HADOOP-9763. Extends LightWeightGSet to support eviction of expired elements. Contributed by Tsz Wo (Nicholas) SZE.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1506405 13f79535-47bb-0310-9956-ffa450edef68

hadoop-common-project/hadoop-common/CHANGES.txt

@@ -369,6 +369,9 @@ Release 2.1.0-beta - 2013-07-02
     HADOOP-8470. Add NetworkTopologyWithNodeGroup, a 4-layer implementation
     of NetworkTopology.  (Junping Du via szetszwo)
 
+    HADOOP-9763. Extends LightWeightGSet to support eviction of expired 
+    elements. (Tsz Wo (Nicholas) SZE via jing9)
+
   IMPROVEMENTS
 
     HADOOP-9164. Print paths of loaded native libraries in

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/GSet.java

@@ -17,6 +17,8 @@
  */
 package org.apache.hadoop.util;
 
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
 import org.apache.hadoop.classification.InterfaceAudience;
 
 /**
@@ -31,6 +33,8 @@ import org.apache.hadoop.classification.InterfaceAudience;
  */
 @InterfaceAudience.Private
 public interface GSet<K, E extends K> extends Iterable<E> {
+  static final Log LOG = LogFactory.getLog(GSet.class);
+
   /**
    * @return The size of this set.
    */

