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Revamp implementation of parEvalMap, fixing numerous issues and increasing performance #2673

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merged 8 commits into from
Dec 3, 2021
Merged

Revamp implementation of parEvalMap, fixing numerous issues and increasing performance #2673

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66fc1e3
par-eval-map
nikiforo Nov 10, 2021
0c15b22
par-eval-v3
nikiforo Nov 16, 2021
4cb3ad4
parEvalMap
nikiforo Nov 17, 2021
a63b61c
par-eval-v3 - remove unneeded cancellation handling
nikiforo Nov 21, 2021
a769b05
Merge remote-tracking branch 'origin/main' into par-eval-v3
nikiforo Nov 30, 2021
b6000cd
par-eval-v3 - added test on issue-2726
nikiforo Nov 30, 2021
5656020
increase timed interval
nikiforo Dec 1, 2021
6ef5b9d
par-eval-v3 - extracted parEvalMap* tests to a dedicated Suite
nikiforo Dec 3, 2021
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4 changes: 4 additions & 0 deletions benchmark/src/main/scala/fs2/benchmark/ParEvalMapBenchmark.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -40,6 +40,10 @@ class ParEvalMapBenchmark {
def evalMap(): Unit =
execute(getStream.evalMap(_ => dummyLoad))

@Benchmark
def parEvalMap10(): Unit =
execute(getStream.parEvalMap(10)(_ => dummyLoad))

@Benchmark
def parEvalMapUnordered10(): Unit =
execute(getStream.parEvalMapUnordered(10)(_ => dummyLoad))
Expand Down
140 changes: 54 additions & 86 deletions core/shared/src/main/scala/fs2/Stream.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -25,8 +25,8 @@ import scala.annotation.{nowarn, tailrec}
import scala.concurrent.TimeoutException
import scala.concurrent.duration._
import cats.{Eval => _, _}
import cats.data.{Ior, NonEmptyList}
import cats.effect.{Concurrent, SyncIO}
import cats.data.Ior
import cats.effect.{Concurrent, IO, SyncIO}
import cats.effect.kernel._
import cats.effect.kernel.implicits._
import cats.effect.std.{Console, Queue, QueueSink, QueueSource, Semaphore}
Expand Down Expand Up @@ -2054,40 +2054,18 @@ final class Stream[+F[_], +O] private[fs2] (private[fs2] val underlying: Pull[F,
*/
def parEvalMap[F2[x] >: F[x], O2](
maxConcurrent: Int
)(f: O => F2[O2])(implicit F: Concurrent[F2]): Stream[F2, O2] =
if (maxConcurrent === 1) evalMap(f)
else {
val fstream: F2[Stream[F2, O2]] = for {
chan <- Channel.bounded[F2, F2[Either[Throwable, O2]]](maxConcurrent)
chanReadDone <- F.deferred[Unit]
} yield {
def forkOnElem(o: O): F2[Stream[F2, Unit]] =
for {
value <- F.deferred[Either[Throwable, O2]]
send = chan.send(value.get).as {
Stream.eval(f(o).attempt.flatMap(value.complete(_).void))
}
eit <- chanReadDone.get.race(send)
} yield eit match {
case Left(()) => Stream.empty
case Right(stream) => stream
}
)(f: O => F2[O2])(implicit F: Concurrent[F2]): Stream[F2, O2] = {

val background = this
.evalMap(forkOnElem)
.parJoin(maxConcurrent)
.onFinalize(chanReadDone.get.race(chan.close).void)
def init(ch: Channel[F2, F2[Either[Throwable, O2]]], release: F2[Unit]) =
Deferred[F2, Either[Throwable, O2]].flatTap { value =>
ch.send(release *> value.get)
}

val foreground =
chan.stream
.evalMap(identity)
.rethrow
.onFinalize(chanReadDone.complete(()).void)
def send(v: Deferred[F2, Either[Throwable, O2]]) =
(el: Either[Throwable, O2]) => v.complete(el).void

foreground.concurrently(background)
}
Stream.eval(fstream).flatten
}
parEvalMapAction(maxConcurrent, f)((ch, release) => init(ch, release).map(send))
}

