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From la...@apache.org
Subject svn commit: r1305582 - in /hbase/branches/0.94/src: main/java/org/apache/hadoop/hbase/io/hfile/ main/java/org/apache/hadoop/hbase/metrics/ main/java/org/apache/hadoop/hbase/metrics/histogram/ main/java/org/apache/hadoop/hbase/regionserver/ main/java/or...
Date Mon, 26 Mar 2012 20:56:46 GMT
Author: larsh
Date: Mon Mar 26 20:56:45 2012
New Revision: 1305582

URL: http://svn.apache.org/viewvc?rev=1305582&view=rev
Log:
HBASE-5533 Add more metrics to HBase (Shaneal Manek)

Added:
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/ExactCounterMetric.java
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/ExponentiallyDecayingSample.java
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/MetricsHistogram.java
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/Sample.java
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/Snapshot.java
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/UniformSample.java
    hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestExactCounterMetric.java
    hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestExponentiallyDecayingSample.java
    hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestMetricsHistogram.java
Modified:
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFile.java
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderV1.java
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderV2.java
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileWriterV1.java
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileWriterV2.java
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/HRegionServer.java
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/metrics/RegionServerMetrics.java
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/wal/HLog.java
    hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/util/Threads.java

Modified: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFile.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFile.java?rev=1305582&r1=1305581&r2=1305582&view=diff
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFile.java (original)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFile.java Mon Mar 26 20:56:45 2012
@@ -24,8 +24,12 @@ import java.io.DataInput;
 import java.io.IOException;
 import java.nio.ByteBuffer;
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.ArrayBlockingQueue;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicLong;
 
@@ -47,14 +51,15 @@ import org.apache.hadoop.hbase.io.HbaseM
 import org.apache.hadoop.hbase.io.encoding.DataBlockEncoding;
 import org.apache.hadoop.hbase.regionserver.metrics.SchemaMetrics;
 import org.apache.hadoop.hbase.regionserver.metrics.SchemaMetrics.SchemaAware;
-import org.apache.hadoop.hbase.util.ChecksumType;
 import org.apache.hadoop.hbase.util.BloomFilterWriter;
 import org.apache.hadoop.hbase.util.Bytes;
+import org.apache.hadoop.hbase.util.ChecksumType;
 import org.apache.hadoop.hbase.util.FSUtils;
 import org.apache.hadoop.io.RawComparator;
 import org.apache.hadoop.io.Writable;
 
 import com.google.common.base.Preconditions;
+import com.google.common.collect.Lists;
 
 /**
  * File format for hbase.
@@ -163,18 +168,100 @@ public class HFile {
   public static final ChecksumType DEFAULT_CHECKSUM_TYPE = ChecksumType.CRC32;
 
   // For measuring latency of "sequential" reads and writes
-  static final AtomicInteger readOps = new AtomicInteger();
-  static final AtomicLong readTimeNano = new AtomicLong();
-  static final AtomicInteger writeOps = new AtomicInteger();
-  static final AtomicLong writeTimeNano = new AtomicLong();
+  private static final AtomicInteger readOps = new AtomicInteger();
+  private static final AtomicLong readTimeNano = new AtomicLong();
+  private static final AtomicInteger writeOps = new AtomicInteger();
+  private static final AtomicLong writeTimeNano = new AtomicLong();
 
   // For measuring latency of pread
-  static final AtomicInteger preadOps = new AtomicInteger();
-  static final AtomicLong preadTimeNano = new AtomicLong();
+  private static final AtomicInteger preadOps = new AtomicInteger();
+  private static final AtomicLong preadTimeNano = new AtomicLong();
 
   // For measuring number of checksum failures
   static final AtomicLong checksumFailures = new AtomicLong();
 
