HBASE-3455. Add memstore-local allocation buffers to combat heap fragmentation in the region server.

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1068148 13f79535-47bb-0310-9956-ffa450edef68

CHANGES.txt

@@ -84,10 +84,17 @@ Release 0.91.0 - Unreleased
    HBASE-3448  RegionSplitter, utility class to manually split tables
    HBASE-2824  A filter that randomly includes rows based on a configured
                chance (Ferdy via Andrew Purtell)
-
+   HBASE-3455  Add memstore-local allocation buffers to combat heap
+               fragmentation in the region server. Enabled by default as of
+               0.91
 
 Release 0.90.1 - Unreleased
 
+  NEW FEATURES
+   HBASE-3455  Add memstore-local allocation buffers to combat heap
+               fragmentation in the region server. Experimental / disabled
+               by default in 0.90.1
+  
   BUG FIXES
    HBASE-3483  Memstore lower limit should trigger asynchronous flushes
 

src/main/java/org/apache/hadoop/hbase/regionserver/MemStore.java

@@ -34,10 +34,13 @@ import java.util.concurrent.locks.ReentrantReadWriteLock;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.hbase.HBaseConfiguration;
 import org.apache.hadoop.hbase.HConstants;
 import org.apache.hadoop.hbase.KeyValue;
 import org.apache.hadoop.hbase.client.Scan;
 import org.apache.hadoop.hbase.io.HeapSize;
+import org.apache.hadoop.hbase.regionserver.MemStoreLAB.Allocation;
 import org.apache.hadoop.hbase.util.Bytes;
 import org.apache.hadoop.hbase.util.ClassSize;
 
@@ -55,6 +58,13 @@ import org.apache.hadoop.hbase.util.ClassSize;
 public class MemStore implements HeapSize {
   private static final Log LOG = LogFactory.getLog(MemStore.class);
 
+  static final String USEMSLAB_KEY =
+    "hbase.hregion.memstore.mslab.enabled";
+  private static final boolean USEMSLAB_DEFAULT = false;
+
+
+  private Configuration conf;
+
   // MemStore.  Use a KeyValueSkipListSet rather than SkipListSet because of the
   // better semantics.  The Map will overwrite if passed a key it already had
   // whereas the Set will not add new KV if key is same though value might be
@@ -80,19 +90,23 @@ public class MemStore implements HeapSize {
 
   TimeRangeTracker timeRangeTracker;
   TimeRangeTracker snapshotTimeRangeTracker;
+  
+  MemStoreLAB allocator;
 
   /**
    * Default constructor. Used for tests.
    */
   public MemStore() {
-    this(KeyValue.COMPARATOR);
+    this(HBaseConfiguration.create(), KeyValue.COMPARATOR);
   }
 
   /**
    * Constructor.
    * @param c Comparator
    */
-  public MemStore(final KeyValue.KVComparator c) {
+  public MemStore(final Configuration conf,
+                  final KeyValue.KVComparator c) {
+    this.conf = conf;
     this.comparator = c;
     this.comparatorIgnoreTimestamp =
       this.comparator.getComparatorIgnoringTimestamps();
@@ -102,6 +116,11 @@ public class MemStore implements HeapSize {
     timeRangeTracker = new TimeRangeTracker();
     snapshotTimeRangeTracker = new TimeRangeTracker();
     this.size = new AtomicLong(DEEP_OVERHEAD);
+    if (conf.getBoolean(USEMSLAB_KEY, USEMSLAB_DEFAULT)) {
+      this.allocator = new MemStoreLAB(conf);
+    } else {
+      this.allocator = null;
+    }
   }
 
