HDFS-13223. Reduce DiffListBySkipList memory usage. Contributed by Shashikant Banerjee

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/DiffListBySkipList.java

@@ -17,6 +17,7 @@
  */
 package org.apache.hadoop.hdfs.server.namenode.snapshot;
 
+import com.google.common.base.Preconditions;
 import org.apache.hadoop.hdfs.server.namenode.INodeDirectory;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.
     DirectoryWithSnapshotFeature.DirectoryDiff;
@@ -30,9 +31,7 @@ import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.Arrays;
 import java.util.Collections;
-import java.util.Random;
 import java.util.Objects;
-import java.util.concurrent.ThreadLocalRandom;
 
 /**
  * SkipList is an implementation of a data structure for storing a sorted list
@@ -72,7 +71,20 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
   public static final Logger LOG =
       LoggerFactory.getLogger(DiffListBySkipList.class);
 
+  static String childrenDiff2String(ChildrenDiff diff) {
+    if (diff == null) {
+      return "null";
+    }
+    return "@" + Integer.toHexString(System.identityHashCode(diff));
+  }
+
+  static String skip2String(SkipListNode skipTo, ChildrenDiff diff) {
+    return "->" + skipTo + ":diff=" + childrenDiff2String(diff);
+  }
+
   private static class SkipDiff {
+    static final SkipDiff[] EMPTY_ARRAY = {};
+
     /**
      * The references to the subsequent nodes.
      */
@@ -104,7 +116,7 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
 
     @Override
     public String toString() {
-      return "->" + skipTo + (diff == null? " (diff==null)": "");
+      return skip2String(skipTo, diff);
     }
   }
 
@@ -112,17 +124,19 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
    * SkipListNode is an implementation of a DirectoryDiff List node,
    * which stores a Directory Diff and references to subsequent nodes.
    */
-  private final static class SkipListNode implements Comparable<Integer> {
+  final static class SkipListNode implements Comparable<Integer> {
 
     /**
      * The data element stored in this node.
      */
-    private DirectoryDiff diff;
+    private final DirectoryDiff diff;
 
+    /** Next node. */
+    private SkipListNode next;
     /**
-     * List containing combined children diffs over a skip interval.
+     * Array containing combined children diffs over a skip interval.
      */
-    private List<SkipDiff> skipDiffList;
+    private SkipDiff[] skips;
 
     /**
      * Constructs a new instance of SkipListNode with the specified data element
@@ -132,20 +146,28 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
      */
     SkipListNode(DirectoryDiff diff, int level) {
       this.diff = diff;
-      skipDiffList = new ArrayList<>(level + 1);
+
+      this.skips = level > 0? new SkipDiff[level]: SkipDiff.EMPTY_ARRAY;
+      for(int i = 0; i < skips.length; i++) {
+        skips[i] = new SkipDiff(null);
+      }
     }
 
     /**
      * Returns the level of this SkipListNode.
      */
     public int level() {
-      return skipDiffList.size() - 1;
+      return skips.length;
     }
 
     void trim() {
-      for (int level = level();
+      int n = skips.length - 1;
-           level > 0 && getSkipNode(level) == null; level--) {
+      for (; n >= 0 && skips[n] == null; n--) {
-        skipDiffList.remove(level);
+        continue;
+      }
+      n++;
+      if (n < skips.length) {
+        skips = n > 0 ? Arrays.copyOf(skips, n) : SkipDiff.EMPTY_ARRAY;
       }
     }
 
@@ -179,40 +201,66 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
     }
 
     public void setSkipDiff(ChildrenDiff cDiff, int level) {
-      if (level < skipDiffList.size()) {
+      Preconditions.checkArgument(level > 0);
-        skipDiffList.get(level).setDiff(cDiff);
+      resize(level);
-      } else {
+      skips[level - 1].setDiff(cDiff);
-        skipDiffList.add(new SkipDiff(cDiff));
+    }
+
+    void setSkipDiff4Target(
+        SkipListNode target, int startLevel, ChildrenDiff childrenDiff) {
+      for(int i = startLevel; i <= level(); i++) {
+        if (getSkipNode(i) != target) {
+          return;
+        }
+        setSkipDiff(childrenDiff, i);
+      }
+    }
+
+    private void resize(int newLevel) {
+      int i = skips.length;
+      if (i < newLevel) {
+        skips = Arrays.copyOf(skips, newLevel);
+        for (; i < newLevel; i++) {
+          skips[i] = new SkipDiff(null);
+        }
       }
     }
 