+  // For getting more detailed stats on FS latencies
+  // If, for some reason, the metrics subsystem stops polling for latencies, 
+  // I don't want data to pile up in a memory leak
+  // so, after LATENCY_BUFFER_SIZE items have been enqueued for processing,
+  // fs latency stats will be dropped (and this behavior will be logged)
+  private static final int LATENCY_BUFFER_SIZE = 5000;
+  private static final BlockingQueue<Long> fsReadLatenciesNanos = 
+      new ArrayBlockingQueue<Long>(LATENCY_BUFFER_SIZE);
+  private static final BlockingQueue<Long> fsWriteLatenciesNanos = 
+      new ArrayBlockingQueue<Long>(LATENCY_BUFFER_SIZE);
+  private static final BlockingQueue<Long> fsPreadLatenciesNanos = 
+      new ArrayBlockingQueue<Long>(LATENCY_BUFFER_SIZE);
+  private static final AtomicLong lastLoggedDataDrop = new AtomicLong(0);
+  
+  // we don't want to fill up the logs with this message, so only log it 
+  // once every 30 seconds at most
+  // I also want to avoid locks on the 'critical path' (the common case will be
+  // uncontended) - hence the CAS
+  private static void logDroppedLatencyStat() {
+    final long now = System.currentTimeMillis();      
+    final long earliestAcceptableLog = now - TimeUnit.SECONDS.toMillis(30L);
+    while (true) {
+      final long lastLog = lastLoggedDataDrop.get();
+      if (lastLog < earliestAcceptableLog) {
+        if (lastLoggedDataDrop.compareAndSet(lastLog, now)) {
+          LOG.warn("Dropping fs latency stats since buffer is full");
+          break;
+        } // otherwise (if the compaseAndSet failed) the while loop retries
+      } else {
+        break;
+      }
+    }    
+  }
+  
+  public static final void offerReadLatency(long latencyNanos, boolean pread) {
+    boolean stored = false;
+    if (pread) {
+      stored = fsPreadLatenciesNanos.offer(latencyNanos);
+      preadOps.incrementAndGet();
+      preadTimeNano.addAndGet(latencyNanos);
+    } else {
+      stored = fsReadLatenciesNanos.offer(latencyNanos);
+      readTimeNano.addAndGet(latencyNanos);
+      readOps.incrementAndGet();
+    }
+
+    if (!stored) { 
+      logDroppedLatencyStat();
+    }    
+  }
+  
+  public static final void offerWriteLatency(long latencyNanos) {
+    final boolean stored = fsWriteLatenciesNanos.offer(latencyNanos);
+    if (!stored) {
+      logDroppedLatencyStat();
+    }
+    
+    writeTimeNano.addAndGet(latencyNanos);
+    writeOps.incrementAndGet();
+  }
+  
+  public static final Collection<Long> getReadLatenciesNanos() {
+    final List<Long> latencies = 
+        Lists.newArrayListWithCapacity(fsReadLatenciesNanos.size());
+    fsReadLatenciesNanos.drainTo(latencies);
+    return latencies;
+  }
+
+  public static final Collection<Long> getPreadLatenciesNanos() {
+    final List<Long> latencies = 
+        Lists.newArrayListWithCapacity(fsPreadLatenciesNanos.size());
+    fsPreadLatenciesNanos.drainTo(latencies);
+    return latencies;
+  }
+  
+  public static final Collection<Long> getWriteLatenciesNanos() {
+    final List<Long> latencies = 
+        Lists.newArrayListWithCapacity(fsWriteLatenciesNanos.size());
+    fsWriteLatenciesNanos.drainTo(latencies);
+    return latencies;
+  }
+
   // for test purpose
   public static volatile AtomicLong dataBlockReadCnt = new AtomicLong(0);
 

Modified: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderV1.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderV1.java?rev=1305582&r1=1305581&r2=1305582&view=diff
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderV1.java (original)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderV1.java Mon Mar 26 20:56:45 2012
@@ -247,9 +247,8 @@ public class HFileReaderV1 extends Abstr
       passSchemaMetricsTo(hfileBlock);
       hfileBlock.expectType(BlockType.META);
 
-      long delta = System.nanoTime() - startTimeNs;
-      HFile.preadTimeNano.addAndGet(delta);
-      HFile.preadOps.incrementAndGet();
+      final long delta = System.nanoTime() - startTimeNs;
+      HFile.offerReadLatency(delta, true);
       getSchemaMetrics().updateOnCacheMiss(effectiveCategory,
           SchemaMetrics.NO_COMPACTION, delta);
 
@@ -326,14 +325,8 @@ public class HFileReaderV1 extends Abstr
       passSchemaMetricsTo(hfileBlock);
       hfileBlock.expectType(BlockType.DATA);
 
