This document summarizes a Pull Request (PR) that improves the performance of receiving from an empty closed channel in Go. It begins by describing the PR title and motivation to speed up non-blocking receives on closed channels. It then provides background on how channels work in Go, explaining their circular queue mechanism and how goroutines can block on channel operations. The document details how the PR implementation checks if the channel is empty and closed before locking, avoiding the overhead of locking in this common case. It concludes with a demo of the performance improvement using benchmarks and profiling tools.

1. 從 source code 學習 golang
channel
G7

2. Agenda
● A PR title : runtime: speed up receive
on empty closed channel
● How channel work
● How the PR do to improve performance
● Analysis

3. A PR title : runtime: speed up receive
on empty closed channel

4. runtime: speed up receive on empty closed channel
Currently, nonblocking receive on an open channel is about
700 times faster than nonblocking receive on a closed channel.
This change makes closed channels equally fast.

5. How channel work

6. channel mechanism: circular queue

7. channel mechanism: block
goroutine would be blocked if executing channel operations.
○ v <- chan
○ chan <- v

8. channel mechanism: non-block
select {
case msg := <-chan
fmt.Println(“Receive msg from channel”)
default:
fmt.Println(“No message received”)
}

9. channel mechanism: send to full channel
● Keep data in goroutine
● Add goroutine to sendq

10. channel mechanism: send to full channel
● Add goroutine to recvq

11. source code: chan <- val
close block full empty recvq return
false false true true false
true - - - panic
false - - false true
false - false true true
false true true true (block until complete)
true

12. source code: val <- chan
close block empty empty sendq return (select,
received)
false false true true (false, false)
true - - true (true, false)
false - - false (true, true)
- - false true (true, true)
false true true true (block until complete)
(true !close)

13. How the PR do to improve performance

14. partial pseudo code in receive: go1.14
if !block && empty && !close
return
lock()
if close && empty
unlock
return true, false

15. partial pseudo code in receive: cb14bd8
if !block && empty
if !close
return
else
return true, false
lock()

16. Analysis

17. Life demo
demo: run benchmark & pporf

18. QA