parEvalMapUnordered performance

benchmark/src/main/scala/fs2/benchmark/ParEvalMapBenchmark.scala

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2013 Functional Streams for Scala
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package fs2
+package benchmark
+
+import cats.effect.IO
+import cats.effect.unsafe.implicits.global
+import org.openjdk.jmh.annotations.{Benchmark, Param, Scope, State}
+
+@State(Scope.Thread)
+class ParEvalMapBenchmark {
+  @Param(Array("100", "10000"))
+  var size: Int = _
+
+  @Param(Array("10", "100"))
+  var chunkSize: Int = _
+
+  private def dummyLoad = IO.delay(())
+
+  @Benchmark
+  def evalMap(): Unit =
+    execute(getStream.evalMap(_ => dummyLoad))
+
+  @Benchmark
+  def parEvalMapUnordered10(): Unit =
+    execute(getStream.parEvalMapUnordered(10)(_ => dummyLoad))
+
+  private def getStream: Stream[IO, Unit] = Stream.constant((), chunkSize).take(size).covary[IO]
+  private def execute(s: Stream[IO, Unit]): Unit = s.compile.drain.unsafeRunSync()
+}

core/shared/src/main/scala/fs2/CompositeFailure.scala

@@ -59,6 +59,12 @@ object CompositeFailure {
     }
   }
 
+  def fromNel(errors: NonEmptyList[Throwable]): Throwable =
+    errors match {
+      case NonEmptyList(hd, Nil)               => hd
+      case NonEmptyList(first, second :: rest) => apply(first, second, rest)
+    }
+
   def fromList(errors: List[Throwable]): Option[Throwable] =
     errors match {
       case Nil                     => None

core/shared/src/main/scala/fs2/Stream.scala

@@ -24,16 +24,14 @@ package fs2
 import scala.annotation.{nowarn, tailrec}
 import scala.concurrent.TimeoutException
 import scala.concurrent.duration._
-
 import cats.{Eval => _, _}
-import cats.data.Ior
+import cats.data.{Ior, NonEmptyList}
 import cats.effect.{Concurrent, SyncIO}
 import cats.effect.kernel._
 import cats.effect.kernel.implicits._
 import cats.effect.std.{Console, Queue, QueueSink, QueueSource, Semaphore}
 import cats.effect.Resource.ExitCase
 import cats.syntax.all._
-
 import fs2.compat._
 import fs2.concurrent._
 import fs2.internal._
@@ -1753,7 +1751,7 @@ final class Stream[+F[_], +O] private[fs2] (private[fs2] val underlying: Pull[F,
   ]: Concurrent, O2](
       maxConcurrent: Int
   )(f: O => F2[O2]): Stream[F2, O2] =
-    map(o => Stream.eval(f(o))).parJoin(maxConcurrent)
+    parEvalMapUnordered[F2, O2](maxConcurrent)(f)
 
   /** Applies the specified pure function to each chunk in this stream.
     *
@@ -2065,7 +2063,79 @@ final class Stream[+F[_], +O] private[fs2] (private[fs2] val underlying: Pull[F,
   ]: Concurrent, O2](
       maxConcurrent: Int
   )(f: O => F2[O2]): Stream[F2, O2] =
-    map(o => Stream.eval(f(o))).parJoin(maxConcurrent)
+    if (maxConcurrent == 1) evalMap(f)
+    else {
+      assert(maxConcurrent > 0, "maxConcurrent must be > 0, was: " + maxConcurrent)
+
+      // One is taken by inner stream read.
+      val concurrency = if (maxConcurrent == Int.MaxValue) Int.MaxValue else maxConcurrent + 1
+      val action =
+        (
+          Semaphore[F2](concurrency.toLong),
+          Channel.bounded[F2, O2](concurrency),
+          Ref[F2].of(none[Either[NonEmptyList[Throwable], Unit]]),
+          Deferred[F2, Unit]
+        ).mapN { (semaphore, channel, result, stopReading) =>
+          val releaseAndCheckCompletion =
+            semaphore.release *>
+              semaphore.available
+                .product(result.get)
+                .flatMap {
+                  case (`concurrency`, Some(_)) => channel.close.void
+                  case _                        => ().pure[F2]
+                }
+
+          val succeed =
+            result.update {
+              case None  => ().asRight.some
+              case other => other
+            }
+
+          val cancelled = stopReading.complete(()) *> succeed
+
+          def failed(ex: Throwable) =
+            stopReading.complete(()) *>
+              result.update {
+                case Some(Left(nel)) => nel.prepend(ex).asLeft.some
+                case _               => NonEmptyList.one(ex).asLeft.some
+              }
+
+          val completeStream =
+            Stream.force {
+              result.get.map {
+                case Some(Left(nel)) => Stream.raiseError[F2](CompositeFailure.fromNel(nel))
+                case _               => Stream.empty
+              }
+            }
+
+          def forkOnElem(el: O): F2[Unit] =
+            semaphore.acquire *>
+              f(el).attempt
+                .race(stopReading.get)
+                .flatMap {
+                  case Left(Left(ex)) => failed(ex)
+                  case Left(Right(a)) => channel.send(a).void
+                  case Right(_)       => ().pure[F2]
+                }
+                .guarantee(releaseAndCheckCompletion)
+                .start
+                .void
+
+          val background =
+            Stream.exec(semaphore.acquire) ++
+              interruptWhen(stopReading.get.map(_.asRight[Throwable]))
+                .foreach(forkOnElem)
+                .onFinalizeCase {
+                  case ExitCase.Succeeded   => succeed *> releaseAndCheckCompletion
+                  case ExitCase.Errored(ex) => failed(ex) *> releaseAndCheckCompletion
+                  case ExitCase.Canceled    => cancelled *> releaseAndCheckCompletion
+                }
+
+          channel.stream.concurrently(background) ++ completeStream
+        }
+
+      Stream.force(action)
+    }
 
   /** Concurrent zip.
     *