-      long delta = System.nanoTime() - startTimeNs;
-      if (pread) {
-        HFile.preadTimeNano.addAndGet(delta);
-        HFile.preadOps.incrementAndGet();
-      } else {
-        HFile.readTimeNano.addAndGet(delta);
-        HFile.readOps.incrementAndGet();
-      }
+      final long delta = System.nanoTime() - startTimeNs;
+      HFile.offerReadLatency(delta, pread);
       getSchemaMetrics().updateOnCacheMiss(BlockCategory.DATA, isCompaction,
           delta);
 

Modified: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderV2.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderV2.java?rev=1305582&r1=1305581&r2=1305582&view=diff
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderV2.java (original)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderV2.java Mon Mar 26 20:56:45 2012
@@ -234,9 +234,8 @@ public class HFileReaderV2 extends Abstr
           blockSize, -1, true);
       passSchemaMetricsTo(metaBlock);
 
-      long delta = System.nanoTime() - startTimeNs;
-      HFile.preadTimeNano.addAndGet(delta);
-      HFile.preadOps.incrementAndGet();
+      final long delta = System.nanoTime() - startTimeNs;
+      HFile.offerReadLatency(delta, true);
       getSchemaMetrics().updateOnCacheMiss(BlockCategory.META, false, delta);
 
       // Cache the block
@@ -333,14 +332,8 @@ public class HFileReaderV2 extends Abstr
       passSchemaMetricsTo(hfileBlock);
       BlockCategory blockCategory = hfileBlock.getBlockType().getCategory();
 
-      long delta = System.nanoTime() - startTimeNs;
-      if (pread) {
-        HFile.preadTimeNano.addAndGet(delta);
-        HFile.preadOps.incrementAndGet();
-      } else {
-        HFile.readTimeNano.addAndGet(delta);
-        HFile.readOps.incrementAndGet();
-      }
+      final long delta = System.nanoTime() - startTimeNs;
+      HFile.offerReadLatency(delta, pread);
       getSchemaMetrics().updateOnCacheMiss(blockCategory, isCompaction, delta);
 
       // Cache the block if necessary

Modified: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileWriterV1.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileWriterV1.java?rev=1305582&r1=1305581&r2=1305582&view=diff
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileWriterV1.java (original)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileWriterV1.java Mon Mar 26 20:56:45 2012
@@ -139,9 +139,8 @@ public class HFileWriterV1 extends Abstr
     blockDataSizes.add(Integer.valueOf(size));
     this.totalUncompressedBytes += size;
 
-    HFile.writeTimeNano.addAndGet(System.nanoTime() - startTimeNs);
-    HFile.writeOps.incrementAndGet();
-
+    HFile.offerWriteLatency(System.nanoTime() - startTimeNs);
+    
     if (cacheConf.shouldCacheDataOnWrite()) {
       baosDos.flush();
       // we do not do data block encoding on disk for HFile v1

Modified: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileWriterV2.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileWriterV2.java?rev=1305582&r1=1305581&r2=1305582&view=diff
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileWriterV2.java (original)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileWriterV2.java Mon Mar 26 20:56:45 2012
@@ -187,9 +187,8 @@ public class HFileWriterV2 extends Abstr
         onDiskSize);
     totalUncompressedBytes += fsBlockWriter.getUncompressedSizeWithHeader();
 
-    HFile.writeTimeNano.addAndGet(System.nanoTime() - startTimeNs);
-    HFile.writeOps.incrementAndGet();
-
+    HFile.offerWriteLatency(System.nanoTime() - startTimeNs);
+    
     if (cacheConf.shouldCacheDataOnWrite()) {
       doCacheOnWrite(lastDataBlockOffset);
     }