     public void setSkipTo(SkipListNode node, int level) {
-      for (int i = skipDiffList.size(); i <= level; i++) {
+      if (level == 0) {
-        skipDiffList.add(new SkipDiff(null));
+        next = node;
+      } else {
+        resize(level);
+        skips[level - 1].setSkipTo(node);
       }
-      skipDiffList.get(level).setSkipTo(node);
     }
 
     public ChildrenDiff getChildrenDiff(int level) {
       if (level == 0) {
-        return diff.getChildrenDiff();
+        return diff != null? diff.getChildrenDiff(): null;
       } else {
-        return skipDiffList.get(level).getDiff();
+        return skips[level - 1].getDiff();
       }
     }
 
     SkipListNode getSkipNode(int level) {
-      if (level >= skipDiffList.size()) {
+      return level == 0? next
-        return null;
+          : level <= skips.length? skips[level - 1].getSkipTo()
-      } else {
+          : null;
-        return skipDiffList.get(level).getSkipTo();
-      }
     }
 
     @Override
     public String toString() {
       return diff != null ? "" + diff.getSnapshotId() : "?";
     }
+
+    StringBuilder appendTo(StringBuilder b) {
+      b.append(this).append(": ").append(skip2String(next, getChildrenDiff(0)));
+      for(int i = 0; i < skips.length; i++) {
+        b.append(", ").append(skips[i]);
+      }
+      return b;
+    }
   }
 
   /**
@@ -220,17 +268,7 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
    * The list will grow linearly once a new Directory diff gets added.
    * All the list inteface defined methods provide a linear view of the list.
    */
-  private List<SkipListNode> skipNodeList;
+  private final List<SkipListNode> skipNodeList;
-
-  /**
-   * The max no of skipLevels.
-   */
-  private final int maxSkipLevels;
-
-  /**
-   * The no of diffs after which the level promotion happens.
-   */
-  private final int skipInterval;
 
   /**
    * The head node to the list.
@@ -240,11 +278,9 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
   /**
    * Constructs a new, empty instance of SkipList.
    */
-  public DiffListBySkipList(int capacity, int interval, int skipLevel) {
+  public DiffListBySkipList(int capacity) {
     skipNodeList = new ArrayList<>(capacity);
     head = new SkipListNode(null, 0);
-    this.maxSkipLevels = skipLevel;
-    this.skipInterval = interval;
   }
 
   /**
@@ -254,13 +290,9 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
    */
   @Override
   public void addFirst(DirectoryDiff diff) {
-    final int nodeLevel = randomLevel(skipInterval, maxSkipLevels);
+    final int nodeLevel = DirectoryDiffListFactory.randomLevel();
     final SkipListNode[] nodePath = new SkipListNode[nodeLevel + 1];
-
     Arrays.fill(nodePath, head);
-    for (int level = head.level() + 1; level <= nodeLevel; level++) {
-      head.skipDiffList.add(new SkipDiff(null));
-    }
 
     final SkipListNode newNode = new SkipListNode(diff, nodeLevel);
     for (int level = 0; level <= nodeLevel; level++) {
@@ -316,15 +348,10 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
    */
   @Override
   public boolean addLast(DirectoryDiff diff) {
-    final int nodeLevel = randomLevel(skipInterval, maxSkipLevels);
+    final int nodeLevel = DirectoryDiffListFactory.randomLevel();
-    final int headLevel = head.level();
     final SkipListNode[] nodePath = findPreviousNodes(null, nodeLevel);
-    for (int level = headLevel + 1; level <= nodeLevel; level++) {
-      head.skipDiffList.add(new SkipDiff(null));
-      nodePath[level] = head;
-    }
 
