diff --git a/akka-bench-jmh-dev/src/main/scala/akka/BenchRunner.scala b/akka-bench-jmh-dev/src/main/scala/akka/BenchRunner.scala
index 184b220786..44e5856ecb 100644
--- a/akka-bench-jmh-dev/src/main/scala/akka/BenchRunner.scala
+++ b/akka-bench-jmh-dev/src/main/scala/akka/BenchRunner.scala
@@ -8,10 +8,11 @@ object BenchRunner {
   def main(args: Array[String]) = {
     import scala.collection.JavaConversions._
 
-    val args2 = args.toList match {
-      case "quick" :: tail => "-i 1 -wi 1 -f1 -t1".split(" ").toList ::: tail
-      case "full" :: tail => "-i 10 -wi 4 -f3 -t1".split(" ").toList ::: tail
-      case other => other
+    val args2 = args.toList.flatMap {
+      case "quick" => "-i 1 -wi 1 -f1 -t1".split(" ").toList
+      case "full" => "-i 10 -wi 4 -f3 -t1".split(" ").toList
+      case "jitwatch" => "-jvmArgs=-XX:+UnlockDiagnosticVMOptions -XX:+TraceClassLoading -XX:+LogCompilation" :: Nil
+      case other => other :: Nil
     }
 
     val opts = new CommandLineOptions(args2: _*)
diff --git a/akka-bench-jmh-dev/src/main/scala/akka/stream/FlowMapBenchmark.scala b/akka-bench-jmh-dev/src/main/scala/akka/stream/FlowMapBenchmark.scala
index 729be6dc1c..8271c61ce5 100644
--- a/akka-bench-jmh-dev/src/main/scala/akka/stream/FlowMapBenchmark.scala
+++ b/akka-bench-jmh-dev/src/main/scala/akka/stream/FlowMapBenchmark.scala
@@ -12,6 +12,7 @@ import org.openjdk.jmh.annotations._
 import scala.concurrent.Lock
 import scala.util.Success
 import akka.stream.impl.fusing.GraphStages
+import org.reactivestreams._
 
 @State(Scope.Benchmark)
 @OutputTimeUnit(TimeUnit.MILLISECONDS)
@@ -25,6 +26,15 @@ class FlowMapBenchmark {
         log-dead-letters-during-shutdown = off
         loglevel = "WARNING"
 
+        actor.default-dispatcher {
+          #executor = "thread-pool-executor"
+          throughput = 1024
+        }
+
+        actor.default-mailbox {
+          mailbox-type = "akka.dispatch.SingleConsumerOnlyUnboundedMailbox"
+        }
+
         test {
           timefactor =  1.0
           filter-leeway = 3s
@@ -40,10 +50,8 @@ class FlowMapBenchmark {
 
   var materializer: ActorMaterializer = _
 
-  final val UseGraphStageIdentity = false
-
-  // manual, and not via @Param, because we want @OperationsPerInvocation on our tests
-  final val data100k = (1 to 100000).toVector
+  @Param(Array("true", "false"))
+  val UseGraphStageIdentity = false
 
   final val successMarker = Success(1)
   final val successFailure = Success(new Exception)
@@ -51,7 +59,7 @@ class FlowMapBenchmark {
   // safe to be benchmark scoped because the flows we construct in this bench are stateless
   var flow: Source[Int, Unit] = _
 
-  @Param(Array("2", "8")) // todo
+  @Param(Array("8", "32", "128"))
   val initialInputBufferSize = 0
 
   @Param(Array("1", "5", "10"))
@@ -60,11 +68,38 @@ class FlowMapBenchmark {
   @Setup
   def setup() {
     val settings = ActorMaterializerSettings(system)
-      .withInputBuffer(initialInputBufferSize, 16)
+      .withInputBuffer(initialInputBufferSize, initialInputBufferSize)
 
     materializer = ActorMaterializer(settings)
 
-    flow = mkMaps(Source(data100k), numberOfMapOps) {
+    // Important to use a synchronous, zero overhead source, otherwise the slowness of the source
+    // might bias the benchmark, since the stream always adjusts the rate to the slowest stage.
+    val syncTestPublisher = new Publisher[Int] {
+      override def subscribe(s: Subscriber[_ >: Int]): Unit = {
+        val sub = new Subscription {
+          var counter = 0 // Piggyback on caller thread, no need for volatile
+
+          override def request(n: Long): Unit = {
+            var i = n
+            while (i > 0) {
+              s.onNext(counter)
+              counter += 1
+              if (counter == 100000) {
+                s.onComplete()
+                return
+              }
+              i -= 1
+            }
+          }
+
+          override def cancel(): Unit = ()
+        }
+
+        s.onSubscribe(sub)
+      }
+    }
+
+    flow = mkMaps(Source(syncTestPublisher), numberOfMapOps) {
       if (UseGraphStageIdentity)
         new GraphStages.Identity[Int]
       else
diff --git a/akka-bench-jmh-dev/src/main/scala/akka/stream/InterpreterBenchmark.scala b/akka-bench-jmh-dev/src/main/scala/akka/stream/InterpreterBenchmark.scala
index 370a54f7a2..c3af314393 100644
--- a/akka-bench-jmh-dev/src/main/scala/akka/stream/InterpreterBenchmark.scala
+++ b/akka-bench-jmh-dev/src/main/scala/akka/stream/InterpreterBenchmark.scala
@@ -51,9 +51,13 @@ class InterpreterBenchmark {
   @Benchmark
   @OperationsPerInvocation(100000)
   def onebounded_interpreter_100k_elements() {
+    val lock = new Lock()
+    lock.acquire()
     val sink = OneBoundedDataSink(data100k.size)
-    // FIXME: This should not be here, this is pure setup overhead
-    val ops = Vector.fill(numberOfIds)(MapStage(identity[Int], Supervision.stoppingDecider))
+    val ops = Vector.fill(numberOfIds)(new PushPullStage[Int, Int] {
+      override def onPull(ctx: _root_.akka.stream.stage.Context[Int]) = ctx.pull()
+      override def onPush(elem: Int, ctx: _root_.akka.stream.stage.Context[Int]) = ctx.push(elem)
+    })
     val interpreter = new OneBoundedInterpreter(OneBoundedDataSource(data100k) +: ops :+ sink,
       (op, ctx, event) ⇒ (),
       Logging(NoopBus, classOf[InterpreterBenchmark]),
@@ -70,6 +74,7 @@ object InterpreterBenchmark {
   case class GraphDataSource[T](override val toString: String, data: Vector[T]) extends UpstreamBoundaryStageLogic[T] {
     var idx = 0
     val out = Outlet[T]("out")
+    out.id = 0
 
     setHandler(out, new OutHandler {
       override def onPull(): Unit = {
@@ -87,6 +92,7 @@ object InterpreterBenchmark {
 
   case class GraphDataSink[T](override val toString: String, var expected: Int) extends DownstreamBoundaryStageLogic[T] {
     val in = Inlet[T]("in")
+    in.id = 0
 
     setHandler(in, new InHandler {
       override def onPush(): Unit = {
diff --git a/akka-http-core/src/main/scala/akka/http/impl/util/package.scala b/akka-http-core/src/main/scala/akka/http/impl/util/package.scala
index 4d90d50aa3..de25d0ee5b 100644
--- a/akka-http-core/src/main/scala/akka/http/impl/util/package.scala
+++ b/akka-http-core/src/main/scala/akka/http/impl/util/package.scala
@@ -138,7 +138,7 @@ package util {
     val out = Outlet[HttpEntity.Strict]("out")
     override val shape = FlowShape(in, out)
 
-    override def createLogic: GraphStageLogic = new GraphStageLogic {
+    override def createLogic: GraphStageLogic = new GraphStageLogic(shape) {
       var bytes = ByteString.newBuilder
       private var emptyStream = false
 
diff --git a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/ActorGraphInterpreterSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/ActorGraphInterpreterSpec.scala
index c204319cb2..3a761595aa 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/ActorGraphInterpreterSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/ActorGraphInterpreterSpec.scala
@@ -47,7 +47,7 @@ class ActorGraphInterpreterSpec extends AkkaSpec {
         val out2 = Outlet[Int]("out2")
         val shape = BidiShape(in1, out1, in2, out2)
 
-        override def createLogic: GraphStageLogic = new GraphStageLogic {
+        override def createLogic: GraphStageLogic = new GraphStageLogic(shape) {
           setHandler(in1, new InHandler {
             override def onPush(): Unit = push(out1, grab(in1))
             override def onUpstreamFinish(): Unit = complete(out1)
@@ -88,7 +88,7 @@ class ActorGraphInterpreterSpec extends AkkaSpec {
         val out2 = Outlet[Int]("out2")
         val shape = BidiShape(in1, out1, in2, out2)
 
-        override def createLogic: GraphStageLogic = new GraphStageLogic {
+        override def createLogic: GraphStageLogic = new GraphStageLogic(shape) {
           setHandler(in1, new InHandler {
             override def onPush(): Unit = push(out1, grab(in1))
 
@@ -134,7 +134,7 @@ class ActorGraphInterpreterSpec extends AkkaSpec {
         val out2 = Outlet[Int]("out2")
         val shape = BidiShape(in1, out1, in2, out2)
 
-        override def createLogic: GraphStageLogic = new GraphStageLogic {
+        override def createLogic: GraphStageLogic = new GraphStageLogic(shape) {
           setHandler(in1, new InHandler {
             override def onPush(): Unit = push(out1, grab(in1))
 
