TcpConnection: add file-io-transferTo-limit config parameter

akka-actor/src/main/resources/reference.conf

@@ -486,7 +486,14 @@ akka {
 
       # Fully qualified config path which holds the dispatcher configuration
       # on which file IO tasks are scheduled
-      file-io-dispatcher = "akka.io.pinned-dispatcher"
+      file-io-dispatcher = "akka.actor.default-dispatcher"
+
+      # The maximum number of bytes (or "unlimited") to transfer in one batch when using
+      # `WriteFile` command which uses `FileChannel.transferTo` to pipe files to a TCP socket.
+      # On some OS like Linux `FileChannel.transferTo` may block for a long time when network
+      # IO is faster than file IO. Decreasing the value may improve fairness while increasing
+      # may improve throughput.
+      file-io-transferTo-limit = 512 KiB
     }
 
     udp {

akka-actor/src/main/scala/akka/io/Tcp.scala

@@ -189,6 +189,10 @@ class TcpExt(system: ExtendedActorSystem) extends IO.Extension {
     }
     val ManagementDispatcher = getString("management-dispatcher")
     val FileIODispatcher = getString("file-io-dispatcher")
+    val TransferToLimit = getString("file-io-transferTo-limit") match {
+      case "unlimited" ⇒ Int.MaxValue
+      case _           ⇒ getIntBytes("file-io-transferTo-limit")
+    }
 
     require(NrOfSelectors > 0, "nr-of-selectors must be > 0")
     require(MaxChannels == -1 || MaxChannels > 0, "max-channels must be > 0 or 'unlimited'")

akka-actor/src/main/scala/akka/io/TcpConnection.scala

@@ -344,7 +344,8 @@ private[io] abstract class TcpConnection(val channel: SocketChannel,
     new Runnable {
       def run() {
         import pendingWrite._
-        val writtenBytes = fileChannel.transferTo(currentPosition, remainingBytes, channel)
+        val toWrite = math.min(remainingBytes, tcp.Settings.TransferToLimit)
+        val writtenBytes = fileChannel.transferTo(currentPosition, toWrite, channel)
 
         if (writtenBytes < remainingBytes) self ! SendBufferFull(pendingWrite.updatedWrite(alreadyWritten + writtenBytes))
         else { // finished