-    final SkipListNode current = new SkipListNode(diff, nodeLevel);
+    final SkipListNode newNode = new SkipListNode(diff, nodeLevel);
     for (int level = 0; level <= nodeLevel; level++) {
       if (level > 0 && nodePath[level] != head) {
         //  suppose the list is like:
@@ -339,22 +366,24 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
         //  level 0:head->    s1->s2->s3->s4->s5->s6->s7->s8->s9---->s10
         //  At level 1, we combine s5, s6, s7, s8, s9 and store as s5'
         //  At level 2, we combine s1', s3', s5' and form s1'' and store at s1.
-        // Note : the last element(elemnt being added) diff is not added while
+        // Note : the last element(element being added) diff is not added while
         // combining the diffs.
-        ChildrenDiff combined = combineDiff(nodePath[level], current, level);
+        ChildrenDiff combined = combineDiff(nodePath[level], newNode, level);
         if (combined != null) {
           nodePath[level].setSkipDiff(combined, level);
         }
       }
-      nodePath[level].setSkipTo(current, level);
+      nodePath[level].setSkipTo(newNode, level);
-      current.setSkipTo(null, level);
+      newNode.setSkipTo(null, level);
     }
-    return skipNodeList.add(current);
+    return skipNodeList.add(newNode);
   }
 
   private static ChildrenDiff combineDiff(SkipListNode from, SkipListNode to,
       int level) {
     ChildrenDiff combined = null;
+    ChildrenDiff first = null;
+
     SkipListNode cur = from;
     for (int i = level - 1; i >= 0; i--) {
       while (cur != to) {
@@ -362,14 +391,20 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
         if (next == null) {
           break;
         }
-        if (combined == null) {
+
-          combined = new ChildrenDiff();
+        if (first == null) {
+          first = cur.getChildrenDiff(i);
+        } else {
+          if (combined == null) {
+            combined = new ChildrenDiff();
+            combined.combinePosterior(first, null);
+          }
+          combined.combinePosterior(cur.getChildrenDiff(i), null);
         }
-        combined.combinePosterior(cur.getChildrenDiff(i), null);
         cur = next;
       }
     }
-    return combined;
+    return combined != null? combined: first;
   }
 
   /**
@@ -383,6 +418,10 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
     return skipNodeList.get(index).getDiff();
   }
 
+  SkipListNode getSkipListNode(int i) {
+    return skipNodeList.get(i);
+  }
+
   /**
    * Removes the element at the specified position in this list.
    *
@@ -391,12 +430,20 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
    */
   @Override
   public DirectoryDiff remove(int index) {
-    SkipListNode node = getNode(index);
+    final SkipListNode node = getNode(index);
+
     int headLevel = head.level();
     int nodeLevel = node.level();
     final SkipListNode[] nodePath = findPreviousNodes(node, nodeLevel);
+
     for (int level = 0; level <= nodeLevel; level++) {
-      if (nodePath[level] != head && level > 0) {
+      final SkipListNode previous = nodePath[level];
+      final SkipListNode next = node.getSkipNode(level);
+      if (level == 0) {
+        if (next != null) {
+          previous.setSkipDiff4Target(next, 1, previous.getChildrenDiff(0));
+        }
+      } else if (previous != head) {
         // if the last snapshot is deleted, for all the skip level nodes
         // pointing to the last one, the combined children diff at each level
         // > 0 should be made null and skip pointers will be updated to null.
@@ -404,8 +451,8 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
         // the diff of deleted node at each level to the previous skip level
         // node at that level and the skip pointers will be updated to point to
         // the skip nodes of the deleted node.
-        if (index == size() - 1) {
+        if (next == null) {
-          nodePath[level].setSkipDiff(null, level);
+          previous.setSkipDiff(null, level);
         } else {
           /* Ideally at level 0, the deleted diff will be combined with
            * the previous diff , and deleted inodes will be cleaned up
@@ -414,12 +461,23 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
            * {@link AbstractINodeDiffList#deleteSnapshotDiff} function.
            */
           if (node.getChildrenDiff(level) != null) {
-            nodePath[level].getChildrenDiff(level)
+            final ChildrenDiff combined;
-                .combinePosterior(node.getChildrenDiff(level), null);
+            if (previous == nodePath[level - 1]
+                && next == node.getSkipNode(level - 1)) {
+              combined = nodePath[level - 1].getChildrenDiff(level - 1);
+              previous.setSkipDiff4Target(next, level + 1, combined);
+            } else if (next == previous.getSkipNode(level + 1)) {
+              combined = previous.getChildrenDiff(level + 1);
+            } else {
+              combined = new ChildrenDiff();
+              combined.combinePosterior(previous.getChildrenDiff(level), null);
+              combined.combinePosterior(node.getChildrenDiff(level), null);
+            }
+            previous.setSkipDiff(combined, level);
           }
         }
       }
-      nodePath[level].setSkipTo(node.getSkipNode(level), level);
+      previous.setSkipTo(next, level);
     }
     if (nodeLevel == headLevel) {
       head.trim();
@@ -478,24 +536,6 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
     return skipNodeList.get(index);
   }
 