Added: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/ExactCounterMetric.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/ExactCounterMetric.java?rev=1305582&view=auto
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/ExactCounterMetric.java (added)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/ExactCounterMetric.java Mon Mar 26 20:56:45 2012
@@ -0,0 +1,154 @@
+/**
+ * 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.metrics;
+
+import java.util.Collections;
+import java.util.Comparator;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
+import org.apache.hadoop.hbase.util.Pair;
+import org.apache.hadoop.metrics.MetricsRecord;
+import org.apache.hadoop.metrics.util.MetricsBase;
+import org.apache.hadoop.metrics.util.MetricsRegistry;
+import org.cliffc.high_scale_lib.Counter;
+
+import com.google.common.base.Function;
+import com.google.common.collect.Lists;
+import com.google.common.collect.MapMaker;
+
+public class ExactCounterMetric extends MetricsBase {
+
+  private static final int DEFAULT_TOP_N = 5;
+
+  // only publish stats on the topN items (default to DEFAULT_TOP_N)
+  private final int topN;
+  private final Map<String, Counter> counts;
+
+  // all access to the 'counts' map should use this lock.
+  // take a write lock iff you want to guarantee exclusive access
+  // (the map stripes locks internally, so it's already thread safe -
+  // this lock is just so you can take a consistent snapshot of data)
+  private final ReadWriteLock lock;
+  
+  
+  /**
+   * Constructor to create a new counter metric
+   * @param nam         the name to publish this metric under
+   * @param registry    where the metrics object will be registered
+   * @param description metrics description
+   * @param topN        how many 'keys' to publish metrics on 
+   */
+  public ExactCounterMetric(final String nam, final MetricsRegistry registry, 
+      final String description, int topN) {
+    super(nam, description);
+
+    this.counts = new MapMaker().makeComputingMap(
+        new Function<String, Counter>() {
+          @Override
+          public Counter apply(String input) {
+            return new Counter();
+          }    
+        });
+
+    this.lock = new ReentrantReadWriteLock();
+    this.topN = topN;
+
+    if (registry != null) {
+      registry.add(nam, this);      
+    }
+  }
+
+  /**
+   * Constructor creates a new ExactCounterMetric
+   * @param nam       the name of the metrics to be used to publish the metric
+   * @param registry  where the metrics object will be registered
+   */
+  public ExactCounterMetric(final String nam, MetricsRegistry registry) {
+    this(nam, registry, NO_DESCRIPTION, DEFAULT_TOP_N);
+  }
+
+  
+  public void update(String type) {
+    this.lock.readLock().lock();
+    try {
+      this.counts.get(type).increment();
+    } finally {
+      this.lock.readLock().unlock();
+    }
+  }
+  
+  public void update(String type, long count) {
+    this.lock.readLock().lock();
+    try {
+      this.counts.get(type).add(count);
+    } finally {
+      this.lock.readLock().unlock();
+    }
+  }
+  
+  public List<Pair<String, Long>> getTop(int n) {
+    final List<Pair<String, Long>> countsSnapshot = 
+        Lists.newArrayListWithCapacity(this.counts.size());
+    
+    // no updates are allowed while I'm holding this lock, so move fast
+    this.lock.writeLock().lock();
+    try {
+      for(Entry<String, Counter> entry : this.counts.entrySet()) {
+        countsSnapshot.add(Pair.newPair(entry.getKey(), 
+            entry.getValue().get()));
+      }
+    } finally {
+      this.lock.writeLock().unlock();
+    }
+    
+    Collections.sort(countsSnapshot, new Comparator<Pair<String, Long>>() {
+      @Override
+      public int compare(Pair<String, Long> a, Pair<String, Long> b) {
+        return b.getSecond().compareTo(a.getSecond());
+      }      
+    });
+    
+    return countsSnapshot.subList(0, Math.min(n, countsSnapshot.size()));
+  }
+  
+  @Override
+  public void pushMetric(MetricsRecord mr) {
+    final List<Pair<String, Long>> topKeys = getTop(Integer.MAX_VALUE);
+    int sum = 0;
+    
+    int counter = 0;
+    for (Pair<String, Long> keyCount : topKeys) {
+      counter++;
+      // only push stats on the topN keys
+      if (counter <= this.topN) {
+        mr.setMetric(getName() + "_" + keyCount.getFirst(), 
+            keyCount.getSecond());        
+      }
+      sum += keyCount.getSecond();
+    }
+    mr.setMetric(getName() + "_map_size", this.counts.size());
+    mr.setMetric(getName() + "_total_count", sum);
+  }
+  
+}