   void dump() {
@@ -134,6 +153,10 @@ public class MemStore implements HeapSize {
           this.timeRangeTracker = new TimeRangeTracker();
           // Reset heap to not include any keys
           this.size.set(DEEP_OVERHEAD);
+          // Reset allocator so we get a fresh buffer for the new memstore
+          if (allocator != null) {
+            this.allocator = new MemStoreLAB(conf);
+          }
         }
       }
     } finally {
@@ -184,18 +207,47 @@ public class MemStore implements HeapSize {
    * @return approximate size of the passed key and value.
    */
   long add(final KeyValue kv) {
-    long s = -1;
     this.lock.readLock().lock();
     try {
-      s = heapSizeChange(kv, this.kvset.add(kv));
-      timeRangeTracker.includeTimestamp(kv);
-      this.size.addAndGet(s);
+      KeyValue toAdd = maybeCloneWithAllocator(kv);
+      return internalAdd(toAdd);
     } finally {
       this.lock.readLock().unlock();
     }
+  }
+  
+  /**
+   * Internal version of add() that doesn't clone KVs with the
+   * allocator, and doesn't take the lock.
+   * 
+   * Callers should ensure they already have the read lock taken
+   */
+  private long internalAdd(final KeyValue toAdd) {
+    long s = heapSizeChange(toAdd, this.kvset.add(toAdd));
+    timeRangeTracker.includeTimestamp(toAdd);
+    this.size.addAndGet(s);
     return s;
   }
 
+  private KeyValue maybeCloneWithAllocator(KeyValue kv) {
+    if (allocator == null) {
+      return kv;
+    }
+
+    int len = kv.getLength();
+    Allocation alloc = allocator.allocateBytes(len);
+    if (alloc == null) {
+      // The allocation was too large, allocator decided
+      // not to do anything with it.
+      return kv;
+    }
+    assert alloc != null && alloc.getData() != null;
+    System.arraycopy(kv.getBuffer(), kv.getOffset(), alloc.getData(), alloc.getOffset(), len);
+    KeyValue newKv = new KeyValue(alloc.getData(), alloc.getOffset(), len);
+    newKv.setMemstoreTS(kv.getMemstoreTS());
+    return newKv;
+  }
+
   /**
    * Write a delete
    * @param delete
@@ -205,8 +257,9 @@ public class MemStore implements HeapSize {
     long s = 0;
     this.lock.readLock().lock();
     try {
-      s += heapSizeChange(delete, this.kvset.add(delete));
-      timeRangeTracker.includeTimestamp(delete);
+      KeyValue toAdd = maybeCloneWithAllocator(delete);
+      s += heapSizeChange(toAdd, this.kvset.add(toAdd));
+      timeRangeTracker.includeTimestamp(toAdd);
     } finally {
       this.lock.readLock().unlock();
     }
@@ -459,12 +512,20 @@ public class MemStore implements HeapSize {
    * <p>
    * If there are any existing KeyValues in this MemStore with the same row,
    * family, and qualifier, they are removed.
+   * <p>
+   * Callers must hold the read lock.
+   * 
    * @param kv
    * @return change in size of MemStore
    */
   private long upsert(KeyValue kv) {
     // Add the KeyValue to the MemStore
-    long addedSize = add(kv);
+    // Use the internalAdd method here since we (a) already have a lock
+    // and (b) cannot safely use the MSLAB here without potentially
+    // hitting OOME - see TestMemStore.testUpsertMSLAB for a
+    // test that triggers the pathological case if we don't avoid MSLAB
+    // here.
+    long addedSize = internalAdd(kv);
 
     // Get the KeyValues for the row/family/qualifier regardless of timestamp.
     // For this case we want to clean up any other puts
@@ -732,7 +793,7 @@ public class MemStore implements HeapSize {
   }
 
   public final static long FIXED_OVERHEAD = ClassSize.align(
-      ClassSize.OBJECT + (9 * ClassSize.REFERENCE));
+      ClassSize.OBJECT + (11 * ClassSize.REFERENCE));
 
   public final static long DEEP_OVERHEAD = ClassSize.align(FIXED_OVERHEAD +
       ClassSize.REENTRANT_LOCK + ClassSize.ATOMIC_LONG +