-  /**
-   * Returns the level of the skipList node.
-   *
-   * @param skipInterval The max interval after which the next level promotion
-   *                     should happen.
-   * @param maxLevel     Maximum no of skip levels
-   * @return A value in the range 0 to maxLevel-1.
-   */
-  static int randomLevel(int skipInterval, int maxLevel) {
-    final Random r = ThreadLocalRandom.current();
-    for (int level = 0; level < maxLevel; level++) {
-      // skip to the next level with probability 1/skipInterval
-      if (r.nextInt(skipInterval) > 0) {
-        return level;
-      }
-    }
-    return maxLevel;
-  }
 
   /**
    * This function returns the minimal set of diffs required to combine in
@@ -535,10 +575,10 @@ public class DiffListBySkipList implements DiffList<DirectoryDiff> {
 
   @Override
   public String toString() {
-    final StringBuilder b = new StringBuilder(getClass().getSimpleName());
+    final StringBuilder b = new StringBuilder().append(" head: ");
-    b.append(" head: ").append(head).append(head.skipDiffList);
+    head.appendTo(b);
     for (SkipListNode n : skipNodeList) {
-      b.append("\n  ").append(n).append(n.skipDiffList);
+      n.appendTo(b.append("\n  "));
     }
     return b.toString();
   }

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/DirectoryDiffListFactory.java

@@ -17,9 +17,11 @@
  */
 package org.apache.hadoop.hdfs.server.namenode.snapshot;
 
-import org.apache.commons.logging.Log;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.DirectoryWithSnapshotFeature.DirectoryDiff;
+import org.slf4j.Logger;
 
+import java.util.Random;
+import java.util.concurrent.ThreadLocalRandom;
 import java.util.function.IntFunction;
 
 /** For creating {@link DiffList} for {@link DirectoryDiff}. */
@@ -28,9 +30,12 @@ public abstract class DirectoryDiffListFactory {
     return constructor.apply(capacity);
   }
 
-  public static void init(int skipInterval, int maxLevels, Log log) {
+  public static void init(int interval, int maxSkipLevels, Logger log) {
+    DirectoryDiffListFactory.skipInterval = interval;
+    DirectoryDiffListFactory.maxLevels = maxSkipLevels;
+
     if (maxLevels > 0) {
-      constructor = c -> new DiffListBySkipList(c, skipInterval, maxLevels);
+      constructor = c -> new DiffListBySkipList(c);
       log.info("SkipList is enabled with skipInterval=" + skipInterval
           + ", maxLevels=" + maxLevels);
     } else {
@@ -42,5 +47,24 @@ public abstract class DirectoryDiffListFactory {
   private static volatile IntFunction<DiffList<DirectoryDiff>> constructor
       = c -> new DiffListByArrayList<>(c);
 
+  private static volatile int skipInterval;
+  private static volatile int maxLevels;
+
+  /**
+   * Returns the level of a skip list node.
+   * @return A value in the range 0 to maxLevels.
+   */
+  public static int randomLevel() {
+    final Random r = ThreadLocalRandom.current();
+    for (int level = 0; level < maxLevels; level++) {
+      // skip to the next level with probability 1/skipInterval
+      if (r.nextInt(skipInterval) > 0) {
+        return level;
+      }
+    }
+    return maxLevels;
+  }
+
+
   private DirectoryDiffListFactory() {}
 }

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/SnapshotManager.java

@@ -36,8 +36,6 @@ import java.util.concurrent.atomic.AtomicInteger;
 import javax.management.ObjectName;
 
 import com.google.common.annotations.VisibleForTesting;
-import org.apache.commons.logging.Log;
-import org.apache.commons.logging.LogFactory;
 import org.apache.hadoop.conf.Configuration;
 import org.apache.hadoop.hdfs.DFSConfigKeys;
 import org.apache.hadoop.hdfs.DFSUtil;
@@ -60,6 +58,8 @@ import org.apache.hadoop.hdfs.server.namenode.LeaseManager;
 import org.apache.hadoop.metrics2.util.MBeans;
 
 import com.google.common.base.Preconditions;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * Manage snapshottable directories and their snapshots.
@@ -74,7 +74,8 @@ import com.google.common.base.Preconditions;
  * if necessary.
  */
 public class SnapshotManager implements SnapshotStatsMXBean {
-  public static final Log LOG = LogFactory.getLog(SnapshotManager.class);
+  public static final Logger LOG =
+      LoggerFactory.getLogger(SnapshotManager.class);
 
   private final FSDirectory fsdir;
   private boolean captureOpenFiles;

