(idle)
G0
G
main
G

13. Init of Another Go Routine
• newproc
• newproc1
• wakep
• startm(nil, true)
• newm
• mstart
M0
M1
P P
G0
G
main
G0
G

14. When to Schedule
• block system call
After Go 1.2:
• call function
• use a channel

15. System Call
• .entersyscallblock
M0
M1
P P
G0
G
main
G0
G(sysc
all)
G

16. System Call
• .entersyscallblock
• handoffp M0
P
G0
G
main
M1
G(sysc
all)
M2
P(idle)
G0
G

17. System Call
• .entersyscallblock
• handoffp
• startm(p
M0
P
G0
G
main
M1
G(sysc
all)
M2
P
G0
G

18. How to Change Current Go
Routine

19. A Demo

20. A Demo
Run with flags:
• GODEBUG=schedtrace=1000,scheddetail=1
• GOMAXPROCS=4

21. Q & A