src/main/java/org/apache/hadoop/hbase/regionserver/MemStoreLAB.java

@@ -0,0 +1,262 @@
+/**
+ * Copyright 2011 The Apache Software Foundation
+ *
+ * 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.hbase.regionserver;
+
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicReference;
+
+import org.apache.hadoop.conf.Configuration;
+import com.google.common.base.Preconditions;
+
+/**
+ * A memstore-local allocation buffer.
+ * <p>
+ * The MemStoreLAB is basically a bump-the-pointer allocator that allocates
+ * big (2MB) byte[] chunks from and then doles it out to threads that request
+ * slices into the array.
+ * <p>
+ * The purpose of this class is to combat heap fragmentation in the
+ * regionserver. By ensuring that all KeyValues in a given memstore refer
+ * only to large chunks of contiguous memory, we ensure that large blocks
+ * get freed up when the memstore is flushed.
+ * <p>
+ * Without the MSLAB, the byte array allocated during insertion end up
+ * interleaved throughout the heap, and the old generation gets progressively
+ * more fragmented until a stop-the-world compacting collection occurs.
+ * <p>
+ * TODO: we should probably benchmark whether word-aligning the allocations
+ * would provide a performance improvement - probably would speed up the
+ * Bytes.toLong/Bytes.toInt calls in KeyValue, but some of those are cached
+ * anyway
+ */
+public class MemStoreLAB {
+  private AtomicReference<Chunk> curChunk = new AtomicReference<Chunk>();
+
+  final static String CHUNK_SIZE_KEY = "hbase.hregion.memstore.mslab.chunksize";
+  final static int CHUNK_SIZE_DEFAULT = 2048 * 1024;
+  final int chunkSize;
+
+  final static String MAX_ALLOC_KEY = "hbase.hregion.memstore.mslab.max.allocation";
+  final static int MAX_ALLOC_DEFAULT = 256  * 1024; // allocs bigger than this don't go through allocator
+  final int maxAlloc;
+
+  public MemStoreLAB() {
+    this(new Configuration());
+  }
+
+  public MemStoreLAB(Configuration conf) {
+    chunkSize = conf.getInt(CHUNK_SIZE_KEY, CHUNK_SIZE_DEFAULT);
+    maxAlloc = conf.getInt(MAX_ALLOC_KEY, MAX_ALLOC_DEFAULT);
+
+    // if we don't exclude allocations >CHUNK_SIZE, we'd infiniteloop on one!
+    Preconditions.checkArgument(
+      maxAlloc <= chunkSize,
+      MAX_ALLOC_KEY + " must be less than " + CHUNK_SIZE_KEY);
+  }
+
+  /**
+   * Allocate a slice of the given length.
+   *
+   * If the size is larger than the maximum size specified for this
+   * allocator, returns null.
+   */
+  public Allocation allocateBytes(int size) {
+    Preconditions.checkArgument(size >= 0, "negative size");
+
+    // Callers should satisfy large allocations directly from JVM since they
+    // don't cause fragmentation as badly.
+    if (size > maxAlloc) {
+      return null;
+    }
+
+    while (true) {
+      Chunk c = getOrMakeChunk();
+
+      // Try to allocate from this chunk
+      int allocOffset = c.alloc(size);
+      if (allocOffset != -1) {
+        // We succeeded - this is the common case - small alloc
+        // from a big buffer
+        return new Allocation(c.data, allocOffset);
+      }
+
+      // not enough space!
+      // try to retire this chunk
+      tryRetireChunk(c);
+    }
+  }
+
+  /**
+   * Try to retire the current chunk if it is still
+   * <code>c</code>. Postcondition is that curChunk.get()
+   * != c
+   */
+  private void tryRetireChunk(Chunk c) {
+    @SuppressWarnings("unused")
+    boolean weRetiredIt = curChunk.compareAndSet(c, null);
+    // If the CAS succeeds, that means that we won the race
+    // to retire the chunk. We could use this opportunity to
+    // update metrics on external fragmentation.
+    //
+    // If the CAS fails, that means that someone else already
+    // retired the chunk for us.
+  }
+
+  /**
+   * Get the current chunk, or, if there is no current chunk,
+   * allocate a new one from the JVM.
+   */
+  private Chunk getOrMakeChunk() {
+    while (true) {
+      // Try to get the chunk
+      Chunk c = curChunk.get();
+      if (c != null) {