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/snapshot/TestDiffListBySkipList.java

@@ -27,6 +27,7 @@ import org.apache.hadoop.hdfs.server.namenode.INode;
 import org.apache.hadoop.hdfs.server.namenode.INodeDirectory;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.DirectoryWithSnapshotFeature.ChildrenDiff;
 import org.apache.hadoop.hdfs.server.namenode.snapshot.DirectoryWithSnapshotFeature.DirectoryDiff;
+import org.apache.hadoop.hdfs.server.namenode.snapshot.DiffListBySkipList.SkipListNode;
 import org.apache.hadoop.hdfs.util.ReadOnlyList;
 import org.junit.After;
 import org.junit.Assert;
@@ -36,13 +37,16 @@ import org.junit.Test;
 import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.ThreadLocalRandom;
-import java.util.function.IntFunction;
+import java.util.function.ToIntBiFunction;
+import java.util.function.ToIntFunction;
 
 /**
  * This class tests the DirectoryDiffList API's.
  */
 public class TestDiffListBySkipList {
   static final int NUM_SNAPSHOTS = 100;
+  static final int MAX_LEVEL = 5;
+
   static {
     SnapshotTestHelper.disableLogs();
   }
@@ -71,6 +75,11 @@ public class TestDiffListBySkipList {
     }
   }
 
+  static DiffListBySkipList newDiffListBySkipList() {
+    DirectoryDiffListFactory.init(3, MAX_LEVEL, DiffListBySkipList.LOG);
+    return new DiffListBySkipList(0);
+  }
+
   static void assertList(List<INode> expected, List<INode> computed) {
     Assert.assertEquals(expected.size(), computed.size());
     for (int index = 0; index < expected.size(); index++) {
@@ -114,6 +123,8 @@ public class TestDiffListBySkipList {
         }
       }
     }
+
+    assertSkipList(skip);
   }
 
   private static ChildrenDiff getCombined(
@@ -146,7 +157,7 @@ public class TestDiffListBySkipList {
     final Path root = new Path("/testAddLast" + n);
     DiffListBySkipList.LOG.info("run " + root);
 
-    final DiffListBySkipList skipList = new DiffListBySkipList(0, 3, 5);
+    final DiffListBySkipList skipList = newDiffListBySkipList();
     final DiffList<DirectoryDiff> arrayList = new DiffListByArrayList<>(0);
     INodeDirectory dir = addDiff(n, skipList, arrayList, root);
     // verify that the both the children list obtained from hdfs and
@@ -180,7 +191,7 @@ public class TestDiffListBySkipList {
     DiffList<DirectoryDiff> diffs = dir.getDiffs().asList();
     List<INode> childrenList = ReadOnlyList.Util.asList(dir.getChildrenList(
         diffs.get(0).getSnapshotId()));
-    final DiffListBySkipList skipList = new DiffListBySkipList(0, 3, 5);
+    final DiffListBySkipList skipList = newDiffListBySkipList();
     final DiffList<DirectoryDiff> arrayList = new DiffListByArrayList<>(0);
     for (int i = diffs.size() - 1; i >= 0; i--) {
       final DirectoryDiff d = diffs.get(i);
@@ -208,6 +219,7 @@ public class TestDiffListBySkipList {
       skipList.addLast(d);
       arrayList.addLast(d);
     }
+    DiffListBySkipList.LOG.info("skipList: " + skipList);
     return dir;
   }
 
@@ -228,19 +240,26 @@ public class TestDiffListBySkipList {
     testRemove("Random", n, i -> ThreadLocalRandom.current().nextInt(n - i));
   }
 
-  static void testRemove(String name, int n, IntFunction<Integer> indexFunction)
+  static void testRemove(String name, int n,
+      ToIntFunction<Integer> indexFunction) throws Exception {
+    testRemove(name, n, (skipList, i) -> indexFunction.applyAsInt(i));
+  }
+
+  static void testRemove(String name, int n,
+      ToIntBiFunction<DiffListBySkipList, Integer> indexFunction)
       throws Exception {
     final Path root = new Path("/testRemove" + name + n);
     DiffListBySkipList.LOG.info("run " + root);
 