/** Like [[Stream#evalMap]], but will evaluate effects in parallel, emitting the results
* downstream. The number of concurrent effects is limited by the `maxConcurrent` parameter.
Expand All @@ -2100,11 +2078,27 @@ final class Stream[+F[_], +O] private[fs2] (private[fs2] val underlying: Pull[F,
* res0: Unit = ()
* }}}
*/
def parEvalMapUnordered[F2[x] >: F[
x
]: Concurrent, O2](
def parEvalMapUnordered[F2[x] >: F[x], O2](
maxConcurrent: Int
)(f: O => F2[O2]): Stream[F2, O2] =
)(f: O => F2[O2])(implicit F: Concurrent[F2]): Stream[F2, O2] = {

val init = ().pure[F2]

def send(ch: Channel[F2, F2[Either[Throwable, O2]]], release: F2[Unit]) =
(el: Either[Throwable, O2]) => release <* ch.send(el.pure[F2])

parEvalMapAction(maxConcurrent, f)((ch, release) => init.as(send(ch, release)))
}

private def parEvalMapAction[F2[x] >: F[x], O2, T](
maxConcurrent: Int,
f: O => F2[O2]
)(
initFork: (
Channel[F2, F2[Either[Throwable, O2]]],
F2[Unit]
) => F2[Either[Throwable, O2] => F2[Unit]]
)(implicit F: Concurrent[F2]): Stream[F2, O2] =
if (maxConcurrent == 1) evalMap(f)
else {
assert(maxConcurrent > 0, "maxConcurrent must be > 0, was: " + maxConcurrent)
Expand All @@ -2114,66 +2108,40 @@ final class Stream[+F[_], +O] private[fs2] (private[fs2] val underlying: Pull[F,
val action =
(
Semaphore[F2](concurrency.toLong),
Channel.bounded[F2, O2](concurrency),
Ref[F2].of(none[Either[NonEmptyList[Throwable], Unit]]),
Channel.bounded[F2, F2[Either[Throwable, O2]]](concurrency),
Deferred[F2, Unit],
Deferred[F2, Unit]
).mapN { (semaphore, channel, result, stopReading) =>
).mapN { (semaphore, channel, stop, end) =>
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
semaphore.available.flatMap {
case `concurrency` => channel.close *> end.complete(()).void
case _ => ().pure[F2]
}
}

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]
F.uncancelable { poll =>
poll(semaphore.acquire) <*
Deferred[F2, Unit].flatMap { pushed =>
val init = initFork(channel, pushed.complete(()).void)
poll(init).onCancel(releaseAndCheckCompletion).flatMap { send =>
val action = f(el).attempt.flatMap(send) *> pushed.get
F.start(stop.get.race(action) *> releaseAndCheckCompletion)
}
}
.guarantee(releaseAndCheckCompletion)
.start
.void
}

val background =
Stream.exec(semaphore.acquire) ++
interruptWhen(stopReading.get.map(_.asRight[Throwable]))
interruptWhen(stop.get.map(_.asRight[Throwable]))
.foreach(forkOnElem)
.onFinalizeCase {
case ExitCase.Succeeded => succeed *> releaseAndCheckCompletion
case ExitCase.Errored(ex) => failed(ex) *> releaseAndCheckCompletion
case ExitCase.Canceled => cancelled *> releaseAndCheckCompletion
case ExitCase.Succeeded => releaseAndCheckCompletion
case _ => stop.complete(()) *> releaseAndCheckCompletion
}

channel.stream.concurrently(background) ++ completeStream
val foreground = channel.stream.evalMap(_.rethrow)
foreground.onFinalize(stop.complete(()) *> end.get).concurrently(background)
}

Stream.force(action)
Expand Down Expand Up @@ -4487,17 +4455,17 @@ object Stream extends StreamLowPriority {
*
* As a quick example, let's write a timed pull which emits the
* string "late!" whenever a chunk of the stream is not emitted
* within 150 milliseconds:
* within 450 milliseconds:
*
* @example {{{
* scala> import cats.effect.IO
* scala> import cats.effect.unsafe.implicits.global
* scala> import scala.concurrent.duration._
* scala> val s = (Stream("elem") ++ Stream.sleep_[IO](200.millis)).repeat.take(3)
* scala> val s = (Stream("elem") ++ Stream.sleep_[IO](600.millis)).repeat.take(3)
* scala> s.pull
* | .timed { timedPull =>
* | def go(timedPull: Pull.Timed[IO, String]): Pull[IO, String, Unit] =
* | timedPull.timeout(150.millis) >> // starts new timeout and stops the previous one
* | timedPull.timeout(450.millis) >> // starts new timeout and stops the previous one
* | timedPull.uncons.flatMap {
* | case Some((Right(elems), next)) => Pull.output(elems) >> go(next)
* | case Some((Left(_), next)) => Pull.output1("late!") >> go(next)
Expand Down
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