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/LightWeightCache.java

@@ -0,0 +1,238 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.hadoop.util;
+
+import java.util.Comparator;
+import java.util.PriorityQueue;
+
+import org.apache.hadoop.HadoopIllegalArgumentException;
+import org.apache.hadoop.classification.InterfaceAudience;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Preconditions;
+
+/**
+ * A low memory footprint Cache which extends {@link LightWeightGSet}.
+ * An entry in the cache is expired if
+ * (1) it is added to the cache longer than the creation-expiration period, and
+ * (2) it is not accessed for the access-expiration period.
+ * When an entry is expired, it may be evicted from the cache.
+ * When the size limit of the cache is set, the cache will evict the entries
+ * with earliest expiration time, even if they are not expired.
+ * 
+ * It is guaranteed that number of entries in the cache is less than or equal
+ * to the size limit.  However, It is not guaranteed that expired entries are
+ * evicted from the cache. An expired entry may possibly be accessed after its
+ * expiration time. In such case, the expiration time may be updated.
+ *
+ * This class does not support null entry.
+ *
+ * This class is not thread safe.
+ *
+ * @param <K> Key type for looking up the entries
+ * @param <E> Entry type, which must be
+ *       (1) a subclass of K, and
+ *       (2) implementing {@link Entry} interface, and
+ */
+@InterfaceAudience.Private
+public class LightWeightCache<K, E extends K> extends LightWeightGSet<K, E> {
+  /** Limit the number of entries in each eviction. */
+  private static final int EVICTION_LIMIT = 1 << 16;
+
+  /**
+   * Entries of {@link LightWeightCache}.
+   */
+  public static interface Entry extends LinkedElement {
+    /** Set the expiration time. */
+    public void setExpirationTime(long timeNano);
+
+    /** Get the expiration time. */
+    public long getExpirationTime();
+  }
+
+  /** Comparator for sorting entries by expiration time in ascending order. */
+  private static final Comparator<Entry> expirationTimeComparator
+      = new Comparator<Entry>() {
+    @Override
+    public int compare(Entry left, Entry right) {
+      final long l = left.getExpirationTime();
+      final long r = right.getExpirationTime();
+      return l > r? 1: l < r? -1: 0;
+    }
+  };
+
+  /** A clock for measuring time so that it can be mocked in unit tests. */
+  static class Clock {
+    /** @return the current time. */
+    long currentTime() {
+      return System.nanoTime();
+    }
+  }
+  
+  private static int updateRecommendedLength(int recommendedLength,
+      int sizeLimit) {
+    return sizeLimit > 0 && sizeLimit < recommendedLength?
+        (sizeLimit/4*3) // 0.75 load factor
+        : recommendedLength;
+  }
+
+  /*
+   * The memory footprint for java.util.PriorityQueue is low but the
+   * remove(Object) method runs in linear time. We may improve it by using a
+   * balanced tree. However, we do not yet have a low memory footprint balanced
+   * tree implementation.
+   */
+  private final PriorityQueue<Entry> queue;
+  private final long creationExpirationPeriod;
+  private final long accessExpirationPeriod;
+  private final int sizeLimit;
+  private final Clock clock;
+
+  /**
+   * @param recommendedLength Recommended size of the internal array.
+   * @param sizeLimit the limit of the size of the cache.
+   *            The limit is disabled if it is <= 0.
+   * @param creationExpirationPeriod the time period C > 0 in nanoseconds that
+   *            the creation of an entry is expired if it is added to the cache
+   *            longer than C.
+   * @param accessExpirationPeriod the time period A >= 0 in nanoseconds that
+   *            the access of an entry is expired if it is not accessed
+   *            longer than A. 
+   */
+  public LightWeightCache(final int recommendedLength,
+      final int sizeLimit,
+      final long creationExpirationPeriod,
+      final long accessExpirationPeriod) {
+    this(recommendedLength, sizeLimit,
+        creationExpirationPeriod, accessExpirationPeriod, new Clock());
+  }
+
+  @VisibleForTesting
+  LightWeightCache(final int recommendedLength,
+      final int sizeLimit,
+      final long creationExpirationPeriod,
+      final long accessExpirationPeriod,
+      final Clock clock) {
+    super(updateRecommendedLength(recommendedLength, sizeLimit));
+
+    this.sizeLimit = sizeLimit;
+
+    if (creationExpirationPeriod <= 0) {
+      throw new IllegalArgumentException("creationExpirationPeriod = "
+          + creationExpirationPeriod + " <= 0");
+    }
+    this.creationExpirationPeriod = creationExpirationPeriod;
+
+    if (accessExpirationPeriod < 0) {
+      throw new IllegalArgumentException("accessExpirationPeriod = "
+          + accessExpirationPeriod + " < 0");
+    }
+    this.accessExpirationPeriod = accessExpirationPeriod;
+
+    this.queue = new PriorityQueue<Entry>(
+        sizeLimit > 0? sizeLimit + 1: 1 << 10, expirationTimeComparator);
+    this.clock = clock;
+  }
+
+  void setExpirationTime(final Entry e, final long expirationPeriod) {
+    e.setExpirationTime(clock.currentTime() + expirationPeriod);
+  }
+
+  boolean isExpired(final Entry e, final long now) {
+    return now > e.getExpirationTime();
+  }
+
+  private E evict() {
+    @SuppressWarnings("unchecked")
+    final E polled = (E)queue.poll();
+    final E removed = super.remove(polled);
+    Preconditions.checkState(removed == polled);
+    return polled;
+  }
+
+  /** Evict expired entries. */
+  private void evictExpiredEntries() {
+    final long now = clock.currentTime();
+    for(int i = 0; i < EVICTION_LIMIT; i++) {
+      final Entry peeked = queue.peek();
+      if (peeked == null || !isExpired(peeked, now)) {
+        return;
+      }
+
+      final E evicted = evict();
+      Preconditions.checkState(evicted == peeked);
+    }
+  }
+
+  /** Evict entries in order to enforce the size limit of the cache. */
+  private void evictEntries() {
+    if (sizeLimit > 0) {
+      for(int i = size(); i > sizeLimit; i--) {
+        evict();
+      }
+    }
+  }
+  
+  @Override
+  public E get(K key) {
+    final E entry = super.get(key);
+    if (entry != null) {
+      if (accessExpirationPeriod > 0) {
+        // update expiration time
+        final Entry existing = (Entry)entry;
+        Preconditions.checkState(queue.remove(existing));
+        setExpirationTime(existing, accessExpirationPeriod);
+        queue.offer(existing);
+      }
+    }
+    return entry;
+  }
+
+  @Override
+  public E put(final E entry) {
+    if (!(entry instanceof Entry)) {
+      throw new HadoopIllegalArgumentException(
+          "!(entry instanceof Entry), entry.getClass()=" + entry.getClass());
+    }
+
+    evictExpiredEntries();
+
+    final E existing = super.put(entry);
+    if (existing != null) {
+      queue.remove(existing);
+    }
+
+    final Entry e = (Entry)entry;
+    setExpirationTime(e, creationExpirationPeriod);
+    queue.offer(e);
+    
+    evictEntries();
+    return existing;
+  }
+
+  @Override
+  public E remove(K key) {
+    evictExpiredEntries();
+
+    final E removed = super.remove(key);
+    if (removed != null) {
+      Preconditions.checkState(queue.remove(removed));
+    }
+    return removed;
+  }
+}

