Kotlin: Is there a tool that allows me to control parallelism when executing suspend functions? - kotlin

Question

I'm trying to execute certain suspend function multiple times, in such a way that never more than N of these are being executed at the same time.
For those acquainted with Akka and Scala Streaming libraries, something like mapAsync.
I did my own implementation using one input channel (as in kotlin channels) and N output channels. But it seems cumbersome and not very efficient.
The code I'm currently using is somewhat like this:
val inChannel = Channel<T>()
val outChannels = (0..n).map{
Channel<T>()
}
launch{
var i = 0
for(t in inChannel){
outChannels[i].offer(t)
i = ((i+1)%n)
}
}
outChannels.forEach{outChannel ->
launch{
for(t in outChannel){
fn(t)
}
}
}
Of course it has error management and everything, but still...
Edit: I did the following test, and it failed.
test("Parallelism is correctly capped") {
val scope = CoroutineScope(Dispatchers.Default.limitedParallelism(3))
var num = 0
(1..100).map {
scope.launch {
num ++
println("started $it")
delay(Long.MAX_VALUE)
}
}
delay(500)
assertEquals(3,num)
}

Answer 1

You can use the limitedParallelism-function on a Dispatcher (experimental in v1.6.0), and use the returned dispatcher to call your asynchronous functions. The function returns a view over the original dispatcher which limits the parallelism to a limit you provide. You can use it like this:
val limit = 2 // Or some other number
val dispatcher = Dispatchers.Default
val limitedDispatcher = dispatcher.limitedParallelism(limit)
for (n in 0..100) {
scope.launch(limitedDispatcher) {
executeTask(n)
}
}

Answer 2

Your question, as asked, calls for #marstran's answer. If what you want is that no more than N coroutines are being actively executed at any given time (in parallel), then limitedParallelism is the way to go:
val maxThreads: Int = TODO("some max number of threads")
val limitedDispatcher = Dispatchers.Default.limitedParallelism(maxThreads)
elements.forEach { elt ->
scope.launch(limitedDispatcher) {
doSomething(elt)
}
}
Now, if what you want is to even limit concurrency, so that at most N coroutines are run concurrently (potentially interlacing), regardless of threads, you could use a Semaphore instead:
val maxConcurrency: Int = TODO("some max number of concurrency coroutines")
val semaphore = Semaphore(maxConcurrency)
elements.forEach { elt ->
scope.async {
semaphore.withPermit {
doSomething(elt)
}
}
}
You can also combine both approaches.

Answer 3

Other answers already explained that it depends whether you need to limit parallelism or concurrency. If you need to limit concurrency, then you can do this similarly to your original solution, but with only a single channel:
val channel = Channel<T>()
repeat(n) {
launch {
for(t in channel){
fn(t)
}
}
}
Also note that offer() in your example does not guarantee that the task will be ever executed. If the next consumer in the round robin is still occupied with the previous task, the new task is simply ignored.