diff --git a/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/counters/FileSystemCounterGroup.java b/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/counters/FileSystemCounterGroup.java
index 046368e96a..ed7f271bcd 100644
--- a/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/counters/FileSystemCounterGroup.java
+++ b/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/counters/FileSystemCounterGroup.java
@@ -61,8 +61,9 @@ public abstract class FileSystemCounterGroup<C extends Counter>
 
   // C[] would need Array.newInstance which requires a Class<C> reference.
   // Just a few local casts probably worth not having to carry it around.
-  private final Map<String, Object[]> map =
-    new ConcurrentSkipListMap<String, Object[]>();
+  // Initialized lazily, since in some situations millions of empty maps can
+  // waste a substantial (e.g. 4% as we observed) portion of the heap
+  private Map<String, Object[]> map;
   private String displayName;
 
   private static final Joiner NAME_JOINER = Joiner.on('_');
@@ -214,6 +215,9 @@ public C findCounter(String counterName) {
   @SuppressWarnings("unchecked")
   public synchronized C findCounter(String scheme, FileSystemCounter key) {
     final String canonicalScheme = checkScheme(scheme);
+    if (map == null) {
+      map = new ConcurrentSkipListMap<>();
+    }
     Object[] counters = map.get(canonicalScheme);
     int ord = key.ordinal();
     if (counters == null) {
@@ -247,10 +251,12 @@ private String checkScheme(String scheme) {
   protected abstract C newCounter(String scheme, FileSystemCounter key);
 
   @Override
-  public int size() {
+  public synchronized int size() {
     int n = 0;
-    for (Object[] counters : map.values()) {
-      n += numSetCounters(counters);
+    if (map != null) {
+      for (Object[] counters : map.values()) {
+        n += numSetCounters(counters);
+      }
     }
     return n;
   }
@@ -271,19 +277,23 @@ public void incrAllCounters(CounterGroupBase<C> other) {
    * FileSystemGroup ::= #scheme (scheme #counter (key value)*)*
    */
   @Override
-  public void write(DataOutput out) throws IOException {
-    WritableUtils.writeVInt(out, map.size()); // #scheme
-    for (Map.Entry<String, Object[]> entry : map.entrySet()) {
-      WritableUtils.writeString(out, entry.getKey()); // scheme
-      // #counter for the above scheme
-      WritableUtils.writeVInt(out, numSetCounters(entry.getValue()));
-      for (Object counter : entry.getValue()) {
-        if (counter == null) continue;
-        @SuppressWarnings("unchecked")
-        FSCounter c = (FSCounter) ((Counter)counter).getUnderlyingCounter();
-        WritableUtils.writeVInt(out, c.key.ordinal());  // key
-        WritableUtils.writeVLong(out, c.getValue());    // value
+  public synchronized void write(DataOutput out) throws IOException {
+    if (map != null) {
+      WritableUtils.writeVInt(out, map.size()); // #scheme
+      for (Map.Entry<String, Object[]> entry : map.entrySet()) {
+        WritableUtils.writeString(out, entry.getKey()); // scheme
+        // #counter for the above scheme
+        WritableUtils.writeVInt(out, numSetCounters(entry.getValue()));
+        for (Object counter : entry.getValue()) {
+          if (counter == null) continue;
+          @SuppressWarnings("unchecked")
+          FSCounter c = (FSCounter) ((Counter) counter).getUnderlyingCounter();
+          WritableUtils.writeVInt(out, c.key.ordinal());  // key
+          WritableUtils.writeVLong(out, c.getValue());    // value
+        }
       }
+    } else {
+      WritableUtils.writeVInt(out, 0);
     }
   }
 