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/LightWeightGSet.java

@@ -21,11 +21,9 @@ import java.io.PrintStream;
 import java.util.ConcurrentModificationException;
 import java.util.Iterator;
 
-import org.apache.commons.logging.Log;
+import org.apache.hadoop.HadoopIllegalArgumentException;
-import org.apache.commons.logging.LogFactory;
 import org.apache.hadoop.classification.InterfaceAudience;
 import org.apache.hadoop.util.StringUtils;
-import org.apache.hadoop.HadoopIllegalArgumentException;
 
 import com.google.common.annotations.VisibleForTesting;
 
@@ -59,7 +57,6 @@ public class LightWeightGSet<K, E extends K> implements GSet<K, E> {
     public LinkedElement getNext();
   }
 
-  public static final Log LOG = LogFactory.getLog(GSet.class);
   static final int MAX_ARRAY_LENGTH = 1 << 30; //prevent int overflow problem
   static final int MIN_ARRAY_LENGTH = 1;
 

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/util/TestGSet.java

@@ -169,9 +169,8 @@ public class TestGSet {
   }
 
   /**
-   * A long test,
+   * A long running test with various data sets and parameters.
-   * which may take ~5 hours,
+   * It may take ~5 hours, 
-   * with various data sets and parameters.
    * If you are changing the implementation,
    * please un-comment the following line in order to run the test.
    */
@@ -327,8 +326,6 @@ public class TestGSet {
       } else {
         Assert.assertEquals(e.id, gset.remove(key).id);
       }
-
-      check();
       return e;
     }
     @Override
@@ -391,7 +388,9 @@ public class TestGSet {
 
     @Override
     public void clear() {
+      expected.clear();
       gset.clear();
+      Assert.assertEquals(0, size());
     }
   }
 

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/util/TestLightWeightCache.java

