How do you make sure goroutines finish in a for-loop using WaitGroup?

Question

I'm running a function in a goroutine each time a for-loop iterates, and I'm using sync.WaitGroup to make sure the goroutines all finish. However, I'm getting weird behavior testing the concurrency with counters. In the example below, I attempt to keep track of the thread count using 4 different techniques (w, x, y, z), and get 4 different results. The only result I understand is x, since it is incremented in the for-loop itself. What am I missing here?

package main

import "fmt"
import "sync"

var w = 0

func main() {
  x := 0
  y := 0
  z := 0
  var wg sync.WaitGroup
  for i := 0; i < 10000; i++ {
    wg.Add(1)
    x++
    go func() {
      z++
      test(&y)
      wg.Done()
    }()
  }
  wg.Wait()
  fmt.Println(w, x, y, z) // 8947 10000 8831 8816
}

func test(y *int) {
  w++
  *y++
}

@CeriseLimón do I need to use a mutex to increment? what is the most efficient way to achieve this? — The Puma
– The Puma, Commented Dec 30, 2017 at 19:45

Pierre Prinetti · Accepted Answer · 2017-12-30 20:18:26Z

9

The sync.Waitgroup is working as expected. w, y and z will not reach 10000 because multiple goroutines are incrementing them concurrently, and Go's increment is not concurrent-safe: it is implemented as a normal fetch-increment-reassign operation.

You have two options.

option 1: mutex

type incrementer struct {
    sync.Mutex
    i int
}

func (i *incrementer) Add(n int) {
    i.Lock()
    defer i.Unlock()
    i.i += n
}

and use this type for w, y and z.

Full example: https://play.golang.org/p/6wWUK2xnOCW

option 2: sync.atomic

var w int32 = 0

go func(){
    // in the loop
    atomic.AddInt32(&w, 1)

}()

Full example: https://play.golang.org/p/oUCGgKYC1-Y

answered Dec 30, 2017 at 20:18

Pierre Prinetti

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Comments

Wakerboy135 · Accepted Answer · 2024-05-03 11:03:50Z

0

The problem to your scenario is that go routines are not thread safe. In other words the values read y, z, w can be totally incorrect.

As another comment mentioned you should use mutex.

package main

import "fmt"
import "sync"

var w = 0

func main() {
  var m sync.Mutex

  x := 0
  y := 0
  z := 0
  var wg sync.WaitGroup
  for i := 0; i < 10000; i++ {
    wg.Add(1)
    x++
    go func() {
      m.Lock()
      defer m.Unlock()
      z++
      test(&y)
      wg.Done()
    }()
  }
  wg.Wait()
  fmt.Println(w, x, y, z) // 10000 10000 10000 10000
}

func test(y *int) {
  w++
  *y++
}

You can test it here: https://play.golang.com/

answered May 3, 2024 at 11:03

Wakerboy135

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How do you make sure goroutines finish in a for-loop using WaitGroup?

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option 1: mutex

option 2: sync.atomic

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2 Answers 2

option 1: mutex

option 2: sync.atomic

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