@@ -183,7 +183,7 @@ class ActorGraphInterpreterSpec extends AkkaSpec {
         val out2 = Outlet[Int]("out2")
         val shape = BidiShape(in1, out1, in2, out2)
 
-        override def createLogic: GraphStageLogic = new GraphStageLogic {
+        override def createLogic: GraphStageLogic = new GraphStageLogic(shape) {
           setHandler(in1, new InHandler {
             override def onPush(): Unit = push(out2, grab(in1))
 
diff --git a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterFailureModesSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterFailureModesSpec.scala
new file mode 100644
index 0000000000..a1b0d01f59
--- /dev/null
+++ b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterFailureModesSpec.scala
@@ -0,0 +1,110 @@
+package akka.stream.impl.fusing
+
+import akka.stream.testkit.Utils.TE
+import akka.testkit.EventFilter
+
+class GraphInterpreterFailureModesSpec extends GraphInterpreterSpecKit {
+
+  "GraphInterpreter" must {
+
+    "handle failure on onPull" in new FailingStageSetup {
+      lastEvents() should be(Set(PreStart(stage)))
+
+      downstream.pull()
+      failOnNextEvent()
+      stepAll()
+
+      lastEvents() should be(Set(Cancel(upstream), OnError(downstream, testException), PostStop(stage)))
+    }
+
+    "handle failure on onPush" in new FailingStageSetup {
+      lastEvents() should be(Set(PreStart(stage)))
+
+      downstream.pull()
+      stepAll()
+      clearEvents()
+      upstream.push(0)
+      failOnNextEvent()
+      stepAll()
+
+      lastEvents() should be(Set(Cancel(upstream), OnError(downstream, testException), PostStop(stage)))
+    }
+
+    "handle failure on onPull while cancel is pending" in new FailingStageSetup {
+      lastEvents() should be(Set(PreStart(stage)))
+
+      downstream.pull()
+      downstream.cancel()
+      failOnNextEvent()
+      stepAll()
+
+      lastEvents() should be(Set(Cancel(upstream), PostStop(stage)))
+    }
+
+    "handle failure on onPush while complete is pending" in new FailingStageSetup {
+      lastEvents() should be(Set(PreStart(stage)))
+
+      downstream.pull()
+      stepAll()
+      clearEvents()
+      upstream.push(0)
+      upstream.complete()
+      failOnNextEvent()
+      stepAll()
+
+      lastEvents() should be(Set(OnError(downstream, testException), PostStop(stage)))
+    }
+
+    "handle failure on onUpstreamFinish" in new FailingStageSetup {
+      lastEvents() should be(Set(PreStart(stage)))
+
+      upstream.complete()
+      failOnNextEvent()
+      stepAll()
+
+      lastEvents() should be(Set(OnError(downstream, testException), PostStop(stage)))
+    }
+
+    "handle failure on onUpstreamFailure" in new FailingStageSetup {
+      lastEvents() should be(Set(PreStart(stage)))
+
+      upstream.fail(TE("another exception")) // this is not the exception that will be propagated
+      failOnNextEvent()
+      stepAll()
+
+      lastEvents() should be(Set(OnError(downstream, testException), PostStop(stage)))
+    }
+
+    "handle failure on onDownstreamFinish" in new FailingStageSetup {
+      lastEvents() should be(Set(PreStart(stage)))
+
+      downstream.cancel()
+      failOnNextEvent()
+      stepAll()
+
+      lastEvents() should be(Set(Cancel(upstream), PostStop(stage)))
+    }
+
+    "handle failure in preStart" in new FailingStageSetup(initFailOnNextEvent = true) {
+      stepAll()
+
+      lastEvents() should be(Set(Cancel(upstream), OnError(downstream, testException), PostStop(stage)))
+    }
+
+    "handle failure in postStop" in new FailingStageSetup {
+      lastEvents() should be(Set(PreStart(stage)))
+
+      upstream.complete()
+      downstream.cancel()
+      failOnPostStop()
+
+      EventFilter.error("Error during postStop in [stage]").intercept {
+        stepAll()
+        lastEvents() should be(Set.empty)
+      }
+
+    }
+
+  }
+
+}
diff --git a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterPortsSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterPortsSpec.scala
index d32c98146c..88a9a1a527 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterPortsSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterPortsSpec.scala
@@ -4,11 +4,14 @@
 package akka.stream.impl.fusing
 
 import akka.stream.testkit.AkkaSpec
+import akka.stream.testkit.Utils._
 
-class GraphInterpreterPortsSpec extends AkkaSpec with GraphInterpreterSpecKit {
+class GraphInterpreterPortsSpec extends GraphInterpreterSpecKit {
 
   "Port states" must {
 
+    // FIXME test failure scenarios
+
     "properly transition on push and pull" in new PortTestSetup {
       lastEvents() should be(Set.empty)
       out.isAvailable should be(false)
@@ -352,6 +355,32 @@ class GraphInterpreterPortsSpec extends AkkaSpec with GraphInterpreterSpecKit {
       in.grab() should ===(0)
     }
 
+    "propagate complete while push is in flight and pulled after the push" in new PortTestSetup {
+      in.pull()
+      stepAll()
+      clearEvents()
+
+      out.push(0)
+      out.complete()
+      step()
+
+      lastEvents() should be(Set(OnNext(in, 0)))
+      in.grab() should ===(0)
+
+      in.pull()
+      stepAll()
+
+      lastEvents() should be(Set(OnComplete(in)))
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+    }
+
     "ignore pull while completing" in new PortTestSetup {
       out.complete()
       in.pull()
@@ -754,6 +783,392 @@ class GraphInterpreterPortsSpec extends AkkaSpec with GraphInterpreterSpecKit {
       an[IllegalArgumentException] should be thrownBy { in.grab() }
     }
 
+    "not allow to grab element before it arrives" in new PortTestSetup {
+      in.pull()
+      stepAll()
+      out.push(0)
+
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+    }
+
+    "not allow to grab element if already cancelled" in new PortTestSetup {
+      in.pull()
+      stepAll()
+
+      out.push(0)
+      in.cancel()
+
+      stepAll()
+
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+    }
+
+    "propagate failure while downstream is active" in new PortTestSetup {
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(false)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(false)
+
+      out.fail(TE("test"))
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(false)
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+
+      stepAll()
+
+      lastEvents() should be(Set(OnError(in, TE("test"))))
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+
+      in.cancel() // This should have no effect now
+      stepAll()
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+
+      out.complete() // This should have no effect now
+      stepAll()
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+    }
+
+    "propagate failure while upstream is active" in new PortTestSetup {
+      in.pull()
+      stepAll()
+
+      lastEvents() should be(Set(RequestOne(out)))
+      out.isAvailable should be(true)
+      out.isClosed should be(false)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(true)
+      in.isClosed should be(false)
+
+      out.fail(TE("test"))
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(true)
+      in.isClosed should be(false)
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+
+      stepAll()
+
+      lastEvents() should be(Set(OnError(in, TE("test"))))
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+
+      in.cancel() // This should have no effect now
+      stepAll()
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+
+      out.complete() // This should have no effect now
+      stepAll()
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+
+    }
+
+    "propagate failure while pull is in flight" in new PortTestSetup {
+      in.pull()
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(false)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(true)
+      in.isClosed should be(false)
+
+      out.fail(TE("test"))
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(true)
+      in.isClosed should be(false)
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+
+      stepAll()
+
+      lastEvents() should be(Set(OnError(in, TE("test"))))
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+
+      in.cancel() // This should have no effect now
+      stepAll()
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+
+      out.complete() // This should have no effect now
+      stepAll()
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+    }
+
+    "propagate failure while push is in flight" in new PortTestSetup {
+      in.pull()
+      stepAll()
+      clearEvents()
+
+      out.push(0)
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(false)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(true)
+      in.isClosed should be(false)
+
+      out.fail(TE("test"))
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(true)
+      in.isClosed should be(false)
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+
+      step()
+
+      lastEvents() should be(Set(OnNext(in, 0)))
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(true)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(false)
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      in.grab() should ===(0)
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+
+      step()
+
+      lastEvents() should be(Set(OnError(in, TE("test"))))
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+
+      out.complete() // This should have no effect now
+      stepAll()
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+    }
+
+    "propagate failure while push is in flight and keep ungrabbed element" in new PortTestSetup {
+      in.pull()
+      stepAll()
+      clearEvents()
+
+      out.push(0)
+      out.fail(TE("test"))
+      step()
+
+      lastEvents() should be(Set(OnNext(in, 0)))
+      step()
+
+      lastEvents() should be(Set(OnError(in, TE("test"))))
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(true)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      in.grab() should ===(0)
+    }
+
+    "ignore pull while failing" in new PortTestSetup {
+      out.fail(TE("test"))
+      in.pull()
+
+      stepAll()
+
+      lastEvents() should be(Set(OnError(in, TE("test"))))
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+    }
+
+    "ignore any failure completion if they are concurrent (cancel first)" in new PortTestSetup {
+      in.cancel()
+      out.fail(TE("test"))
+
+      stepAll()
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+    }
+
+    "ignore any failure completion if they are concurrent (complete first)" in new PortTestSetup {
+      out.fail(TE("test"))
+      in.cancel()
+
+      stepAll()
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+    }
+
+    "ignore failure from a push-then-fail if cancelled while in flight" in new PortTestSetup {
+      in.pull()
+      stepAll()
+      clearEvents()
+
+      out.push(0)
+      out.fail(TE("test"))
+      in.cancel()
+
+      stepAll()
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+    }
+
+    "ignore failure from a push-then-fail if cancelled after onPush" in new PortTestSetup {
+      in.pull()
+      stepAll()
+      clearEvents()
+
+      out.push(0)
+      out.fail(TE("test"))
+
+      step()
+
+      lastEvents() should be(Set(OnNext(in, 0)))
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(true)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(false)
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      in.grab() should ===(0)
+
+      in.cancel()
+      stepAll()
+
+      lastEvents() should be(Set.empty)
+      out.isAvailable should be(false)
+      out.isClosed should be(true)
+      in.isAvailable should be(false)
+      in.hasBeenPulled should be(false)
+      in.isClosed should be(true)
+      an[IllegalArgumentException] should be thrownBy { in.pull() }
+      an[IllegalArgumentException] should be thrownBy { out.push(0) }
+      an[IllegalArgumentException] should be thrownBy { in.grab() }
+    }
+
   }
 
