Go language Chanel memory model

Memory model

Go The memory model describes “ In a groutine Reading variables in can detect in other gorountine Write operation of change amount in ” Conditions .

happen-before Definition

To specify the requirements of reads and writes, we define happens before, a partial order on the execution of memory operations in a Go program. If event e1 happens before event e2, then we say that e2 happens after e1. Also, if e1 does not happen before e2 and does not happen after e2, then we say that e1 and e2 happen concurrently.

This is a Happens Before The definition of , If e1 It happened in e2 Before , So let's say e2 It happened in e1 after , If e1 Neither in e2 front , Not at all e2 after , Let's say the two are concurrent .

About channel Of happens-before stay Go There are three cases mentioned in the memory model ：

• case1: To a channel Send operation of happens-before The corresponding channel The receive operation for is completed
• case2: Close a channel happens-before From the Channel Received the last return value 0
• case3: Unbuffered channel Receiving operation of happens-before The corresponding channel Before the send operation

case1: To a channel Send operation of happens-before The corresponding channel The receive operation for is completed

Test code ：

import "testing"

var c = make(chan int, 10)
var a string

func f() {
 a = "hello, world" // (1)
 c <- 0             // (2)  Write operations   Sending operation 
}
func TestMemoryModel(t *testing.T) {
 go f()
 <-c      // (3) // Receive operation 
 print(a) // (4)
}

The above code , Will be guaranteed to print out hello world . There's a cushion channel The write operation occurs before the receive operation .

Unbuffered channel Receiving operation of happens-before The corresponding channel Before the send operation

var c1 = make(chan int)
var a1 string

func f1() {
 a1 = "hello, world" // (1)
 <-c1                // (2)  Receive operation 
}

func TestMemoryModel1(t *testing.T) {
 go f1()
 c1 <- 0   // (3)  Sending operation 
 print(a1) // (4)
}

Running results ：

=== RUN   TestMemoryModel1
hello, world--- PASS: TestMemoryModel1 (0.00s)
PASS

The above code will ensure that it will print hello world . because

According to the third rule above (2) happens-before (3), Finally, we can guarantee (1) happens-before (4).

No buffer channel The receive operation occurs before the write operation .

Let's take another example

var c2= make(chan int, 1)
var a2 string
func f2() {
 a2 = "hello, world"  // (1)
 <-c2   //   Receive operation 
}
//  There is no guarantee   Print out  "hello, world"
func TestMemoryModel2(t *testing.T) {
 go f2()
 c2 <- 0  // (3)
 print(a2)  // (4)   Write operations 
 //var day  time.Time
 //print(day.Format("20060102"))
}

The above code is not guaranteed to print hello world , Because of the output channel It's buffered , There is no guarantee that the receive operation occurs before the write operation , However, it can ensure that the write operation occurs before the receive operation .

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Reference material

• https://golang.org/ref/mem
• https://lailin.xyz/post/go-training-week3-go-memory-model.html
• https://www.jdon.com/concurrent/golang-memory.html