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 = {
+    hostNameCache.putIfAbsent(host, host)
+    hostNameCache.get(host)
+  }
 }
 
 /**