Added: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/ExponentiallyDecayingSample.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/ExponentiallyDecayingSample.java?rev=1305582&view=auto
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/ExponentiallyDecayingSample.java (added)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/ExponentiallyDecayingSample.java Mon Mar 26 20:56:45 2012
@@ -0,0 +1,212 @@
+/**
+ * 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.metrics.histogram;
+
+import java.util.ArrayList;
+import java.util.Random;
+import java.util.concurrent.ConcurrentSkipListMap;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
+import org.apache.hadoop.hbase.util.Threads;
+
+/**
+ * An exponentially-decaying random sample of {@code long}s. 
+ * Uses Cormode et al's forward-decaying priority reservoir sampling method 
+ * to produce a statistically representative sample, exponentially biased 
+ * towards newer entries.
+ *
+ * see Cormode et al. 
+ * Forward Decay: A Practical Time Decay Model for Streaming Systems. ICDE '09
+ */
+public class ExponentiallyDecayingSample implements Sample {
+
+  private static final Random RANDOM = new Random();  
+  private static final long RESCALE_THRESHOLD = TimeUnit.HOURS.toNanos(1);
+
+  private static final ScheduledExecutorService TICK_SERVICE = 
+      Executors.newScheduledThreadPool(1, 
+          Threads.getNamedThreadFactory("decayingSampleTick"));
+
+  private static volatile long CURRENT_TICK = 
+      TimeUnit.MILLISECONDS.toSeconds(System.currentTimeMillis());
+
+  static {
+    // sample at twice our signal's frequency (1Hz) per the Nyquist theorem
+    TICK_SERVICE.scheduleAtFixedRate(new Runnable() {
+      @Override
+      public void run() {
+        CURRENT_TICK = 
+            TimeUnit.MILLISECONDS.toSeconds(System.currentTimeMillis());
+      }
+    }, 0, 500, TimeUnit.MILLISECONDS);
+  }
+  
+  private final ConcurrentSkipListMap<Double, Long> values = 
+      new ConcurrentSkipListMap<Double, Long>();
+  private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
+  private final AtomicLong count = new AtomicLong(0);
+  private final AtomicLong nextScaleTime = new AtomicLong(0);
+
+  private final double alpha;
+  private final int reservoirSize;
+  private volatile long startTime;
+
+  /**
+   * Constructor for an ExponentiallyDecayingSample.
+   *
+   * @param reservoirSize the number of samples to keep in the reservoir
+   * @param alpha         the exponential decay factor; the higher this is, 
+   *                      the more biased the sample will be towards newer 
+   *                      values
+   */
+  public ExponentiallyDecayingSample(int reservoirSize, double alpha) {
+    this.alpha = alpha;
+    this.reservoirSize = reservoirSize;
+    clear();
+  }
+
+  @Override
+  public void clear() {
+    lockForRescale();
+    try {
+      values.clear();
+      count.set(0);
+      this.startTime = CURRENT_TICK;
+      nextScaleTime.set(System.nanoTime() + RESCALE_THRESHOLD);
+    } finally {
+      unlockForRescale();
+    }
+  }
+
+  @Override
+  public int size() {
+    return (int) Math.min(reservoirSize, count.get());
+  }
+
+  @Override
+  public void update(long value) {
+    update(value, CURRENT_TICK);
+  }
+
+  /**
+   * Adds an old value with a fixed timestamp to the sample.
+   *
+   * @param value     the value to be added
+   * @param timestamp the epoch timestamp of {@code value} in seconds
+   */
+  public void update(long value, long timestamp) {
+    lockForRegularUsage();
+    try {
+      final double priority = weight(timestamp - startTime) 
+          / RANDOM.nextDouble();
+      final long newCount = count.incrementAndGet();
+      if (newCount <= reservoirSize) {
+        values.put(priority, value);
+      } else {
+        Double first = values.firstKey();
+        if (first < priority) {
+          if (values.putIfAbsent(priority, value) == null) {
+            // ensure we always remove an item
+            while (values.remove(first) == null) {
+              first = values.firstKey();
+            }
+          }
+        }
+      }
+    } finally {
+      unlockForRegularUsage();
+    }
+
+    final long now = System.nanoTime();
+    final long next = nextScaleTime.get();
+    if (now >= next) {
+      rescale(now, next);
+    }
+  }
+
+  @Override
+  public Snapshot getSnapshot() {
+    lockForRegularUsage();
+    try {
+      return new Snapshot(values.values());
+    } finally {
+      unlockForRegularUsage();
+    }
+  }
+
+  private double weight(long t) {
+    return Math.exp(alpha * t);
+  }
+
+  /* "A common feature of the above techniques—indeed, the key technique that
+   * allows us to track the decayed weights efficiently—is that they maintain
+   * counts and other quantities based on g(ti − L), and only scale by g(t − L)
+   * at query time. But while g(ti −L)/g(t−L) is guaranteed to lie between zero
+   * and one, the intermediate values of g(ti − L) could become very large. For
+   * polynomial functions, these values should not grow too large, and should 
+   * be effectively represented in practice by floating point values without 
+   * loss of precision. For exponential functions, these values could grow 
+   * quite large as new values of (ti − L) become large, and potentially 
+   * exceed the capacity of common floating point types. However, since the 
+   * values stored by the algorithms are linear combinations of g values 
+   * (scaled sums), they can be rescaled relative to a new landmark. That is, 
+   * by the analysis of exponential decay in Section III-A, the choice of L 
+   * does not affect the final result. We can therefore multiply each value 
+   * based on L by a factor of exp(−α(L′ − L)), and obtain the correct value 
+   * as if we had instead computed relative to a new landmark L′ (and then use 
+   * this new L′ at query time). This can be done with a linear pass over 
+   * whatever data structure is being used."
+   */
+  private void rescale(long now, long next) {
+    if (nextScaleTime.compareAndSet(next, now + RESCALE_THRESHOLD)) {
+      lockForRescale();
+      try {
+        final long oldStartTime = startTime;
+        this.startTime = CURRENT_TICK;
+        final ArrayList<Double> keys = new ArrayList<Double>(values.keySet());
+        for (Double key : keys) {
+          final Long value = values.remove(key);
+          values.put(key * Math.exp(-alpha * (startTime - oldStartTime)), 
+              value);
+        }
+      } finally {
+        unlockForRescale();
+      }
+    }
+  }
+
+  private void unlockForRescale() {
+    lock.writeLock().unlock();
+  }
+
+  private void lockForRescale() {
+    lock.writeLock().lock();
+  }
+
+  private void lockForRegularUsage() {
+    lock.readLock().lock();
+  }
+
+  private void unlockForRegularUsage() {
+    lock.readLock().unlock();
+  }
+}