 }
diff --git a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpec.scala
index ae3944d14a..fed8396499 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpec.scala
@@ -7,7 +7,7 @@ import akka.stream.testkit.AkkaSpec
 import akka.stream.scaladsl.{ Merge, Broadcast, Balance, Zip }
 import GraphInterpreter._
 
-class GraphInterpreterSpec extends AkkaSpec with GraphInterpreterSpecKit {
+class GraphInterpreterSpec extends GraphInterpreterSpecKit {
   import GraphStages._
 
   "GraphInterpreter" must {
@@ -273,8 +273,6 @@ class GraphInterpreterSpec extends AkkaSpec with GraphInterpreterSpecKit {
       lastEvents() should ===(Set(OnNext(sink2, 2)))
     }
 
-    "implement bidi-stage" in pending
-
     "implement non-divergent cycle" in new TestSetup {
       val source = new UpstreamProbe[Int]("source")
       val sink = new DownstreamProbe[Int]("sink")
diff --git a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpecKit.scala b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpecKit.scala
index d1b40d3324..273583ba91 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpecKit.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpecKit.scala
@@ -3,11 +3,14 @@
  */
 package akka.stream.impl.fusing
 
-import akka.stream.{ NoMaterializer, Outlet, Inlet, Shape }
-import akka.stream.impl.fusing.GraphInterpreter.{ GraphAssembly, DownstreamBoundaryStageLogic, UpstreamBoundaryStageLogic }
+import akka.event.Logging
+import akka.stream._
+import akka.stream.impl.fusing.GraphInterpreter.{ Failed, GraphAssembly, DownstreamBoundaryStageLogic, UpstreamBoundaryStageLogic }
 import akka.stream.stage.{ InHandler, OutHandler, GraphStage, GraphStageLogic }
+import akka.stream.testkit.AkkaSpec
+import akka.stream.testkit.Utils.TE
 
-trait GraphInterpreterSpecKit {
+trait GraphInterpreterSpecKit extends AkkaSpec {
 
   sealed trait TestEvent {
     def source: GraphStageLogic
@@ -20,11 +23,17 @@ trait GraphInterpreterSpecKit {
   case class RequestOne(source: GraphStageLogic) extends TestEvent
   case class RequestAnother(source: GraphStageLogic) extends TestEvent
 
+  case class PreStart(source: GraphStageLogic) extends TestEvent
+  case class PostStop(source: GraphStageLogic) extends TestEvent
+
   abstract class TestSetup {
     protected var lastEvent: Set[TestEvent] = Set.empty
     private var _interpreter: GraphInterpreter = _
     protected def interpreter: GraphInterpreter = _interpreter
 
+    def stepAll(): Unit = interpreter.execute(eventLimit = Int.MaxValue)
+    def step(): Unit = interpreter.execute(eventLimit = 1)
+
     class AssemblyBuilder(stages: Seq[GraphStage[_ <: Shape]]) {
       var upstreams = Vector.empty[(UpstreamBoundaryStageLogic[_], Inlet[_])]
       var downstreams = Vector.empty[(Outlet[_], DownstreamBoundaryStageLogic[_])]
@@ -59,7 +68,7 @@ trait GraphInterpreterSpecKit {
           (Vector.fill(upstreams.size)(-1) ++ outOwners).toArray)
 
         val (inHandlers, outHandlers, logics, _) = assembly.materialize()
-        _interpreter = new GraphInterpreter(assembly, NoMaterializer, inHandlers, outHandlers, logics, (_, _, _) ⇒ ())
+        _interpreter = new GraphInterpreter(assembly, NoMaterializer, Logging(system, classOf[TestSetup]), inHandlers, outHandlers, logics, (_, _, _) ⇒ ())
 
         for ((upstream, i) ← upstreams.zipWithIndex) {
           _interpreter.attachUpstreamBoundary(i, upstream._1)
@@ -75,7 +84,7 @@ trait GraphInterpreterSpecKit {
 
     def manualInit(assembly: GraphAssembly): Unit = {
       val (inHandlers, outHandlers, logics, _) = assembly.materialize()
-      _interpreter = new GraphInterpreter(assembly, NoMaterializer, inHandlers, outHandlers, logics, (_, _, _) ⇒ ())
+      _interpreter = new GraphInterpreter(assembly, NoMaterializer, Logging(system, classOf[TestSetup]), inHandlers, outHandlers, logics, (_, _, _) ⇒ ())
     }
 
     def builder(stages: GraphStage[_ <: Shape]*): AssemblyBuilder = new AssemblyBuilder(stages.toSeq)
@@ -90,10 +99,11 @@ trait GraphInterpreterSpecKit {
 
     class UpstreamProbe[T](override val toString: String) extends UpstreamBoundaryStageLogic[T] {
       val out = Outlet[T]("out")
+      out.id = 0
 
       setHandler(out, new OutHandler {
         override def onPull(): Unit = lastEvent += RequestOne(UpstreamProbe.this)
-        override def onDownstreamFinish() = lastEvent += Cancel(UpstreamProbe.this)
+        override def onDownstreamFinish(): Unit = lastEvent += Cancel(UpstreamProbe.this)
       })
 
       def onNext(elem: T, eventLimit: Int = Int.MaxValue): Unit = {
@@ -105,11 +115,12 @@ trait GraphInterpreterSpecKit {
 
     class DownstreamProbe[T](override val toString: String) extends DownstreamBoundaryStageLogic[T] {
       val in = Inlet[T]("in")
+      in.id = 0
 
       setHandler(in, new InHandler {
         override def onPush(): Unit = lastEvent += OnNext(DownstreamProbe.this, grab(in))
-        override def onUpstreamFinish() = lastEvent += OnComplete(DownstreamProbe.this)
-        override def onUpstreamFailure(ex: Throwable) = OnError(DownstreamProbe.this, ex)
+        override def onUpstreamFinish(): Unit = lastEvent += OnComplete(DownstreamProbe.this)
+        override def onUpstreamFailure(ex: Throwable): Unit = lastEvent += OnError(DownstreamProbe.this, ex)
       })
 
       def requestOne(eventLimit: Int = Int.MaxValue): Unit = {
@@ -146,20 +157,20 @@ trait GraphInterpreterSpecKit {
       setHandler(in, new InHandler {
 
         // Modified onPush that does not grab() automatically the element. This accesses some internals.
-        override def onPush(): Unit =
-          lastEvent +=
-            OnNext(
-              DownstreamPortProbe.this,
-              interpreter.connectionStates(inToConn(in)))
+        override def onPush(): Unit = {
+          val internalEvent = interpreter.connectionSlots(inToConn(in.id))
+
+          internalEvent match {
+            case Failed(_, elem) ⇒ lastEvent += OnNext(DownstreamPortProbe.this, elem)
+            case elem            ⇒ lastEvent += OnNext(DownstreamPortProbe.this, elem)
+          }
+        }
 
         override def onUpstreamFinish() = lastEvent += OnComplete(DownstreamPortProbe.this)
-        override def onUpstreamFailure(ex: Throwable) = OnError(DownstreamPortProbe.this, ex)
+        override def onUpstreamFailure(ex: Throwable) = lastEvent += OnError(DownstreamPortProbe.this, ex)
       })
     }
 
-    def stepAll(): Unit = interpreter.execute(eventLimit = Int.MaxValue)
-    def step(): Unit = interpreter.execute(eventLimit = 1)
-
     private val assembly = GraphAssembly(
       stages = Array.empty,
       ins = Array(null),
@@ -173,4 +184,73 @@ trait GraphInterpreterSpecKit {
     interpreter.init()
   }
 