+        return c;
+      }
+
+      // No current chunk, so we want to allocate one. We race
+      // against other allocators to CAS in an uninitialized chunk
+      // (which is cheap to allocate)
+      c = new Chunk(chunkSize);
+      if (curChunk.compareAndSet(null, c)) {
+        // we won race - now we need to actually do the expensive
+        // allocation step
+        c.init();
+        return c;
+      }
+      // someone else won race - that's fine, we'll try to grab theirs
+      // in the next iteration of the loop.
+    }
+  }
+
+  /**
+   * A chunk of memory out of which allocations are sliced.
+   */
+  private static class Chunk {
+    /** Actual underlying data */
+    private byte[] data;
+
+    private static final int UNINITIALIZED = -1;
+    /**
+     * Offset for the next allocation, or the sentinel value -1
+     * which implies that the chunk is still uninitialized.
+     * */
+    private AtomicInteger nextFreeOffset = new AtomicInteger(UNINITIALIZED);
+
+    /** Total number of allocations satisfied from this buffer */
+    private AtomicInteger allocCount = new AtomicInteger();
+
+    /** Size of chunk in bytes */
+    private final int size;
+
+    /**
+     * Create an uninitialized chunk. Note that memory is not allocated yet, so
+     * this is cheap.
+     * @param size in bytes
+     */
+    private Chunk(int size) {
+      this.size = size;
+    }
+
+    /**
+     * Actually claim the memory for this chunk. This should only be called from
+     * the thread that constructed the chunk. It is thread-safe against other
+     * threads calling alloc(), who will block until the allocation is complete.
+     */
+    public void init() {
+      assert nextFreeOffset.get() == UNINITIALIZED;
+      data = new byte[size];
+      // Mark that it's ready for use
+      boolean initted = nextFreeOffset.compareAndSet(
+          UNINITIALIZED, 0);
+      // We should always succeed the above CAS since only one thread
+      // calls init()!
+      Preconditions.checkState(initted,
+          "Multiple threads tried to init same chunk");
+    }
+
+    /**
+     * Try to allocate <code>size</code> bytes from the chunk.
+     * @return the offset of the successful allocation, or -1 to indicate not-enough-space
+     */
+    public int alloc(int size) {
+      while (true) {
+        int oldOffset = nextFreeOffset.get();
+        if (oldOffset == UNINITIALIZED) {
+          // The chunk doesn't have its data allocated yet.
+          // Since we found this in curChunk, we know that whoever
+          // CAS-ed it there is allocating it right now. So spin-loop
+          // shouldn't spin long!
+          Thread.yield();
+          continue;
+        }
+
+        if (oldOffset + size > data.length) {
+          return -1; // alloc doesn't fit
+        }
+
+        // Try to atomically claim this chunk
+        if (nextFreeOffset.compareAndSet(oldOffset, oldOffset + size)) {
+          // we got the alloc
+          allocCount.incrementAndGet();
+          return oldOffset;
+        }
+        // we raced and lost alloc, try again
+      }
+    }
+
+    @Override
+    public String toString() {
+      return "Chunk@" + System.identityHashCode(this) +
+        " allocs=" + allocCount.get() + "waste=" +
+        (data.length - nextFreeOffset.get());
+    }
+  }
+
+  /**
+   * The result of a single allocation. Contains the chunk that the
+   * allocation points into, and the offset in this array where the
+   * slice begins.
+   */
+  public static class Allocation {
+    private final byte[] data;
+    private final int offset;
+
+    private Allocation(byte[] data, int off) {
+      this.data = data;
+      this.offset = off;
+    }
+
+    @Override
+    public String toString() {
+      return "Allocation(data=" + data +
+        " with capacity=" + data.length +
+        ", off=" + offset + ")";
+    }
+
+    byte[] getData() {
+      return data;
+    }
+
+    int getOffset() {
+      return offset;
+    }
+  }
+}

