Why "manual" batching is faster than using inputBuffer

Akka Libraries Akka Streams & Alpakka
1

Hi,
I am doing experiments with async + inputBuffers and I can not understand why “manual” batching is faster than using inputBuffer attribute (in my laptop in five times faster). Can someone explain ?

Code:

import akka.actor.ActorSystem
import akka.stream.Attributes
import akka.stream.scaladsl.{Sink, Source}

import scala.concurrent.Await
import scala.concurrent.duration.Duration

object TimedUtil {
  def timed[T](prefix: String)(cmd: => T): T = {
    val t0 = System.currentTimeMillis()
    val res = cmd
    println(s"duration of $prefix: ${System.currentTimeMillis() - t0}")
    res
  }
}

object Async extends App {
  implicit val system = ActorSystem()
  val numberOfIterations = 10000000

  TimedUtil.timed("async") {
    val source = Source
      .fromIterator(() => (0 to numberOfIterations).iterator)
      .async
      .withAttributes(Attributes.inputBuffer(1024, 1024))
      .map(a => a + 1)
      .runWith(Sink.ignore)

    Await.result(source, Duration.Inf)
  }

  TimedUtil.timed("async-batch") {
    val source = Source
      .fromIterator(() => (0 to numberOfIterations).iterator)
      .batch(1024, el => Vector.newBuilder += el)({ (acc, el) =>
        acc += el
      })
      .map(_.result())
      .async
      .mapConcat(identity)
      .map(a => a + 1)
      .runWith(Sink.ignore)

    Await.result(source, Duration.Inf)
  }

  system.terminate()
}