+  abstract class FailingStageSetup(initFailOnNextEvent: Boolean = false) extends TestSetup {
+
+    val upstream = new UpstreamPortProbe[Int]
+    val downstream = new DownstreamPortProbe[Int]
+
+    private var _failOnNextEvent: Boolean = initFailOnNextEvent
+    private var _failOnPostStop: Boolean = false
+
+    def failOnNextEvent(): Unit = _failOnNextEvent = true
+    def failOnPostStop(): Unit = _failOnPostStop = true
+
+    def testException = TE("test")
+
+    private val stagein = Inlet[Int]("sandwitch.in")
+    private val stageout = Outlet[Int]("sandwitch.out")
+    private val stageshape = FlowShape(stagein, stageout)
+
+    // Must be lazy because I turned this stage "inside-out" therefore changing initialization order
+    // to make tests a bit more readable
+    lazy val stage: GraphStageLogic = new GraphStageLogic(stageshape) {
+      private def mayFail(task: ⇒ Unit): Unit = {
+        if (!_failOnNextEvent) task
+        else {
+          _failOnNextEvent = false
+          throw testException
+        }
+      }
+
+      setHandler(stagein, new InHandler {
+        override def onPush(): Unit = mayFail(push(stageout, grab(stagein)))
+        override def onUpstreamFinish(): Unit = mayFail(completeStage())
+        override def onUpstreamFailure(ex: Throwable): Unit = mayFail(failStage(ex))
+      })
+
+      setHandler(stageout, new OutHandler {
+        override def onPull(): Unit = mayFail(pull(stagein))
+        override def onDownstreamFinish(): Unit = mayFail(completeStage())
+      })
+
+      override def preStart(): Unit = mayFail(lastEvent += PreStart(stage))
+      override def postStop(): Unit =
+        if (!_failOnPostStop) lastEvent += PostStop(stage)
+        else throw testException
+
+      override def toString = "stage"
+    }
+
+    private val sandwitchStage = new GraphStage[FlowShape[Int, Int]] {
+      override def shape = stageshape
+      override def createLogic: GraphStageLogic = stage
+    }
+
+    class UpstreamPortProbe[T] extends UpstreamProbe[T]("upstreamPort") {
+      def push(elem: T): Unit = push(out, elem)
+      def complete(): Unit = complete(out)
+      def fail(ex: Throwable): Unit = fail(out, ex)
+    }
+
+    class DownstreamPortProbe[T] extends DownstreamProbe[T]("upstreamPort") {
+      def pull(): Unit = pull(in)
+      def cancel(): Unit = cancel(in)
+    }
+
+    builder(sandwitchStage)
+      .connect(upstream, stagein)
+      .connect(stageout, downstream)
+      .init()
+  }
+
 }
diff --git a/akka-stream/src/main/boilerplate/akka/stream/scaladsl/UnzipWithApply.scala.template b/akka-stream/src/main/boilerplate/akka/stream/scaladsl/UnzipWithApply.scala.template
index 80061988f9..10ac47fa63 100644
--- a/akka-stream/src/main/boilerplate/akka/stream/scaladsl/UnzipWithApply.scala.template
+++ b/akka-stream/src/main/boilerplate/akka/stream/scaladsl/UnzipWithApply.scala.template
@@ -49,7 +49,7 @@ class UnzipWith1[In, [#A1#]](unzipper: In ⇒ ([#A1#])) extends GraphStage[FanOu
   [#def out0: Outlet[A1] = shape.out0#
   ]
 
-  override def createLogic: GraphStageLogic = new GraphStageLogic {
+  override def createLogic: GraphStageLogic = new GraphStageLogic(shape) {
     var pendingCount = 1
     var downstreamRunning = 1
 
diff --git a/akka-stream/src/main/boilerplate/akka/stream/scaladsl/ZipWithApply.scala.template b/akka-stream/src/main/boilerplate/akka/stream/scaladsl/ZipWithApply.scala.template
index 672dbb2466..9f40ba4c8b 100644
--- a/akka-stream/src/main/boilerplate/akka/stream/scaladsl/ZipWithApply.scala.template
+++ b/akka-stream/src/main/boilerplate/akka/stream/scaladsl/ZipWithApply.scala.template
@@ -30,7 +30,7 @@ class ZipWith1[[#A1#], O] (zipper: ([#A1#]) ⇒ O) extends GraphStage[FanInShape
   [#val in0: Inlet[A1] = shape.in0#
   ]
 
-  override def createLogic: GraphStageLogic = new GraphStageLogic {
+  override def createLogic: GraphStageLogic = new GraphStageLogic(shape) {
     var pending = 1
 
     private def pushAll(): Unit = push(out, zipper([#grab(in0)#]))
diff --git a/akka-stream/src/main/scala/akka/stream/Shape.scala b/akka-stream/src/main/scala/akka/stream/Shape.scala
index 63101e5689..a7ee5ffeee 100644
--- a/akka-stream/src/main/scala/akka/stream/Shape.scala
+++ b/akka-stream/src/main/scala/akka/stream/Shape.scala
@@ -15,8 +15,13 @@ import scala.collection.JavaConverters._
  * for otherwise unreasonable existential types.
  */
 sealed abstract class InPort { self: Inlet[_] ⇒
-  final override def hashCode: Int = System.identityHashCode(this)
+  final override def hashCode: Int = super.hashCode
   final override def equals(that: Any): Boolean = this eq that.asInstanceOf[AnyRef]
+
+  /**
+   * INTERNAL API
+   */
+  private[stream] var id: Int = -1
 }
 /**
  * An output port of a StreamLayout.Module. This type logically belongs
@@ -25,8 +30,13 @@ sealed abstract class InPort { self: Inlet[_] ⇒
  * for otherwise unreasonable existential types.
  */
 sealed abstract class OutPort { self: Outlet[_] ⇒
-  final override def hashCode: Int = System.identityHashCode(this)
+  final override def hashCode: Int = super.hashCode
   final override def equals(that: Any): Boolean = this eq that.asInstanceOf[AnyRef]
+
+  /**
+   * INTERNAL API
+   */
+  private[stream] var id: Int = -1
 }
 
 /**
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/ActorGraphInterpreter.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/ActorGraphInterpreter.scala
index bc41e2492a..2799439985 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/ActorGraphInterpreter.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/ActorGraphInterpreter.scala
@@ -100,6 +100,7 @@ private[stream] object ActorGraphInterpreter {
     private var batchRemaining = requestBatchSize
 
     val out: Outlet[Any] = Outlet[Any]("UpstreamBoundary" + id)
+    out.id = 0
 
     private def dequeue(): Any = {
       val elem = inputBuffer(nextInputElementCursor)
@@ -195,6 +196,7 @@ private[stream] object ActorGraphInterpreter {
 
   class ActorOutputBoundary(actor: ActorRef, id: Int) extends DownstreamBoundaryStageLogic[Any] {
     val in: Inlet[Any] = Inlet[Any]("UpstreamBoundary" + id)
+    in.id = 0
 
     private var exposedPublisher: ActorPublisher[Any] = _
 
@@ -299,6 +301,7 @@ private[stream] class ActorGraphInterpreter(
   val interpreter = new GraphInterpreter(
     assembly,
     mat,
+    Logging(this),
     inHandlers,
     outHandlers,
     logics,
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphInterpreter.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphInterpreter.scala
index 84d472bdc2..611ddfd39e 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphInterpreter.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphInterpreter.scala
@@ -3,6 +3,8 @@
  */
 package akka.stream.impl.fusing
 
+import akka.event.LoggingAdapter
+import akka.io.Tcp.Closed
 import akka.stream.stage._
 import akka.stream.{ Materializer, Shape, Inlet, Outlet }
 
@@ -19,37 +21,35 @@ private[stream] object GraphInterpreter {
    */
   final val Debug = false
 
+  final val NoEvent = -1
+  final val Boundary = -1
+
+  final val InReady = 1
+  final val Pulling = 2
+  final val Pushing = 4
+  final val OutReady = 8
+
+  final val InClosed = 16
+  final val OutClosed = 32
+  final val InFailed = 64
+
+  final val PullStartFlip = 3 // 0011
+  final val PullEndFlip = 10 // 1010
+  final val PushStartFlip = 12 //1100
+  final val PushEndFlip = 5 //0101
+
   /**
    * Marker object that indicates that a port holds no element since it was already grabbed. The port is still pullable,
    * but there is no more element to grab.
    */
   case object Empty
+  final case class Failed(ex: Throwable, previousElem: Any)
 
-  sealed trait ConnectionState
-  case object Pulled extends ConnectionState
-
-  sealed trait HasElementState
-
-  sealed trait CompletingState extends ConnectionState
-  final case class CompletedHasElement(element: Any) extends CompletingState with HasElementState
-  final case class PushCompleted(element: Any) extends CompletingState with HasElementState
-  case object Completed extends CompletingState
-  case object Cancelled extends CompletingState
-  final case class Failed(ex: Throwable) extends CompletingState
-
-  val NoEvent = -1
-  val Boundary = -1
-
-  sealed trait PortState
-  case object InFlight extends PortState
-  case object Available extends PortState
-  case object Closed extends PortState
-
-  abstract class UpstreamBoundaryStageLogic[T] extends GraphStageLogic {
+  abstract class UpstreamBoundaryStageLogic[T] extends GraphStageLogic(inCount = 0, outCount = 1) {
     def out: Outlet[T]
   }
 
-  abstract class DownstreamBoundaryStageLogic[T] extends GraphStageLogic {
+  abstract class DownstreamBoundaryStageLogic[T] extends GraphStageLogic(inCount = 1, outCount = 0) {
     def in: Inlet[T]
   }
 
@@ -107,27 +107,52 @@ private[stream] object GraphInterpreter {
       val logics = Array.ofDim[GraphStageLogic](stages.length)
       var finalMat: Any = ()
 
-      for (i ← stages.indices) {
-        // FIXME: Support for materialized values in fused islands is not yet figured out!
-        val (logic, mat) = stages(i).createLogicAndMaterializedValue
-        // FIXME: Current temporary hack to support non-fused stages. If there is one stage that will be under index 0.
-        if (i == 0) finalMat = mat
+      var i = 0
+      while (i < stages.length) {
+        // Port initialization loops, these must come first
+        val shape = stages(i).asInstanceOf[GraphStageWithMaterializedValue[Shape, _]].shape
 