@@ -0,0 +1,456 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.hadoop.util;
+
+import java.util.Date;
+import java.util.Iterator;
+import java.util.Random;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+/** Testing {@link LightWeightCache} */
+public class TestLightWeightCache {
+  private static final long starttime = Time.now();
+  private static final Random ran = new Random(starttime);
+  static {
+    println("Start time = " + new Date(starttime) + ", seed=" +  starttime);
+  }
+
+  private static void print(Object s) {
+    System.out.print(s);
+    System.out.flush();
+  }
+
+  private static void println(Object s) {
+    System.out.println(s);
+  }
+
+  @Test
+  public void testLightWeightCache() {
+    // test randomized creation expiration with zero access expiration 
+    {
+      final long creationExpiration = ran.nextInt(1024) + 1;
+      check(1, creationExpiration, 0L, 1 << 10, 65537);
+      check(17, creationExpiration, 0L, 1 << 16, 17);
+      check(255, creationExpiration, 0L, 1 << 16, 65537);
+    }
+
+    // test randomized creation/access expiration periods
+    for(int i = 0; i < 3; i++) {
+      final long creationExpiration = ran.nextInt(1024) + 1;
+      final long accessExpiration = ran.nextInt(1024) + 1;
+      
+      check(1, creationExpiration, accessExpiration, 1 << 10, 65537);
+      check(17, creationExpiration, accessExpiration, 1 << 16, 17);
+      check(255, creationExpiration, accessExpiration, 1 << 16, 65537);
+    }
+  
+    // test size limit
+    final int dataSize = 1 << 16;
+    for(int i = 0; i < 10; i++) {
+      final int modulus = ran.nextInt(1024) + 1;
+      final int sizeLimit = ran.nextInt(modulus);
+      checkSizeLimit(sizeLimit, dataSize, modulus);
+    }
+  }
+
+  private static void checkSizeLimit(final int sizeLimit, final int datasize,
+      final int modulus) {
+    final LightWeightCacheTestCase test = new LightWeightCacheTestCase(
+        sizeLimit, sizeLimit, 1L << 32, 1L << 32, datasize, modulus);
+
+    // keep putting entries and check size limit
+    print("  check size ................. ");
+    for(int i = 0; i < test.data.size(); i++) {
+      test.cache.put(test.data.get(i));
+      Assert.assertTrue(test.cache.size() <= sizeLimit);
+    }
+    println("DONE " + test.stat());
+  }
+  
+  /** 
+   * Test various createionExpirationPeriod and accessExpirationPeriod.
+   * It runs ~2 minutes. If you are changing the implementation,
+   * please un-comment the following line in order to run the test.
+   */
+//  @Test
+  public void testExpirationPeriods() {
+    for(int k = -4; k < 10; k += 4) {
+      final long accessExpirationPeriod = k < 0? 0L: (1L << k); 
+      for(int j = 0; j < 10; j += 4) {
+        final long creationExpirationPeriod = 1L << j; 
+        runTests(1, creationExpirationPeriod, accessExpirationPeriod);
+        for(int i = 1; i < Integer.SIZE - 1; i += 8) {
+          runTests((1 << i) + 1, creationExpirationPeriod, accessExpirationPeriod);
+        }
+      }
+    }
+  }
+
+  /** Run tests with various table lengths. */
+  private static void runTests(final int modulus,
+      final long creationExpirationPeriod,
+      final long accessExpirationPeriod) {
+    println("\n\n\n*** runTest: modulus=" + modulus
+        + ", creationExpirationPeriod=" + creationExpirationPeriod
+        + ", accessExpirationPeriod=" + accessExpirationPeriod);
+    for(int i = 0; i <= 16; i += 4) {
+      final int tablelength = (1 << i);
+
+      final int upper = i + 2;
+      final int steps = Math.max(1, upper/3);
+
+      for(int j = upper; j > 0; j -= steps) {
+        final int datasize = 1 << j;
+        check(tablelength, creationExpirationPeriod, accessExpirationPeriod,
+            datasize, modulus);
+      }
+    }
+  }
+
+  private static void check(int tablelength, long creationExpirationPeriod,
+      long accessExpirationPeriod, int datasize, int modulus) {
+    check(new LightWeightCacheTestCase(tablelength, -1,
+        creationExpirationPeriod, accessExpirationPeriod, datasize, modulus));
+  }
+
+  /** 
+   * check the following operations
+   * (1) put
+   * (2) remove & put
+   * (3) remove
+   * (4) remove & put again
+   */
+  private static void check(final LightWeightCacheTestCase test) {
+    //check put
+    print("  check put .................. ");
+    for(int i = 0; i < test.data.size()/2; i++) {