src/main/java/org/apache/hadoop/hbase/regionserver/Store.java

@@ -178,7 +178,7 @@ public class Store implements HeapSize {
       // second -> ms adjust for user data
       this.ttl *= 1000;
     }
-    this.memstore = new MemStore(this.comparator);
+    this.memstore = new MemStore(conf, this.comparator);
     this.storeNameStr = Bytes.toString(this.family.getName());
 
     // By default, compact if storefile.count >= minFilesToCompact

src/main/resources/hbase-default.xml

@@ -359,6 +359,16 @@
     worse, we OOME.
     </description>
   </property>
+  <property>
+    <name>hbase.hregion.memstore.mslab.enabled</name>
+    <value>true</value>
+    <description>
+      Enables the MemStore-Local Allocation Buffer,
+      a feature which works to prevent heap fragmentation under
+      heavy write loads. This can reduce the frequency of stop-the-world
+      GC pauses on large heaps.
+    </description>
+  </property>
   <property>
     <name>hbase.hregion.max.filesize</name>
     <value>268435456</value>

src/test/java/org/apache/hadoop/hbase/regionserver/TestMemStore.java

@@ -20,6 +20,8 @@
 package org.apache.hadoop.hbase.regionserver;
 
 import java.io.IOException;
+import java.lang.management.ManagementFactory;
+import java.lang.management.MemoryMXBean;
 import java.rmi.UnexpectedException;
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -32,6 +34,8 @@ import junit.framework.TestCase;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.hbase.HBaseConfiguration;
 import org.apache.hadoop.hbase.HConstants;
 import org.apache.hadoop.hbase.KeyValue;
 import org.apache.hadoop.hbase.KeyValueTestUtil;
@@ -470,7 +474,7 @@ public class TestMemStore extends TestCase {
   }
 
   public void testMultipleVersionsSimple() throws Exception {
-    MemStore m = new MemStore(KeyValue.COMPARATOR);
+    MemStore m = new MemStore(new Configuration(), KeyValue.COMPARATOR);
     byte [] row = Bytes.toBytes("testRow");
     byte [] family = Bytes.toBytes("testFamily");
     byte [] qf = Bytes.toBytes("testQualifier");
@@ -490,7 +494,7 @@ public class TestMemStore extends TestCase {
   }
 
   public void testBinary() throws IOException {
-    MemStore mc = new MemStore(KeyValue.ROOT_COMPARATOR);
+    MemStore mc = new MemStore(new Configuration(), KeyValue.ROOT_COMPARATOR);
     final int start = 43;
     final int end = 46;
     for (int k = start; k <= end; k++) {
@@ -757,7 +761,6 @@ public class TestMemStore extends TestCase {
     assertEquals(delete, memstore.kvset.first());
   }
 
-
   ////////////////////////////////////
   //Test for timestamps
   ////////////////////////////////////
@@ -790,7 +793,52 @@ public class TestMemStore extends TestCase {
     //assertTrue(!memstore.shouldSeek(scan));
   }
 
