How to wait until buffered channel (semaphore) is empty?
Solution 1
You can't use a semaphore (channel in this case) in that manner. There's no guarantee it won't be empty any point while you are processing values and dispatching more goroutines. That's not a concern in this case specifically since you're dispatching work synchronously, but because there's no race-free way to check a channel's length, there's no primitive to wait for a channel's length to reach 0.
Use a sync.WaitGroup
to wait for all goroutines to complete
sem := make(chan struct{}, 2)
var wg sync.WaitGroup
for _, i := range ints {
wg.Add(1)
// acquire semaphore
sem <- struct{}{}
// start long running go routine
go func(id int) {
defer wg.Done()
// do something
// release semaphore
<-sem
}(i)
}
wg.Wait()
Solution 2
Use "worker pool" to process you data. It is cheeper than run goroutine for each int, allocate memory for variables inside it and so on...
ints := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
ch := make(chan int)
var wg sync.WaitGroup
// run worker pool
for i := 2; i > 0; i-- {
wg.Add(1)
go func() {
defer wg.Done()
for id := range ch {
// do something
fmt.Println(id)
}
}()
}
// send ints to workers
for _, i := range ints {
ch <- i
}
close(ch)
wg.Wait()
Comments
-
Kiril almost 2 years
I have a slice of integers, which are manipulated concurrently:
ints := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
I'm using a buffered channel as semaphore in order to have a an upper bound of concurrently running go routines:
sem := make(chan struct{}, 2) for _, i := range ints { // acquire semaphore sem <- struct{}{} // start long running go routine go func(id int, sem chan struct{}) { // do something // release semaphore <- sem }(i, sem) }
The code above works pretty well until the last or last two integers are reached, because the program ends before those last go routines are finished.
Question: how do I wait for the buffered channel to drain?
-
Kiril over 7 yearsThanks, I also thought about using a
WaitGroup
. This feels like the right way! -
Kiril over 7 yearsThank you, I was looking for how to avoid that, and a
WorkGroup
works perfectly. JimB's answers clarified this