@@ -310,8 +320,8 @@ public void readFields(DataInput in) throws IOException {
   @Override
   public Iterator<C> iterator() {
     return new AbstractIterator<C>() {
-      Iterator<Object[]> it = map.values().iterator();
-      Object[] counters = it.hasNext() ? it.next() : null;
+      Iterator<Object[]> it = map != null ? map.values().iterator() : null;
+      Object[] counters = (it != null && it.hasNext()) ? it.next() : null;
       int i = 0;
       @Override
       protected C computeNext() {
@@ -322,7 +332,7 @@ protected C computeNext() {
             if (counter != null) return counter;
           }
           i = 0;
-          counters = it.hasNext() ? it.next() : null;
+          counters = (it != null && it.hasNext()) ? it.next() : null;
         }
         return endOfData();
       }
@@ -343,8 +353,10 @@ public synchronized boolean equals(Object genericRight) {
   public synchronized int hashCode() {
     // need to be deep as counters is an array
     int hash = FileSystemCounter.class.hashCode();
-    for (Object[] counters : map.values()) {
-      if (counters != null) hash ^= Arrays.hashCode(counters);
+    if (map != null) {
+      for (Object[] counters : map.values()) {
+        if (counters != null) hash ^= Arrays.hashCode(counters);
+      }
     }
     return hash;
   }
diff --git a/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/jobhistory/JobHistoryParser.java b/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/jobhistory/JobHistoryParser.java
index 6efb4f7e02..57c58ba35a 100644
--- a/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/jobhistory/JobHistoryParser.java
+++ b/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/jobhistory/JobHistoryParser.java
@@ -600,7 +600,7 @@ public static class TaskInfo {
     public TaskInfo() {
       startTime = finishTime = -1;
       error = splitLocations = "";
-      attemptsMap = new HashMap<TaskAttemptID, TaskAttemptInfo>();
+      attemptsMap = new HashMap<TaskAttemptID, TaskAttemptInfo>(2);
     }
     
     public void printAll() {
diff --git a/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-hs/src/main/java/org/apache/hadoop/mapreduce/v2/hs/CompletedTask.java b/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-hs/src/main/java/org/apache/hadoop/mapreduce/v2/hs/CompletedTask.java
index 81fddaf42c..63b3600bef 100644
--- a/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-hs/src/main/java/org/apache/hadoop/mapreduce/v2/hs/CompletedTask.java
+++ b/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-hs/src/main/java/org/apache/hadoop/mapreduce/v2/hs/CompletedTask.java
@@ -20,7 +20,6 @@
 
 import java.util.ArrayList;
 import java.util.LinkedHashMap;
-import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicBoolean;
@@ -48,11 +47,11 @@ public class CompletedTask implements Task {
   private final TaskInfo taskInfo;
   private TaskReport report;
   private TaskAttemptId successfulAttempt;
-  private List<String> reportDiagnostics = new LinkedList<String>();
+  private List<String> reportDiagnostics = new ArrayList<String>(2);
   private Lock taskAttemptsLock = new ReentrantLock();
   private AtomicBoolean taskAttemptsLoaded = new AtomicBoolean(false);
   private final Map<TaskAttemptId, TaskAttempt> attempts =
-    new LinkedHashMap<TaskAttemptId, TaskAttempt>();
+      new LinkedHashMap<TaskAttemptId, TaskAttempt>(2);
 
   CompletedTask(TaskId taskId, TaskInfo taskInfo) {
     //TODO JobHistoryParser.handleTaskFailedAttempt should use state from the event.
diff --git a/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-hs/src/main/java/org/apache/hadoop/mapreduce/v2/hs/CompletedTaskAttempt.java b/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-hs/src/main/java/org/apache/hadoop/mapreduce/v2/hs/CompletedTaskAttempt.java
index c87d82b856..17ec0177c4 100644
--- a/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-hs/src/main/java/org/apache/hadoop/mapreduce/v2/hs/CompletedTaskAttempt.java
+++ b/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-hs/src/main/java/org/apache/hadoop/mapreduce/v2/hs/CompletedTaskAttempt.java
@@ -39,7 +39,7 @@ public class CompletedTaskAttempt implements TaskAttempt {
   private final TaskAttemptInfo attemptInfo;
   private final TaskAttemptId attemptId;
   private final TaskAttemptState state;
-  private final List<String> diagnostics = new ArrayList<String>();
+  private final List<String> diagnostics = new ArrayList<String>(2);
   private TaskAttemptReport report;
 
   private String localDiagMessage;