YARN-4690. Skip object allocation in FSAppAttempt#getResourceUsage when possible (Ming Ma via sjlee)

(cherry picked from commit 7de70680fe44967e2afc92ba4c92f8e7afa7b151)

hadoop-yarn-project/CHANGES.txt

@@ -2312,6 +2312,9 @@ Release 2.6.5 - UNRELEASED
 
   OPTIMIZATIONS
 
+    YARN-4690. Skip object allocation in FSAppAttempt#getResourceUsage when
+    possible (Ming Ma via sjlee)
+
   BUG FIXES
 
 Release 2.6.4 - 2016-02-11

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/fair/FSAppAttempt.java

@@ -890,7 +890,12 @@ public class FSAppAttempt extends SchedulerApplicationAttempt
   public Resource getResourceUsage() {
     // Here the getPreemptedResources() always return zero, except in
     // a preemption round
-    return Resources.subtract(getCurrentConsumption(), getPreemptedResources());
+    // In the common case where preempted resource is zero, return the
+    // current consumption Resource object directly without calling
+    // Resources.subtract which creates a new Resource object for each call.
+    return getPreemptedResources().equals(Resources.none()) ?
+        getCurrentConsumption() :
+        Resources.subtract(getCurrentConsumption(), getPreemptedResources());
   }
 
   @Override