+      test.put(test.data.get(i));
+    }
+    for(int i = 0; i < test.data.size(); i++) {
+      test.put(test.data.get(i));
+    }
+    println("DONE " + test.stat());
+
+    //check remove and put
+    print("  check remove & put ......... ");
+    for(int j = 0; j < 10; j++) {
+      for(int i = 0; i < test.data.size()/2; i++) {
+        final int r = ran.nextInt(test.data.size());
+        test.remove(test.data.get(r));
+      }
+      for(int i = 0; i < test.data.size()/2; i++) {
+        final int r = ran.nextInt(test.data.size());
+        test.put(test.data.get(r));
+      }
+    }
+    println("DONE " + test.stat());
+
+    //check remove
+    print("  check remove ............... ");
+    for(int i = 0; i < test.data.size(); i++) {
+      test.remove(test.data.get(i));
+    }
+    Assert.assertEquals(0, test.cache.size());
+    println("DONE " + test.stat());
+
+    //check remove and put again
+    print("  check remove & put again ... ");
+    for(int j = 0; j < 10; j++) {
+      for(int i = 0; i < test.data.size()/2; i++) {
+        final int r = ran.nextInt(test.data.size());
+        test.remove(test.data.get(r));
+      }
+      for(int i = 0; i < test.data.size()/2; i++) {
+        final int r = ran.nextInt(test.data.size());
+        test.put(test.data.get(r));
+      }
+    }
+    println("DONE " + test.stat());
+
+    final long s = (Time.now() - starttime)/1000L;
+    println("total time elapsed=" + s + "s\n");
+  }
+
+  /**
+   * The test case contains two data structures, a cache and a hashMap.
+   * The hashMap is used to verify the correctness of the cache.  Note that
+   * no automatic eviction is performed in the hashMap.  Thus, we have
+   * (1) If an entry exists in cache, it MUST exist in the hashMap.
+   * (2) If an entry does not exist in the cache, it may or may not exist in the
+   *     hashMap.  If it exists, it must be expired.
+   */
+  private static class LightWeightCacheTestCase implements GSet<IntEntry, IntEntry> {
+    /** hashMap will not evict entries automatically. */
+    final GSet<IntEntry, IntEntry> hashMap
+        = new GSetByHashMap<IntEntry, IntEntry>(1024, 0.75f);
+
+    final LightWeightCache<IntEntry, IntEntry> cache;
+    final IntData data;
+
+    final String info;
+    final long starttime = Time.now();
+    /** Determine the probability in {@link #check()}. */
+    final int denominator;
+    int iterate_count = 0;
+    int contain_count = 0;
+
+    private long currentTestTime = ran.nextInt();
+
+    LightWeightCacheTestCase(int tablelength, int sizeLimit,
+        long creationExpirationPeriod, long accessExpirationPeriod,
+        int datasize, int modulus) {
+      denominator = Math.min((datasize >> 7) + 1, 1 << 16);
+      info = getClass().getSimpleName() + "(" + new Date(starttime)
+          + "): tablelength=" + tablelength
+          + ", creationExpirationPeriod=" + creationExpirationPeriod
+          + ", accessExpirationPeriod=" + accessExpirationPeriod
+          + ", datasize=" + datasize
+          + ", modulus=" + modulus
+          + ", denominator=" + denominator;
+      println(info);
+
+      data = new IntData(datasize, modulus);
+      cache = new LightWeightCache<IntEntry, IntEntry>(tablelength, sizeLimit,
+          creationExpirationPeriod, 0, new LightWeightCache.Clock() {
+        @Override
+        long currentTime() {
+          return currentTestTime;
+        }
+      });
+
+      Assert.assertEquals(0, cache.size());
+    }
+
+    private boolean containsTest(IntEntry key) {
+      final boolean c = cache.contains(key);
+      if (c) {
+        Assert.assertTrue(hashMap.contains(key));
+      } else {
+        final IntEntry h = hashMap.remove(key);
+        if (h != null) {
+          Assert.assertTrue(cache.isExpired(h, currentTestTime));
+        }
+      }
+      return c;
+    }
+    @Override
+    public boolean contains(IntEntry key) {
+      final boolean e = containsTest(key);
+      check();
+      return e;
+    }
+
+    private IntEntry getTest(IntEntry key) {
+      final IntEntry c = cache.get(key);
+      if (c != null) {
+        Assert.assertEquals(hashMap.get(key).id, c.id);
+      } else {
+        final IntEntry h = hashMap.remove(key);
+        if (h != null) {
+          Assert.assertTrue(cache.isExpired(h, currentTestTime));
+        }
+      }
+      return c;
+    }
+    @Override
+    public IntEntry get(IntEntry key) {