Added: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/MetricsHistogram.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/MetricsHistogram.java?rev=1305582&view=auto
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/MetricsHistogram.java (added)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/MetricsHistogram.java Mon Mar 26 20:56:45 2012
@@ -0,0 +1,225 @@
+/**
+ * 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.metrics.histogram;
+
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.atomic.AtomicReference;
+
+import org.apache.hadoop.metrics.MetricsRecord;
+import org.apache.hadoop.metrics.util.MetricsBase;
+import org.apache.hadoop.metrics.util.MetricsRegistry;
+
+public class MetricsHistogram extends MetricsBase {
+  
+  // 1028 items implies 99.9% CI w/ 5% margin of error 
+  // (assuming a normal distribution on the underlying data)
+  private static final int DEFAULT_SAMPLE_SIZE = 1028;
+
+  // the bias towards sampling from more recent data. 
+  // Per Cormode et al. an alpha of 0.015 strongly biases to the last 5 minutes
+  private static final double DEFAULT_ALPHA = 0.015;
+
+  /**
+   * Constructor to create a new histogram metric
+   * @param nam           the name to publish the metric under
+   * @param registry      where the metrics object will be registered
+   * @param description   the metric's description
+   * @param forwardBiased true if you want this histogram to give more 
+   *                      weight to recent data, 
+   *                      false if you want all data to have uniform weight
+   */
+  public MetricsHistogram(final String nam, final MetricsRegistry registry, 
+      final String description, boolean forwardBiased) {
+    super(nam, description);
+
+    this.min = new AtomicLong();
+    this.max = new AtomicLong();
+    this.sum = new AtomicLong();
+    this.sample = forwardBiased ? 
+        new ExponentiallyDecayingSample(DEFAULT_SAMPLE_SIZE, DEFAULT_ALPHA) 
+    : new UniformSample(DEFAULT_SAMPLE_SIZE);
+
+    this.variance =  new AtomicReference<double[]>(new double[]{-1, 0});
+    this.count = new AtomicLong();
+
+    this.clear();
+
+    if (registry != null) {
+      registry.add(nam, this);      
+    }
+  }
+
+  /**
+   * Constructor create a new (forward biased) histogram metric
+   * @param nam         the name to publish the metric under
+   * @param registry    where the metrics object will be registered
+   * @param description the metric's description
+   */
+  public MetricsHistogram(final String nam, MetricsRegistry registry, 
+      final String description) {
+    this(nam, registry, NO_DESCRIPTION, true);
+  }
+    
+  /**
+   * Constructor - create a new (forward biased) histogram metric
+   * @param nam the name of the metrics to be used to publish the metric
+   * @param registry - where the metrics object will be registered
+   */
+  public MetricsHistogram(final String nam, MetricsRegistry registry) {
+    this(nam, registry, NO_DESCRIPTION);
+  }
+
+  private final Sample sample;
+  private final AtomicLong min;
+  private final AtomicLong max;
+  private final AtomicLong sum;
+
+  // these are for computing a running-variance, 
+  // without letting floating point errors accumulate via Welford's algorithm
+  private final AtomicReference<double[]> variance;
+  private final AtomicLong count;
+
+  /**
+   * Clears all recorded values.
+   */
+  public void clear() {
+    this.sample.clear();
+    this.count.set(0);
+    this.max.set(Long.MIN_VALUE);
+    this.min.set(Long.MAX_VALUE);
