Github user tgravescs commented on a diff in the pull request:
https://github.com/apache/spark/pull/12113#discussion_r58378772
--- Diff: core/src/main/scala/org/apache/spark/MapOutputTracker.scala ---
@@ -296,10 +290,89 @@ private[spark] class MapOutputTrackerMaster(conf:
SparkConf)
protected val mapStatuses = new ConcurrentHashMap[Int,
Array[MapStatus]]().asScala
private val cachedSerializedStatuses = new ConcurrentHashMap[Int,
Array[Byte]]().asScala
+ private val maxRpcMessageSize = RpcUtils.maxMessageSizeBytes(conf)
+
+ // Kept in sync with cachedSerializedStatuses explicitly
+ // This is required so that the Broadcast variable remains in scope
until we remove
+ // the shuffleId explicitly or implicitly.
+ private val cachedSerializedBroadcast = new HashMap[Int,
Broadcast[Array[Byte]]]()
+
+ // This is to prevent multiple serializations of the same shuffle -
which happens when
+ // there is a request storm when shuffle start.
+ private val shuffleIdLocks = new ConcurrentHashMap[Int, AnyRef]()
+
+ // requests for map output statuses
+ private val mapOutputRequests = new
LinkedBlockingQueue[GetMapOutputMessage]
+
+ // Thread pool used for handling map output status requests. This is a
separate thread pool
+ // to ensure we don't block the normal dispatcher threads.
+ private val threadpool: ThreadPoolExecutor = {
+ val numThreads =
conf.getInt("spark.shuffle.mapOutput.dispatcher.numThreads", 8)
+ val pool = ThreadUtils.newDaemonFixedThreadPool(numThreads,
"map-output-dispatcher")
+ for (i <- 0 until numThreads) {
+ pool.execute(new MessageLoop)
+ }
+ pool
+ }
+
+ def post(message: GetMapOutputMessage): Unit = {
+ mapOutputRequests.offer(message)
+ }
+
+ /** Message loop used for dispatching messages. */
+ private class MessageLoop extends Runnable {
+ override def run(): Unit = {
+ try {
+ while (true) {
+ try {
+ val data = mapOutputRequests.take()
+ if (data == PoisonPill) {
+ // Put PoisonPill back so that other MessageLoops can see it.
+ mapOutputRequests.offer(PoisonPill)
+ return
+ }
+ val context = data.context
+ val shuffleId = data.shuffleId
+ val hostPort = context.senderAddress.hostPort
+ logDebug("Handling request to send map output locations for
shuffle " + shuffleId +
+ " to " + hostPort)
+ val mapOutputStatuses =
getSerializedMapOutputStatuses(shuffleId)
--- End diff --
can you clarify your comment/concern here?
Yes the getSerializedMapOutputStatuses can block but it only blocks for
roughly the time it takes to serialize the map output status. Obviously could
take a little longer if its backed up in the queue but that should be very
minimal since at that point its cached. This is really no different then the
behavior before this change, either way you have to serialize the map output
statuses
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