+      final IntEntry e = getTest(key);
+      check();
+      return e;
+    }
+
+    private IntEntry putTest(IntEntry entry) {
+      final IntEntry c = cache.put(entry);
+      if (c != null) {
+        Assert.assertEquals(hashMap.put(entry).id, c.id);
+      } else {
+        final IntEntry h = hashMap.put(entry);
+        if (h != null && h != entry) {
+          // if h == entry, its expiration time is already updated
+          Assert.assertTrue(cache.isExpired(h, currentTestTime));
+        }
+      }
+      return c;
+    }
+    @Override
+    public IntEntry put(IntEntry entry) {
+      final IntEntry e = putTest(entry);
+      check();
+      return e;
+    }
+
+    private IntEntry removeTest(IntEntry key) {
+      final IntEntry c = cache.remove(key);
+      if (c != null) {
+        Assert.assertEquals(c.id, hashMap.remove(key).id);
+      } else {
+        final IntEntry h = hashMap.remove(key);
+        if (h != null) {
+          Assert.assertTrue(cache.isExpired(h, currentTestTime));
+        }
+      }
+      return c;
+    }
+    @Override
+    public IntEntry remove(IntEntry key) {
+      final IntEntry e = removeTest(key);
+      check();
+      return e;
+    }
+
+    private int sizeTest() {
+      final int c = cache.size();
+      Assert.assertTrue(hashMap.size() >= c);
+      return c;
+    }
+    @Override
+    public int size() {
+      final int s = sizeTest();
+      check();
+      return s;
+    }
+
+    @Override
+    public Iterator<IntEntry> iterator() {
+      throw new UnsupportedOperationException();
+    }
+
+    boolean tossCoin() {
+      return ran.nextInt(denominator) == 0;
+    }
+
+    void check() {
+      currentTestTime += ran.nextInt() & 0x3;
+
+      //test size
+      sizeTest();
+
+      if (tossCoin()) {
+        //test get(..), check content and test iterator
+        iterate_count++;
+        for(IntEntry i : cache) {
+          getTest(i);
+        }
+      }
+
+      if (tossCoin()) {
+        //test contains(..)
+        contain_count++;
+        final int count = Math.min(data.size(), 1000);
+        if (count == data.size()) {
+          for(IntEntry i : data.integers) {
+            containsTest(i);
+          }
+        } else {
+          for(int j = 0; j < count; j++) {
+            containsTest(data.get(ran.nextInt(data.size())));
+          }
+        }
+      }
+    }
+
+    String stat() {
+      final long t = Time.now() - starttime;
+      return String.format(" iterate=%5d, contain=%5d, time elapsed=%5d.%03ds",
+          iterate_count, contain_count, t/1000, t%1000);
+    }
+
+    @Override
+    public void clear() {
+      hashMap.clear();
+      cache.clear();
+      Assert.assertEquals(0, size());
+    }
+  }
+
+  private static class IntData {
+    final IntEntry[] integers;
+
+    IntData(int size, int modulus) {
+      integers = new IntEntry[size];
+      for(int i = 0; i < integers.length; i++) {
+        integers[i] = new IntEntry(i, ran.nextInt(modulus));
+      }
+    }
+
+    IntEntry get(int i) {
+      return integers[i];
+    }
+
+    int size() {
+      return integers.length;
+    }
+  }
+
+  /** Entries of {@link LightWeightCache} in this test */
+  private static class IntEntry implements LightWeightCache.Entry,
+      Comparable<IntEntry> {
+    private LightWeightGSet.LinkedElement next;
+    final int id;
+    final int value;
+    private long expirationTime = 0;
+
+    IntEntry(int id, int value) {
+      this.id = id;
+      this.value = value;
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+      return obj != null && obj instanceof IntEntry
+          && value == ((IntEntry)obj).value;
+    }
+
+    @Override
+    public int hashCode() {
+      return value;
+    }
+
+    @Override
+    public int compareTo(IntEntry that) {
+      return value - that.value;
+    }
+
+    @Override
+    public String toString() {
+      return id + "#" + value + ",expirationTime=" + expirationTime;
+    }
+
+    @Override
+    public LightWeightGSet.LinkedElement getNext() {
+      return next;
+    }
+
+    @Override
+    public void setNext(LightWeightGSet.LinkedElement e) {
+      next = e;
+    }
+
+    @Override
+    public void setExpirationTime(long timeNano) {
+      this.expirationTime = timeNano;
+    }
+
+    @Override
+    public long getExpirationTime() {
+      return expirationTime;
+    }
+  }
+}