-        logics(i) = logic
+        var idx = 0
+        val inletItr = shape.inlets.iterator
+        while (inletItr.hasNext) {
+          val inlet = inletItr.next()
+          require(inlet.id == -1 || inlet.id == idx, s"Inlet $inlet was shared among multiple stages. This is illegal.")
+          inlet.id = idx
+          idx += 1
+        }
+
+        idx = 0
+        val outletItr = shape.outlets.iterator
+        while (outletItr.hasNext) {
+          val outlet = outletItr.next()
+          require(outlet.id == -1 || outlet.id == idx, s"Outlet $outlet was shared among multiple stages. This is illegal.")
+          outlet.id = idx
+          idx += 1
+        }
+
+        // FIXME: Support for materialized values in fused islands is not yet figured out!
+        val logicAndMat = stages(i).createLogicAndMaterializedValue
+        // FIXME: Current temporary hack to support non-fused stages. If there is one stage that will be under index 0.
+        if (i == 0) finalMat = logicAndMat._2
+
+        logics(i) = logicAndMat._1
+        i += 1
       }
 
       val inHandlers = Array.ofDim[InHandler](connectionCount)
       val outHandlers = Array.ofDim[OutHandler](connectionCount)
 
-      for (i ← 0 until connectionCount) {
+      i = 0
+      while (i < connectionCount) {
         if (ins(i) ne null) {
-          inHandlers(i) = logics(inOwners(i)).inHandlers(ins(i))
-          logics(inOwners(i)).inToConn += ins(i) -> i
+          inHandlers(i) = logics(inOwners(i)).inHandlers(ins(i).id)
+          logics(inOwners(i)).inToConn(ins(i).id) = i
         }
         if (outs(i) ne null) {
-          outHandlers(i) = logics(outOwners(i)).outHandlers(outs(i))
-          logics(outOwners(i)).outToConn += outs(i) -> i
+          outHandlers(i) = logics(outOwners(i)).outHandlers(outs(i).id)
+          logics(outOwners(i)).outToConn(outs(i).id) = i
         }
+        i += 1
       }
 
       (inHandlers, outHandlers, logics, finalMat)
@@ -168,12 +193,10 @@ private[stream] object GraphInterpreter {
  * connection represents an output-input port pair (an analogue for a connected RS Publisher-Subscriber pair),
  * while in the practical sense a connection is a number which represents slots in certain arrays.
  * In particular
- *  - connectionStates is a mapping from a connection id to a current (or future) state of the connection
- *  - inStates is a mapping from a connection to a [[akka.stream.impl.fusing.GraphInterpreter.PortState]]
- *    that indicates whether the input corresponding
- *    to the connection is currently pullable or completed
- *  - outStates is a mapping from a connection to a [[akka.stream.impl.fusing.GraphInterpreter.PortState]]
- *    that indicates whether the input corresponding to the connection is currently pushable or completed
+ *  - portStates contains a bitfield that tracks the states of the ports (output-input) corresponding to this
+ *    connection. This bitfield is used to decode the event that is in-flight.
+ *  - connectionSlots is a mapping from a connection id to a potential element or exception that accompanies the
+ *    event encoded in the portStates bitfield
  *  - inHandlers is a mapping from a connection id to the [[InHandler]] instance that handles the events corresponding
  *    to the input port of the connection
  *  - outHandlers is a mapping from a connection id to the [[OutHandler]] instance that handles the events corresponding
@@ -181,20 +204,39 @@ private[stream] object GraphInterpreter {
  *
  * On top of these lookup tables there is an eventQueue, represented as a circular buffer of integers. The integers
  * it contains represents connections that have pending events to be processed. The pending event itself is encoded
- * in the connectionStates table. This implies that there can be only one event in flight for a given connection, which
- * is true in almost all cases, except a complete-after-push which is therefore handled with a special event
- * [[GraphInterpreter#PushCompleted]].
+ * in the portStates bitfield. This implies that there can be only one event in flight for a given connection, which
+ * is true in almost all cases, except a complete-after-push or fail-after-push.
+ *
+ * The layout of the portStates bitfield is the following:
+ *
+ *             |- state machn.-| Only one bit is hot among these bits
+ *  64  32  16 | 8   4   2   1 |
+ * +---+---+---|---+---+---+---|
+ *   |   |   |   |   |   |   |
+ *   |   |   |   |   |   |   |  From the following flags only one is active in any given time. These bits encode
+ *   |   |   |   |   |   |   |  state machine states, and they are "moved" around using XOR masks to keep other bits
+ *   |   |   |   |   |   |   |  intact.
+ *   |   |   |   |   |   |   |
+ *   |   |   |   |   |   |   +- InReady:  The input port is ready to be pulled
+ *   |   |   |   |   |   +----- Pulling:  A pull is active, but have not arrived yet (queued)
+ *   |   |   |   |   +--------- Pushing:  A push is active, but have not arrived yet (queued)
+ *   |   |   |   +------------- OutReady: The output port is ready to be pushed
+ *   |   |   |
+ *   |   |   +----------------- InClosed:  The input port is closed and will not receive any events.
+ *   |   |                                 A push might be still in flight which will be then processed first.
+ *   |   +--------------------- OutClosed: The output port is closed and will not receive any events.
+ *   +------------------------- InFailed:  Always set in conjunction with InClosed. Indicates that the close event
+ *                                         is a failure
  *
  * Sending an event is usually the following sequence:
  *  - An action is requested by a stage logic (push, pull, complete, etc.)
- *  - the availability of the port is set on the sender side to Limbo (inStates or outStates)
- *  - the scheduled event is put in the slot of the connection in the connectionStates table
+ *  - the state machine in portStates is transitioned from a ready state to a pending event
  *  - the id of the affected connection is enqueued
  *
  * Receiving an event is usually the following sequence:
  *  - id of connection to be processed is dequeued
- *  - the type of the event is determined by the object in the corresponding connectionStates slot
- *  - the availability of the port is set on the receiver side to be Available (inStates or outStates)
+ *  - the type of the event is determined from the bits set on portStates
+ *  - the state machine in portStates is transitioned to a ready state
  *  - using the inHandlers/outHandlers table the corresponding callback is called on the stage logic.
  *
  * Because of the FIFO construction of the queue the interpreter is fair, i.e. a pending event is always executed
@@ -205,52 +247,48 @@ private[stream] object GraphInterpreter {
 private[stream] final class GraphInterpreter(
   private val assembly: GraphInterpreter.GraphAssembly,
   val materializer: Materializer,
+  val log: LoggingAdapter,
   val inHandlers: Array[InHandler], // Lookup table for the InHandler of a connection
   val outHandlers: Array[OutHandler], // Lookup table for the outHandler of the connection
   val logics: Array[GraphStageLogic], // Array of stage logics
   val onAsyncInput: (GraphStageLogic, Any, (Any) ⇒ Unit) ⇒ Unit) {
   import GraphInterpreter._
 
-  // Maintains the next event (and state) of the connection.
-  // Technically the connection cannot be considered being in the state that is encoded here before the enqueued
-  // connection event has been processed. The inStates and outStates arrays usually protect access to this
-  // field while it is in transient state.
-  val connectionStates = Array.fill[Any](assembly.connectionCount)(Empty)
+  // Maintains additional information for events, basically elements in-flight, or failure.
+  // Other events are encoded in the portStates bitfield.
+  val connectionSlots = Array.fill[Any](assembly.connectionCount)(Empty)
 
-  // Indicates whether the input port is pullable. After pulling it becomes false
-  // Be aware that when inAvailable goes to false outAvailable does not become true immediately, only after
-  // the corresponding event in the queue has been processed
-  val inStates = Array.fill[PortState](assembly.connectionCount)(Available)
+  // Bitfield encoding pending events and various states for efficient querying and updates. See the documentation
+  // of the class for a full description.
+  val portStates = Array.fill[Int](assembly.connectionCount)(InReady)
 
-  // Indicates whether the output port is pushable. After pushing it becomes false
-  // Be aware that when inAvailable goes to false outAvailable does not become true immediately, only after
-  // the corresponding event in the queue has been processed
-  val outStates = Array.fill[PortState](assembly.connectionCount)(InFlight)
+  private[this] var activeStageId = Boundary
 
   // The number of currently running stages. Once this counter reaches zero, the interpreter is considered to be
   // completed
-  private var runningStages = assembly.stages.length
+  private[this] var runningStages = assembly.stages.length
 
   // Counts how many active connections a stage has. Once it reaches zero, the stage is automatically stopped.
-  private val shutdownCounter = Array.tabulate(assembly.stages.length) { i ⇒
+  private[this] val shutdownCounter = Array.tabulate(assembly.stages.length) { i ⇒
     val shape = assembly.stages(i).shape.asInstanceOf[Shape]
     shape.inlets.size + shape.outlets.size
   }
 
   // An event queue implemented as a circular buffer
-  private val eventQueue = Array.ofDim[Int](256)
-  private val mask = eventQueue.length - 1
-  private var queueHead: Int = 0
-  private var queueTail: Int = 0
+  // FIXME: This calculates the maximum size ever needed, but most assemblies can run on a smaller queue
+  private[this] val eventQueue = Array.ofDim[Int](1 << Integer.highestOneBit(assembly.connectionCount))
+  private[this] val mask = eventQueue.length - 1
+  private[this] var queueHead: Int = 0
+  private[this] var queueTail: Int = 0
 
   /**
    * Assign the boundary logic to a given connection. This will serve as the interface to the external world
    * (outside the interpreter) to process and inject events.
    */
   def attachUpstreamBoundary(connection: Int, logic: UpstreamBoundaryStageLogic[_]): Unit = {
-    logic.outToConn += logic.out -> connection
+    logic.outToConn(logic.out.id) = connection
     logic.interpreter = this
-    outHandlers(connection) = logic.outHandlers.head._2
+    outHandlers(connection) = logic.outHandlers(0)
   }
 