-
+  ////////////////////////////////////
+  //Test for upsert with MSLAB
+  ////////////////////////////////////
+  
+  /**
+   * Test a pathological pattern that shows why we can't currently
+   * use the MSLAB for upsert workloads. This test inserts data
+   * in the following pattern:
+   * 
+   * - row0001 through row1000 (fills up one 2M Chunk)
+   * - row0002 through row1001 (fills up another 2M chunk, leaves one reference
+   *   to the first chunk
+   * - row0003 through row1002 (another chunk, another dangling reference)
+   * 
+   * This causes OOME pretty quickly if we use MSLAB for upsert
+   * since each 2M chunk is held onto by a single reference.
+   */
+  public void testUpsertMSLAB() throws Exception {
+    Configuration conf = HBaseConfiguration.create();
+    conf.setBoolean(MemStore.USEMSLAB_KEY, true);
+    memstore = new MemStore(conf, KeyValue.COMPARATOR);
+    
+    int ROW_SIZE = 2048;
+    byte[] qualifier = new byte[ROW_SIZE - 4];
+    
+    MemoryMXBean bean = ManagementFactory.getMemoryMXBean();
+    for (int i = 0; i < 3; i++) { System.gc(); }
+    long usageBefore = bean.getHeapMemoryUsage().getUsed();
+    
+    long size = 0;
+    long ts=0;
+    
+    for (int newValue = 0; newValue < 1000; newValue++) {
+      for (int row = newValue; row < newValue + 1000; row++) {
+        byte[] rowBytes = Bytes.toBytes(row);
+        size += memstore.updateColumnValue(rowBytes, FAMILY, qualifier, newValue, ++ts);
+      }
+    }
+    System.out.println("Wrote " + ts + " vals");
+    for (int i = 0; i < 3; i++) { System.gc(); }
+    long usageAfter = bean.getHeapMemoryUsage().getUsed();
+    System.out.println("Memory used: " + (usageAfter - usageBefore)
+        + " (heapsize: " + memstore.heapSize() + 
+        " size: " + size + ")");
+  }
+  
   //////////////////////////////////////////////////////////////////////////////
   // Helpers
   //////////////////////////////////////////////////////////////////////////////

