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Author: JoshRosen

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

@@ -203,6 +203,8 @@ class TaskMetrics private[spark] () extends Serializable {
 
   import InternalAccumulator._
   @transient private[spark] lazy val nameToAccums = {
+    // The construction of this map is a performance hotspot in the JobProgressListener, so we
+    // optimize this by using a pre-sized Java hashmap; see SPARK-20776 for more details.
     val mapEntries = Array[(String, AccumulatorV2[_, _])](
       EXECUTOR_DESERIALIZE_TIME -> _executorDeserializeTime,
       EXECUTOR_DESERIALIZE_CPU_TIME -> _executorDeserializeCpuTime,