   /**
@@ -258,9 +296,9 @@ private[stream] final class GraphInterpreter(
    * (outside the interpreter) to process and inject events.
    */
   def attachDownstreamBoundary(connection: Int, logic: DownstreamBoundaryStageLogic[_]): Unit = {
-    logic.inToConn += logic.in -> connection
+    logic.inToConn(logic.in.id) = connection
     logic.interpreter = this
-    inHandlers(connection) = logic.inHandlers.head._2
+    inHandlers(connection) = logic.inHandlers(0)
   }
 
   /**
@@ -279,10 +317,15 @@ private[stream] final class GraphInterpreter(
   def init(): Unit = {
     var i = 0
     while (i < logics.length) {
-      logics(i).stageId = i
-      logics(i).interpreter = this
-      logics(i).beforePreStart()
-      logics(i).preStart()
+      val logic = logics(i)
+      logic.stageId = i
+      logic.interpreter = this
+      try {
+        logic.beforePreStart()
+        logic.preStart()
+      } catch {
+        case NonFatal(e) ⇒ logic.failStage(e)
+      }
       i += 1
     }
   }
@@ -293,10 +336,7 @@ private[stream] final class GraphInterpreter(
   def finish(): Unit = {
     var i = 0
     while (i < logics.length) {
-      if (!isStageCompleted(i)) {
-        logics(i).postStop()
-        logics(i).afterPostStop()
-      }
+      if (!isStageCompleted(i)) finalizeStage(logics(i))
       i += 1
     }
   }
@@ -323,16 +363,8 @@ private[stream] final class GraphInterpreter(
       try processEvent(connection)
       catch {
         case NonFatal(e) ⇒
-          val stageId = connectionStates(connection) match {
-            case Failed(ex)          ⇒ throw new IllegalStateException("Double fault. Failure while handling failure.", e)
-            case Pulled              ⇒ assembly.outOwners(connection)
-            case Completed           ⇒ assembly.inOwners(connection)
-            case Cancelled           ⇒ assembly.outOwners(connection)
-            case PushCompleted(elem) ⇒ assembly.inOwners(connection)
-            case pushedElem          ⇒ assembly.inOwners(connection)
-          }
-          if (stageId == Boundary) throw e
-          else logics(stageId).failStage(e)
+          if (activeStageId == Boundary) throw e
+          else logics(activeStageId).failStage(e)
       }
       eventsRemaining -= 1
       if (eventsRemaining > 0) connection = dequeue()
@@ -344,61 +376,62 @@ private[stream] final class GraphInterpreter(
   private def processEvent(connection: Int): Unit = {
 
     def processElement(elem: Any): Unit = {
-      if (!isStageCompleted(assembly.inOwners(connection))) {
-        if (GraphInterpreter.Debug) println(s"PUSH ${outOwnerName(connection)} -> ${inOwnerName(connection)}, $elem")
-        inStates(connection) = Available
-        inHandlers(connection).onPush()
+      if (GraphInterpreter.Debug) println(s"PUSH ${outOwnerName(connection)} -> ${inOwnerName(connection)}, $elem")
+      activeStageId = assembly.inOwners(connection)
+      portStates(connection) ^= PushEndFlip
+      inHandlers(connection).onPush()
+    }
+
+    val code = portStates(connection)
+
+    // Manual fast decoding, fast paths are PUSH and PULL
+    //   PUSH
+    if ((code & (Pushing | InClosed | OutClosed)) == Pushing) {
+      processElement(connectionSlots(connection))
+
+      // PULL
+    } else if ((code & (Pulling | OutClosed | InClosed)) == Pulling) {
+      if (GraphInterpreter.Debug) println(s"PULL ${inOwnerName(connection)} -> ${outOwnerName(connection)}")
+      portStates(connection) ^= PullEndFlip
+      activeStageId = assembly.outOwners(connection)
+      outHandlers(connection).onPull()
+
+      // CANCEL
+    } else if ((code & (OutClosed | InClosed)) == InClosed) {
+      val stageId = assembly.outOwners(connection)
+      if (GraphInterpreter.Debug) println(s"CANCEL ${inOwnerName(connection)} -> ${outOwnerName(connection)}")
+      portStates(connection) |= OutClosed
+      activeStageId = assembly.outOwners(connection)
+      outHandlers(connection).onDownstreamFinish()
+      completeConnection(stageId)
+    } else if ((code & (OutClosed | InClosed)) == OutClosed) {
+      // COMPLETIONS
+
+      val stageId = assembly.inOwners(connection)
+
+      if ((code & Pushing) == 0) {
+        // Normal completion (no push pending)
+        if (GraphInterpreter.Debug) println(s"COMPLETE ${outOwnerName(connection)} -> ${inOwnerName(connection)}")
+        portStates(connection) |= InClosed
+        activeStageId = assembly.inOwners(connection)
+        if ((portStates(connection) & InFailed) == 0) inHandlers(connection).onUpstreamFinish()
+        else inHandlers(connection).onUpstreamFailure(connectionSlots(connection).asInstanceOf[Failed].ex)
+        completeConnection(stageId)
+      } else {
+        // Push is pending, first process push, then re-enqueue closing event
+        // Non-failure case
+        val code = portStates(connection) & (InClosed | InFailed)
+        if (code == 0) {
+          processElement(connectionSlots(connection))
+          enqueue(connection)
+        } else if (code == InFailed) {
+          // Failure case
+          processElement(connectionSlots(connection).asInstanceOf[Failed].previousElem)
+          enqueue(connection)
+        }
       }
-    }
-
-    connectionStates(connection) match {
-      case Pulled ⇒
-        if (!isStageCompleted(assembly.outOwners(connection))) {
-          if (GraphInterpreter.Debug) println(s"PULL ${inOwnerName(connection)} -> ${outOwnerName(connection)}")
-          outStates(connection) = Available
-          outHandlers(connection).onPull()
-        }
-      case Completed | CompletedHasElement(_) ⇒
-        val stageId = assembly.inOwners(connection)
-        if (!isStageCompleted(stageId) && inStates(connection) != Closed) {
-          if (GraphInterpreter.Debug) println(s"COMPLETE ${outOwnerName(connection)} -> ${inOwnerName(connection)}")
-          inStates(connection) = Closed
-          inHandlers(connection).onUpstreamFinish()
-          completeConnection(stageId)
-        }
-      case Failed(ex) ⇒
-        val stageId = assembly.inOwners(connection)
-        if (!isStageCompleted(stageId) && inStates(connection) != Closed) {
-          if (GraphInterpreter.Debug) println(s"FAIL ${outOwnerName(connection)} -> ${inOwnerName(connection)}")
-          inStates(connection) = Closed
-          inHandlers(connection).onUpstreamFailure(ex)
-          completeConnection(stageId)
-        }
-      case Cancelled ⇒
-        val stageId = assembly.outOwners(connection)
-        if (!isStageCompleted(stageId) && outStates(connection) != Closed) {
-          if (GraphInterpreter.Debug) println(s"CANCEL ${inOwnerName(connection)} -> ${outOwnerName(connection)}")
-          outStates(connection) = Closed
-          outHandlers(connection).onDownstreamFinish()
-          completeConnection(stageId)
-        }
-      case PushCompleted(elem) ⇒
-        val stageId = assembly.inOwners(connection)
-        if (!isStageCompleted(stageId) && inStates(connection) != Closed) {
-          inStates(connection) = Available
-          connectionStates(connection) = elem
-          processElement(elem)
-          val elemAfter = connectionStates(connection)
-          if (elemAfter == Empty) enqueue(connection, Completed)
-          else enqueue(connection, CompletedHasElement(elemAfter))
-        } else {
-          connectionStates(connection) = Completed
-        }
-
-      case pushedElem ⇒ processElement(pushedElem)
 
     }
-
   }
 
   private def dequeue(): Int = {
@@ -412,29 +445,14 @@ private[stream] final class GraphInterpreter(
     }
   }
 
-  private def enqueue(connection: Int, event: Any): Unit = {
-    connectionStates(connection) = event
+  private def enqueue(connection: Int): Unit = {
     eventQueue(queueTail & mask) = connection
     queueTail += 1
   }
 
-  // Returns true if a connection has been completed *or if the completion event is already enqueued*. This is useful
-  // to prevent redundant completion events in case of concurrent invocation on both sides of the connection.
-  // I.e. when one side already enqueued the completion event, then the other side will not enqueue the event since
-  // there is noone to process it anymore.
-  def isConnectionCompleting(connection: Int): Boolean = connectionStates(connection).isInstanceOf[CompletingState]
-
   // Returns true if the given stage is alredy completed
   def isStageCompleted(stageId: Int): Boolean = stageId != Boundary && shutdownCounter(stageId) == 0
 
-  private def isPushInFlight(connection: Int): Boolean =
-    (inStates(connection) == InFlight) && // Other side has not been notified
-      hasElement(connection)
-
-  private def hasElement(connection: Int): Boolean =
-    !connectionStates(connection).isInstanceOf[ConnectionState] &&
-      connectionStates(connection) != Empty
-
   // Register that a connection in which the given stage participated has been completed and therefore the stage
   // itself might stop, too.
   private def completeConnection(stageId: Int): Unit = {
@@ -445,53 +463,66 @@ private[stream] final class GraphInterpreter(
         // This was the last active connection keeping this stage alive
         if (activeConnections == 1) {
           runningStages -= 1
-          logics(stageId).postStop()
-          logics(stageId).afterPostStop()
+          finalizeStage(logics(stageId))
         }
       }
     }
   }
 