src/test/java/org/apache/hadoop/hbase/regionserver/TestMemStoreLAB.java

@@ -0,0 +1,177 @@
+/**
+ * Copyright 2011 The Apache Software Foundation
+ *
+ * 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.hbase.regionserver;
+
+import static org.junit.Assert.*;
+
+import java.util.List;
+import java.util.Map;
+import java.util.Random;
+import java.util.TreeMap;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.hbase.MultithreadedTestUtil;
+import org.apache.hadoop.hbase.MultithreadedTestUtil.TestThread;
+import org.apache.hadoop.hbase.regionserver.MemStoreLAB.Allocation;
+import org.junit.Test;
+
+import com.google.common.base.Function;
+import com.google.common.collect.Iterables;
+import com.google.common.collect.Lists;
+import com.google.common.collect.MapMaker;
+import com.google.common.collect.Maps;
+import com.google.common.primitives.Ints;
+import com.google.common.primitives.Longs;
+
+public class TestMemStoreLAB {
+
+  /**
+   * Test a bunch of random allocations
+   */
+  @Test
+  public void testLABRandomAllocation() {
+    Random rand = new Random();
+    MemStoreLAB mslab = new MemStoreLAB();
+    int expectedOff = 0;
+    byte[] lastBuffer = null;
+    // 100K iterations by 0-1K alloc -> 50MB expected
+    // should be reasonable for unit test and also cover wraparound
+    // behavior
+    for (int i = 0; i < 100000; i++) {
+      int size = rand.nextInt(1000);
+      Allocation alloc = mslab.allocateBytes(size);
+      
+      if (alloc.getData() != lastBuffer) {
+        expectedOff = 0;
+        lastBuffer = alloc.getData();
+      }
+      assertEquals(expectedOff, alloc.getOffset());
+      assertTrue("Allocation " + alloc + " overruns buffer",
+          alloc.getOffset() + size <= alloc.getData().length);
+      expectedOff += size;
+    }
+  }
+
+  @Test
+  public void testLABLargeAllocation() {
+    MemStoreLAB mslab = new MemStoreLAB();
+    Allocation alloc = mslab.allocateBytes(2*1024*1024);
+    assertNull("2MB allocation shouldn't be satisfied by LAB.",
+      alloc);
+  } 
+
+  /**
+   * Test allocation from lots of threads, making sure the results don't
+   * overlap in any way
+   */
+  @Test
+  public void testLABThreading() throws Exception {
+    Configuration conf = new Configuration();
+    MultithreadedTestUtil.TestContext ctx =
+      new MultithreadedTestUtil.TestContext(conf);
+    
+    final AtomicInteger totalAllocated = new AtomicInteger();
+    
+    final MemStoreLAB mslab = new MemStoreLAB();
+    List<List<AllocRecord>> allocations = Lists.newArrayList();
+    
+    for (int i = 0; i < 10; i++) {
+      final List<AllocRecord> allocsByThisThread = Lists.newLinkedList();
+      allocations.add(allocsByThisThread);
+      
+      TestThread t = new MultithreadedTestUtil.RepeatingTestThread(ctx) {
+        private Random r = new Random();
+        @Override
+        public void doAnAction() throws Exception {
+          int size = r.nextInt(1000);
+          Allocation alloc = mslab.allocateBytes(size);
+          totalAllocated.addAndGet(size);
+          allocsByThisThread.add(new AllocRecord(alloc, size));
+        }
+      };
+      ctx.addThread(t);
+    }
+    
+    ctx.startThreads();
+    while (totalAllocated.get() < 50*1024*1024 && ctx.shouldRun()) {
+      Thread.sleep(10);
+    }
+    ctx.stop();
+    
+    // Partition the allocations by the actual byte[] they point into,
+    // make sure offsets are unique for each chunk
+    Map<byte[], Map<Integer, AllocRecord>> mapsByChunk =
+      Maps.newHashMap();
+    
+    int sizeCounted = 0;
+    for (AllocRecord rec : Iterables.concat(allocations)) {
+      sizeCounted += rec.size;
+      if (rec.size == 0) continue;
+      
+      Map<Integer, AllocRecord> mapForThisByteArray =
+        mapsByChunk.get(rec.alloc.getData());
+      if (mapForThisByteArray == null) {
+        mapForThisByteArray = Maps.newTreeMap();
+        mapsByChunk.put(rec.alloc.getData(), mapForThisByteArray);
+      }
+      AllocRecord oldVal = mapForThisByteArray.put(rec.alloc.getOffset(), rec);
+      assertNull("Already had an entry " + oldVal + " for allocation " + rec,
+          oldVal);
+    }
+    assertEquals("Sanity check test", sizeCounted, totalAllocated.get());
+    
+    // Now check each byte array to make sure allocations don't overlap
+    for (Map<Integer, AllocRecord> allocsInChunk : mapsByChunk.values()) {
+      int expectedOff = 0;
+      for (AllocRecord alloc : allocsInChunk.values()) {
+        assertEquals(expectedOff, alloc.alloc.getOffset());
+        assertTrue("Allocation " + alloc + " overruns buffer",
+            alloc.alloc.getOffset() + alloc.size <= alloc.alloc.getData().length);
+        expectedOff += alloc.size;
+      }
+    }
+
+  }
+  
+  private static class AllocRecord implements Comparable<AllocRecord>{
+    private final Allocation alloc;
+    private final int size;
+    public AllocRecord(Allocation alloc, int size) {
+      super();
+      this.alloc = alloc;
+      this.size = size;
+    }
+
+    @Override
+    public int compareTo(AllocRecord e) {
+      if (alloc.getData() != e.alloc.getData()) {
+        throw new RuntimeException("Can only compare within a particular array");
+      }
+      return Ints.compare(alloc.getOffset(), e.alloc.getOffset());
+    }
+    
+    @Override
+    public String toString() {
+      return "AllocRecord(alloc=" + alloc + ", size=" + size + ")";
+    }
+    
+  }
+}