-    final DiffListBySkipList skipList = new DiffListBySkipList(0, 3, 5);
+    final DiffListBySkipList skipList = newDiffListBySkipList();
     final DiffList<DirectoryDiff> arrayList = new DiffListByArrayList<>(0);
     final INodeDirectory dir = addDiff(n, skipList, arrayList, root);
     Assert.assertEquals(n, arrayList.size());
     Assert.assertEquals(n, skipList.size());
 
-    for(int i = 0; i < n; i++) {
+    for (int i = 0; i < n; i++) {
-      final int index = indexFunction.apply(i);
+      DiffListBySkipList.LOG.debug("i={}: {}", i, skipList);
+      final int index = indexFunction.applyAsInt(skipList, i);
       final DirectoryDiff diff = remove(index, skipList, arrayList);
       hdfs.deleteSnapshot(root, "s" + diff.getSnapshotId());
       verifyChildrenList(skipList, dir);
@@ -248,10 +267,58 @@ public class TestDiffListBySkipList {
     }
   }
 
+  @Test
+  public void testRemoveFromLowerLevel() throws Exception {
+    testRemove("FromLowerLevel", NUM_SNAPSHOTS,
+        new ToIntBiFunction<DiffListBySkipList, Integer>() {
+          private int level = 0;
+
+          @Override
+          public int applyAsInt(DiffListBySkipList skipList, Integer integer) {
+            for (; level <= MAX_LEVEL; level++) {
+              final int index = findIndex(skipList, level);
+              if (index != -1) {
+                return index;
+              }
+            }
+            return -1;
+          }
+        });
+  }
+
+  @Test
+  public void testRemoveFromUpperLevel() throws Exception {
+    testRemove("FromUpperLevel", NUM_SNAPSHOTS,
+        new ToIntBiFunction<DiffListBySkipList, Integer>() {
+      private int level = MAX_LEVEL;
+      @Override
+      public int applyAsInt(DiffListBySkipList skipList, Integer integer) {
+        for(; level >= 0; level--) {
+          final int index = findIndex(skipList, level);
+          if (index != -1) {
+            return index;
+          }
+          DiffListBySkipList.LOG.info("change from level " + level);
+        }
+        return -1;
+      }
+    });
+  }
+
+  static int findIndex(DiffListBySkipList skipList, int level) {
+    for (int i = 0; i < skipList.size(); i++) {
+      if (skipList.getSkipListNode(i).level() == level) {
+        return i;
+      }
+    }
+    return -1;
+  }
+
   static DirectoryDiff remove(int i, DiffListBySkipList skip,
       DiffList<DirectoryDiff> array) {
-    DiffListBySkipList.LOG.info("remove " + i);
     final DirectoryDiff expected = array.remove(i);
+    DiffListBySkipList.LOG
+        .info("remove " + i + ", snapshotId=" + expected.getSnapshotId());
     final DirectoryDiff computed = skip.remove(i);
     assertDirectoryDiff(expected, computed);
     return expected;
@@ -261,4 +328,30 @@ public class TestDiffListBySkipList {
       DirectoryDiff computed) {
     Assert.assertEquals(expected.getSnapshotId(), computed.getSnapshotId());
   }
+
+  static void assertSkipList(DiffListBySkipList skipList) {
+    for(int i = 0; i < skipList.size(); i++) {
+      assertSkipListNode(skipList.getSkipListNode(i));
+    }
+  }
+
+  static void assertSkipListNode(SkipListNode n) {
+    for (int i = 1; i <= n.level(); i++) {
+      final SkipListNode target = n.getSkipNode(i);
+      final ChildrenDiff diff = n.getChildrenDiff(i);
+      if (target == null) {
+        if (diff != null) {
+          throw new AssertionError(
+              "Target is null but children diff is not at i=" + i + n
+                  .appendTo(new StringBuilder(": ")));
+        }
+      } else if (target == n.getSkipNode(i - 1)) {
+        if (diff != n.getChildrenDiff(i - 1)) {
+          throw new AssertionError(
+              "Same target but different children diff at i=" + i + n
+                  .appendTo(new StringBuilder(": ")));
+        }
+      }
+    }
+  }
 }