+  private def finalizeStage(logic: GraphStageLogic): Unit = {
+    try {
+      logic.postStop()
+      logic.afterPostStop()
+    } catch {
+      case NonFatal(e) ⇒
+        log.error(s"Error during postStop in [${assembly.stages(logic.stageId)}]", e)
+    }
+  }
+
   private[stream] def push(connection: Int, elem: Any): Unit = {
-    if (!(inStates(connection) eq Closed)) {
-      outStates(connection) = InFlight
-      enqueue(connection, elem)
+    if ((portStates(connection) & InClosed) == 0) {
+      portStates(connection) ^= PushStartFlip
+      connectionSlots(connection) = elem
+      enqueue(connection)
     }
   }
 
   private[stream] def pull(connection: Int): Unit = {
-    if (!(outStates(connection) eq Closed)) {
-      inStates(connection) = InFlight
-      enqueue(connection, Pulled)
+    if ((portStates(connection) & OutClosed) == 0) {
+      portStates(connection) ^= PullStartFlip
+      enqueue(connection)
     }
   }
 
   private[stream] def complete(connection: Int): Unit = {
-    outStates(connection) = Closed
-    if (!isConnectionCompleting(connection) && (inStates(connection) ne Closed)) {
-      if (hasElement(connection)) {
-        // There is a pending push, we change the signal to be a PushCompleted (there can be only one signal in flight
-        // for a connection)
-        if (inStates(connection) == InFlight)
-          connectionStates(connection) = PushCompleted(connectionStates(connection))
-        else enqueue(connection, CompletedHasElement(connectionStates(connection)))
-      } else enqueue(connection, Completed)
+    val currentState = portStates(connection)
+    portStates(connection) = portStates(connection) | OutClosed
+    if ((currentState & InClosed) == 0) {
+      if ((currentState & Pushing) != 0) {} // FIXME: Fold into previous condition
+      else if (connectionSlots(connection) != Empty)
+        enqueue(connection)
+      else
+        enqueue(connection)
     }
     completeConnection(assembly.outOwners(connection))
   }
 
   private[stream] def fail(connection: Int, ex: Throwable): Unit = {
-    outStates(connection) = Closed
-    if (!isConnectionCompleting(connection) && (inStates(connection) ne Closed))
-      enqueue(connection, Failed(ex))
+    portStates(connection) |= (OutClosed | InFailed)
+    if ((portStates(connection) & InClosed) == 0) {
+      connectionSlots(connection) = Failed(ex, connectionSlots(connection))
+      enqueue(connection)
+    }
 
     completeConnection(assembly.outOwners(connection))
   }
 
   private[stream] def cancel(connection: Int): Unit = {
-    inStates(connection) = Closed
-    if (!isConnectionCompleting(connection) && (outStates(connection) ne Closed))
-      enqueue(connection, Cancelled)
+    portStates(connection) |= InClosed
+    if ((portStates(connection) & OutClosed) == 0) {
+      connectionSlots(connection) = Empty
+      enqueue(connection)
+    }
 
     completeConnection(assembly.inOwners(connection))
   }
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphStages.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphStages.scala
index 8fe324ee1a..6370e2a6a2 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphStages.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphStages.scala
@@ -25,10 +25,9 @@ object GraphStages {
     val out = Outlet[T]("out")
     override val shape = FlowShape(in, out)
 
-    protected abstract class SimpleLinearStageLogic extends GraphStageLogic {
+    protected abstract class SimpleLinearStageLogic extends GraphStageLogic(shape) {
       setHandler(out, new OutHandler {
         override def onPull(): Unit = pull(in)
-        override def onDownstreamFinish(): Unit = completeStage()
       })
     }
 
@@ -36,11 +35,9 @@ object GraphStages {
 
   class Identity[T] extends SimpleLinearGraphStage[T] {
 
-    override def createLogic: GraphStageLogic = new SimpleLinearStageLogic {
+    override def createLogic: GraphStageLogic = new SimpleLinearStageLogic() {
       setHandler(in, new InHandler {
         override def onPush(): Unit = push(out, grab(in))
-        override def onUpstreamFinish(): Unit = completeStage()
-        override def onUpstreamFailure(ex: Throwable): Unit = failStage(ex)
       })
     }
 
@@ -52,7 +49,7 @@ object GraphStages {
     val out = Outlet[T]("out")
     override val shape = FlowShape(in, out)
 
-    override def createLogic: GraphStageLogic = new GraphStageLogic {
+    override def createLogic: GraphStageLogic = new GraphStageLogic(shape) {
       var initialized = false
 
       setHandler(in, new InHandler {
@@ -108,7 +105,7 @@ object GraphStages {
       val cancelled = new AtomicBoolean(false)
       val cancellable = new TickSourceCancellable(cancelled)
 
-      val logic = new GraphStageLogic {
+      val logic = new GraphStageLogic(shape) {
         override def preStart() = {
           schedulePeriodicallyWithInitialDelay("TickTimer", initialDelay, interval)
           val callback = getAsyncCallback[Unit]((_) ⇒ {
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala
index 4538f3a957..545104b56c 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala
@@ -744,7 +744,7 @@ private[stream] class GroupedWithin[T](n: Int, d: FiniteDuration) extends GraphS
   val out = Outlet[immutable.Seq[T]]("out")
   val shape = FlowShape(in, out)
 
-  override def createLogic: GraphStageLogic = new GraphStageLogic {
+  override def createLogic: GraphStageLogic = new GraphStageLogic(shape) {
     private val buf: VectorBuilder[T] = new VectorBuilder
     // True if:
     // - buf is nonEmpty
diff --git a/akka-stream/src/main/scala/akka/stream/io/Timeouts.scala b/akka-stream/src/main/scala/akka/stream/io/Timeouts.scala
index a373129a4d..3fd3716ce2 100644
--- a/akka-stream/src/main/scala/akka/stream/io/Timeouts.scala
+++ b/akka-stream/src/main/scala/akka/stream/io/Timeouts.scala
@@ -129,7 +129,7 @@ object Timeouts {
 
     override def toString = "IdleTimeoutBidi"
 
-    override def createLogic: GraphStageLogic = new GraphStageLogic {
+    override def createLogic: GraphStageLogic = new GraphStageLogic(shape) {
       private var nextDeadline: Deadline = Deadline.now + timeout
 
       setHandler(in1, new InHandler {
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Graph.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Graph.scala
index 59cc1a5adc..5eec534865 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Graph.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Graph.scala
@@ -40,7 +40,7 @@ class Merge[T](val inputPorts: Int) extends GraphStage[UniformFanInShape[T, T]]
   val out: Outlet[T] = Outlet[T]("Merge.out")
   override val shape: UniformFanInShape[T, T] = UniformFanInShape(out, in: _*)
 
-  override def createLogic: GraphStageLogic = new GraphStageLogic {
+  override def createLogic: GraphStageLogic = new GraphStageLogic(shape) {
     private var initialized = false
 
     private val pendingQueue = Array.ofDim[Inlet[T]](inputPorts)
@@ -143,7 +143,7 @@ class MergePreferred[T] private (secondaryPorts: Int) extends GraphStage[MergePr
   def preferred: Inlet[T] = shape.preferred
 
   // FIXME: Factor out common stuff with Merge
-  override def createLogic: GraphStageLogic = new GraphStageLogic {
+  override def createLogic: GraphStageLogic = new GraphStageLogic(shape) {
     private var initialized = false
 
     private val pendingQueue = Array.ofDim[Inlet[T]](secondaryPorts)
@@ -252,7 +252,7 @@ class Broadcast[T](private val outputPorts: Int, eagerCancel: Boolean) extends G
   val out: immutable.IndexedSeq[Outlet[T]] = Vector.tabulate(outputPorts)(i ⇒ Outlet[T]("Broadcast.out" + i))
   override val shape: UniformFanOutShape[T, T] = UniformFanOutShape(in, out: _*)
 
-  override def createLogic: GraphStageLogic = new GraphStageLogic {
+  override def createLogic: GraphStageLogic = new GraphStageLogic(shape) {
     private var pendingCount = outputPorts
     private val pending = Array.fill[Boolean](outputPorts)(true)
     private var downstreamsRunning = outputPorts
@@ -350,7 +350,7 @@ class Balance[T](val outputPorts: Int, waitForAllDownstreams: Boolean) extends G
   val out: immutable.IndexedSeq[Outlet[T]] = Vector.tabulate(outputPorts)(i ⇒ Outlet[T]("Balance.out" + i))
   override val shape: UniformFanOutShape[T, T] = UniformFanOutShape[T, T](in, out: _*)
 
-  override def createLogic: GraphStageLogic = new GraphStageLogic {
+  override def createLogic: GraphStageLogic = new GraphStageLogic(shape) {
     private val pendingQueue = Array.ofDim[Outlet[T]](outputPorts)
     private var pendingHead: Int = 0
     private var pendingTail: Int = 0
@@ -522,7 +522,7 @@ class Concat[T](inputCount: Int) extends GraphStage[UniformFanInShape[T, T]] {
   val out: Outlet[T] = Outlet[T]("Concat.out")
   override val shape: UniformFanInShape[T, T] = UniformFanInShape(out, in: _*)
 
-  override def createLogic = new GraphStageLogic {
+  override def createLogic = new GraphStageLogic(shape) {
     var activeStream: Int = 0
 
