diff --git a/hadoop-mapreduce-project/CHANGES.txt b/hadoop-mapreduce-project/CHANGES.txt
index 490c0a1d4d..0c065c5694 100644
--- a/hadoop-mapreduce-project/CHANGES.txt
+++ b/hadoop-mapreduce-project/CHANGES.txt
@@ -279,6 +279,9 @@ Release 2.6.0 - UNRELEASED
     MAPREDUCE-6075. HistoryServerFileSystemStateStore can create zero-length
     files (jlowe)
 
+    MAPREDUCE-6070. yarn.app.am.resource.mb/cpu-vcores affects uber mode but
+    is not documented (Tsuyoshi OZAWA via jlowe)
+
 Release 2.5.1 - UNRELEASED
 
   INCOMPATIBLE CHANGES
diff --git a/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/resources/mapred-default.xml b/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/resources/mapred-default.xml
index 703a103956..6cefdc97b5 100644
--- a/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/resources/mapred-default.xml
+++ b/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/resources/mapred-default.xml
@@ -508,7 +508,12 @@
   <description>Whether to enable the small-jobs "ubertask" optimization,
   which runs "sufficiently small" jobs sequentially within a single JVM.
   "Small" is defined by the following maxmaps, maxreduces, and maxbytes
-  settings.  Users may override this value.
+  settings. Note that configurations for application masters also affect
+  the "Small" definition - yarn.app.mapreduce.am.resource.mb must be
+  larger than both mapreduce.map.memory.mb and mapreduce.reduce.memory.mb,
+  and yarn.app.mapreduce.am.resource.cpu-vcores must be larger than
+  both mapreduce.map.cpu.vcores and mapreduce.reduce.cpu.vcores to enable
+  ubertask. Users may override this value.
   </description>
 </property>