Shreyas Patil Does that mean, the pool created by Executors.newFixedThreadPool will live "forever", independent of where it is defined? Doesn't it get GC collected? Thanks for your help :)

Hi, why not create the dispatcher within the extension function? Like: fun <T, R> Iterable<T>.map( concurrency: Int, transform: (T) -> R ): List<R> = runBlocking { val dispatcher = Executors.newFixedThreadPool(concurrency).asCoroutineDispatcher() //... That way we do not need semaphores, but can limit the thread count for each call to the extension function.