[SPARK-5523] [CORE] [STREAMING] Add a cache for hostname in TaskMetrics to decrease the memory usage and GC overhead

Hostname in TaskMetrics will be created through deserialization, mostly the number of hostname is only the order of number of cluster node, so adding a cache layer to dedup the object could reduce the memory usage and alleviate GC overhead, especially for long-running and fast job generation applications like Spark Streaming.

Author: jerryshao <[email protected]>
Author: Saisai Shao <[email protected]>

Closes apache#5064 from jerryshao/SPARK-5523 and squashes the following commits:

3e2412a [jerryshao] Address the comments
b092a81 [Saisai Shao] Add a pool to cache the hostname

Author: jerryshao

core/src/main/scala/org/apache/spark/executor/TaskMetrics.scala

@@ -17,11 +17,15 @@
 
 package org.apache.spark.executor
 
+import java.io.{IOException, ObjectInputStream}
+import java.util.concurrent.ConcurrentHashMap
+
 import scala.collection.mutable.ArrayBuffer
 
 import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.executor.DataReadMethod.DataReadMethod
 import org.apache.spark.storage.{BlockId, BlockStatus}
+import org.apache.spark.util.Utils
 
 /**
  * :: DeveloperApi ::
@@ -210,10 +214,26 @@ class TaskMetrics extends Serializable {
   private[spark] def updateInputMetrics(): Unit = synchronized {
     inputMetrics.foreach(_.updateBytesRead())
   }
+
+  @throws(classOf[IOException])
+  private def readObject(in: ObjectInputStream): Unit = Utils.tryOrIOException {
+    in.defaultReadObject()
+    // Get the hostname from cached data, since hostname is the order of number of nodes in
+    // cluster, so using cached hostname will decrease the object number and alleviate the GC
+    // overhead.
+    _hostname = TaskMetrics.getCachedHostName(_hostname)
+  }
 }
 
 private[spark] object TaskMetrics {
+  private val hostNameCache = new ConcurrentHashMap[String, String]()
+
   def empty: TaskMetrics = new TaskMetrics
+
+  def getCachedHostName(host: String): String = {
+    val canonicalHost = hostNameCache.putIfAbsent(host, host)
+    if (canonicalHost != null) canonicalHost else host
+  }
 }
 
 /**