YARN-3430. Made headroom data available on app attempt page of RM WebUI. Contributed by Xuan Gong.

(cherry picked from commit 8366a36ad356e6318b8ce6c5c96e201149f811bd)

hadoop-yarn-project/CHANGES.txt

@@ -825,6 +825,9 @@ Release 2.7.0 - UNRELEASED
     removing inconsistencies in the default values. (Junping Du and Karthik
     Kambatla via vinodkv)
 
+    YARN-3430. Made headroom data available on app attempt page of RM WebUI.
+    (Xuan Gong via zjshen)
+
 Release 2.6.0 - 2014-11-18
 
   INCOMPATIBLE CHANGES

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/webapp/RMAppAttemptBlock.java

@@ -166,10 +166,12 @@ public class RMAppAttemptBlock extends AppAttemptBlock{
     if (attempt != null) {
       if (!isApplicationInFinalState(YarnApplicationAttemptState
           .valueOf(attempt.getAppAttemptState().toString()))) {
+        RMAppAttemptMetrics metrics = attempt.getRMAppAttemptMetrics();
         DIV<Hamlet> pdiv = html._(InfoBlock.class).div(_INFO_WRAP);
         info("Application Attempt Overview").clear();
         info("Application Attempt Metrics")._(
-          "Application Attempt Headroom : ", 0);
+          "Application Attempt Headroom : ", metrics == null ? "N/A" :
+            metrics.getApplicationAttemptHeadroom());
         pdiv._();
       }
     }