     {
diff --git a/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala b/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala
index 1da600c696..3007504760 100644
--- a/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala
+++ b/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala
@@ -85,10 +85,12 @@ private object TimerMessages {
  *  The stage logic is always stopped once all its input and output ports have been closed, i.e. it is not possible to
  *  keep the stage alive for further processing once it does not have any open ports.
  */
-abstract class GraphStageLogic {
+abstract class GraphStageLogic private[stream] (inCount: Int, outCount: Int) {
   import GraphInterpreter._
   import TimerMessages._
 
+  def this(shape: Shape) = this(shape.inlets.size, shape.outlets.size)
+
   private val keyToTimers = mutable.Map[Any, Timer]()
   private val timerIdGen = Iterator from 1
   private var queuedTimerEvents = List.empty[Queued]
@@ -109,20 +111,20 @@ abstract class GraphStageLogic {
   /**
    * INTERNAL API
    */
-  private[stream] var inHandlers = scala.collection.Map.empty[Inlet[_], InHandler]
+  private[stream] var inHandlers = Array.ofDim[InHandler](inCount)
   /**
    * INTERNAL API
    */
-  private[stream] var outHandlers = scala.collection.Map.empty[Outlet[_], OutHandler]
+  private[stream] var outHandlers = Array.ofDim[OutHandler](outCount)
 
   /**
    * INTERNAL API
    */
-  private[stream] var inToConn = scala.collection.Map.empty[Inlet[_], Int]
+  private[stream] var inToConn = Array.ofDim[Int](inHandlers.length)
   /**
    * INTERNAL API
    */
-  private[stream] var outToConn = scala.collection.Map.empty[Outlet[_], Int]
+  private[stream] var outToConn = Array.ofDim[Int](outHandlers.length)
 
   /**
    * INTERNAL API
@@ -134,18 +136,18 @@ abstract class GraphStageLogic {
    */
   final protected def setHandler(in: Inlet[_], handler: InHandler): Unit = {
     handler.ownerStageLogic = this
-    inHandlers += in -> handler
+    inHandlers(in.id) = handler
   }
   /**
    * Assigns callbacks for the events for an [[Outlet]]
    */
   final protected def setHandler(out: Outlet[_], handler: OutHandler): Unit = {
     handler.ownerStageLogic = this
-    outHandlers += out -> handler
+    outHandlers(out.id) = handler
   }
 
-  private def conn[T](in: Inlet[T]): Int = inToConn(in)
-  private def conn[T](out: Outlet[T]): Int = outToConn(out)
+  private def conn[T](in: Inlet[T]): Int = inToConn(in.id)
+  private def conn[T](out: Outlet[T]): Int = outToConn(out.id)
 
   /**
    * Requests an element on the given port. Calling this method twice before an element arrived will fail.
@@ -153,9 +155,13 @@ abstract class GraphStageLogic {
    * query whether pull is allowed to be called or not.
    */
   final protected def pull[T](in: Inlet[T]): Unit = {
-    require(!hasBeenPulled(in), "Cannot pull port twice")
-    require(!isClosed(in), "Cannot pull closed port")
-    interpreter.pull(conn(in))
+    if ((interpreter.portStates(conn(in)) & (InReady | InClosed)) == InReady) {
+      interpreter.pull(conn(in))
+    } else {
+      // Detailed error information should not add overhead to the hot path
+      require(!isClosed(in), "Cannot pull closed port")
+      require(!hasBeenPulled(in), "Cannot pull port twice")
+    }
   }
 
   /**
@@ -171,17 +177,20 @@ abstract class GraphStageLogic {
    * The method [[isAvailable()]] can be used to query if the port has an element that can be grabbed or not.
    */
   final protected def grab[T](in: Inlet[T]): T = {
-    require(isAvailable(in), "Cannot get element from already empty input port")
     val connection = conn(in)
-    val elem = interpreter.connectionStates(connection)
-
-    elem match {
-      case CompletedHasElement(realElem) ⇒
-        interpreter.connectionStates(connection) = Completed
-        realElem.asInstanceOf[T]
-      case _ ⇒
-        interpreter.connectionStates(connection) = Empty
-        elem.asInstanceOf[T]
+    // Fast path
+    if ((interpreter.portStates(connection) & (InReady | InFailed)) == InReady &&
+      (interpreter.connectionSlots(connection).asInstanceOf[AnyRef] ne Empty)) {
+      val elem = interpreter.connectionSlots(connection)
+      interpreter.connectionSlots(connection) = Empty
+      elem.asInstanceOf[T]
+    } else {
+      // Slow path
+      require(isAvailable(in), "Cannot get element from already empty input port")
+      val failed = interpreter.connectionSlots(connection).asInstanceOf[Failed]
+      val elem = failed.previousElem.asInstanceOf[T]
+      interpreter.connectionSlots(connection) = Failed(failed.ex, Empty)
+      elem
     }
   }
 
@@ -189,7 +198,7 @@ abstract class GraphStageLogic {
    * Indicates whether there is already a pending pull for the given input port. If this method returns true
    * then [[isAvailable()]] must return false for that same port.
    */
-  final protected def hasBeenPulled[T](in: Inlet[T]): Boolean = interpreter.inStates(conn(in)) eq InFlight
+  final protected def hasBeenPulled[T](in: Inlet[T]): Boolean = (interpreter.portStates(conn(in)) & (InReady | InClosed)) == 0
 
   /**
    * Indicates whether there is an element waiting at the given input port. [[grab()]] can be used to retrieve the
@@ -199,19 +208,26 @@ abstract class GraphStageLogic {
    */
   final protected def isAvailable[T](in: Inlet[T]): Boolean = {
     val connection = conn(in)
-    val state = interpreter.connectionStates(connection)
 
-    val arrived = interpreter.inStates(connection) ne InFlight
-    val hasElementState = state.isInstanceOf[HasElementState]
-    val rawElement = (state != Empty) && !state.isInstanceOf[ConnectionState]
+    val normalArrived = (interpreter.portStates(conn(in)) & (InReady | InFailed)) == InReady
 
-    arrived && (hasElementState || rawElement)
+    // Fast path
+    if (normalArrived) interpreter.connectionSlots(connection).asInstanceOf[AnyRef] ne Empty
+    else {
+      // Slow path on failure
+      if ((interpreter.portStates(conn(in)) & (InReady | InFailed)) == (InReady | InFailed)) {
+        interpreter.connectionSlots(connection) match {
+          case Failed(_, elem) ⇒ elem.asInstanceOf[AnyRef] ne Empty
+          case _               ⇒ false // This can only be Empty actually (if a cancel was concurrent with a failure)
+        }
+      } else false
+    }
   }
 
   /**
    * Indicates whether the port has been closed. A closed port cannot be pulled.
    */
-  final protected def isClosed[T](in: Inlet[T]): Boolean = interpreter.inStates(conn(in)) eq Closed
+  final protected def isClosed[T](in: Inlet[T]): Boolean = (interpreter.portStates(conn(in)) & InClosed) != 0
 
   /**
    * Emits an element through the given output port. Calling this method twice before a [[pull()]] has been arrived
@@ -219,9 +235,13 @@ abstract class GraphStageLogic {
    * used to check if the port is ready to be pushed or not.
    */
   final protected def push[T](out: Outlet[T], elem: T): Unit = {
-    require(isAvailable(out), "Cannot push port twice")
-    require(!isClosed(out), "Cannot pull closed port")
-    interpreter.push(conn(out), elem)
+    if ((interpreter.portStates(conn(out)) & (OutReady | OutClosed)) == OutReady) {
+      interpreter.push(conn(out), elem)
+    } else {
+      // Detailed error information should not add overhead to the hot path
+      require(isAvailable(out), "Cannot push port twice")
+      require(!isClosed(out), "Cannot pull closed port")
+    }
   }
 
   /**
@@ -239,8 +259,16 @@ abstract class GraphStageLogic {
    * then stops the stage, then [[postStop()]] is called.
    */
   final def completeStage(): Unit = {
-    inToConn.valuesIterator.foreach(interpreter.cancel)
-    outToConn.valuesIterator.foreach(interpreter.complete)
+    var i = 0
+    while (i < inToConn.length) {
+      interpreter.cancel(inToConn(i))
+      i += 1
+    }
+    i = 0
+    while (i < outToConn.length) {
+      interpreter.complete(outToConn(i))
+      i += 1
+    }
   }
 
   /**
@@ -248,19 +276,28 @@ abstract class GraphStageLogic {
    * then stops the stage, then [[postStop()]] is called.
    */
   final def failStage(ex: Throwable): Unit = {
-    inToConn.valuesIterator.foreach(interpreter.cancel)
-    outToConn.valuesIterator.foreach(interpreter.fail(_, ex))
+    var i = 0
+    while (i < inToConn.length) {
+      interpreter.cancel(inToConn(i))
+      i += 1
+    }
+    i = 0
+    while (i < outToConn.length) {
+      interpreter.fail(outToConn(i), ex)
+      i += 1
+    }
   }
 
   /**
    * Return true if the given output port is ready to be pushed.
    */
-  final def isAvailable[T](out: Outlet[T]): Boolean = interpreter.outStates(conn(out)) eq Available
+  final def isAvailable[T](out: Outlet[T]): Boolean =
+    (interpreter.portStates(conn(out)) & (OutReady | OutClosed)) == OutReady
 
   /**
    * Indicates whether the port has been closed. A closed port cannot be pushed.
    */
-  final protected def isClosed[T](out: Outlet[T]): Boolean = interpreter.outStates(conn(out)) eq Closed
+  final protected def isClosed[T](out: Outlet[T]): Boolean = (interpreter.portStates(conn(out)) & OutClosed) != 0
 
   /**
    * Obtain a callback object that can be used asynchronously to re-enter the