+    this.sum.set(0);
+    variance.set(new double[]{-1, 0});
+  }
+
+  public void update(int val) {
+    update((long) val);
+  }
+
+  public void update(final long val) {
+    count.incrementAndGet();
+    sample.update(val);
+    setMax(val);
+    setMin(val);
+    sum.getAndAdd(val);
+    updateVariance(val);
+  }
+
+  private void setMax(final long potentialMax) {
+    boolean done = false;
+    while (!done) {
+      final long currentMax = max.get();
+      done = currentMax >= potentialMax 
+          || max.compareAndSet(currentMax, potentialMax);
+    }
+  }
+
+  private void setMin(long potentialMin) {
+    boolean done = false;
+    while (!done) {
+      final long currentMin = min.get();
+      done = currentMin <= potentialMin 
+          || min.compareAndSet(currentMin, potentialMin);
+    }
+  }
+
+  private void updateVariance(long value) {
+    boolean done = false;
+    while (!done) {
+      final double[] oldValues = variance.get();
+      final double[] newValues = new double[2];
+      if (oldValues[0] == -1) {
+        newValues[0] = value;
+        newValues[1] = 0;
+      } else {
+        final double oldM = oldValues[0];
+        final double oldS = oldValues[1];
+
+        final double newM = oldM + ((value - oldM) / getCount());
+        final double newS = oldS + ((value - oldM) * (value - newM));
+
+        newValues[0] = newM;
+        newValues[1] = newS;
+      }
+      done = variance.compareAndSet(oldValues, newValues);
+    }
+  }
+
+
+  public long getCount() {
+    return count.get();
+  }
+
+  public long getMax() {
+    if (getCount() > 0) {
+      return max.get();
+    }
+    return 0L;
+  }
+
+  public long getMin() {
+    if (getCount() > 0) {
+      return min.get();
+    }
+    return 0L;
+  }
+
+  public double getMean() {
+    if (getCount() > 0) {
+      return sum.get() / (double) getCount();
+    }
+    return 0.0;
+  }
+
+  public double getStdDev() {
+    if (getCount() > 0) {
+      return Math.sqrt(getVariance());
+    }
+    return 0.0;
+  }
+
+  public Snapshot getSnapshot() {
+    return sample.getSnapshot();
+  }
+
+  private double getVariance() {
+    if (getCount() <= 1) {
+      return 0.0;
+    }
+    return variance.get()[1] / (getCount() - 1);
+  }
+
+  @Override
+  public void pushMetric(MetricsRecord mr) {
+    final Snapshot s = this.getSnapshot();
+    mr.setMetric(getName() + "_num_ops", this.getCount());
+    mr.setMetric(getName() + "_min", this.getMin());
+    mr.setMetric(getName() + "_max", this.getMax());
+    
+    mr.setMetric(getName() + "_mean", (float) this.getMean());
+    mr.setMetric(getName() + "_std_dev", (float) this.getStdDev());
+    
+    mr.setMetric(getName() + "_median", (float) s.getMedian());
+    mr.setMetric(getName() + "_75th_percentile", 
+        (float) s.get75thPercentile());
+    mr.setMetric(getName() + "_95th_percentile", 
+        (float) s.get95thPercentile());
+    mr.setMetric(getName() + "_99th_percentile", 
+        (float) s.get99thPercentile());
+  }
+}

Added: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/Sample.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/Sample.java?rev=1305582&view=auto
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/Sample.java (added)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/Sample.java Mon Mar 26 20:56:45 2012
@@ -0,0 +1,49 @@
+/**
+ * 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.metrics.histogram;
+
+/**
+ * A statistically representative sample of items from a stream.
+ */
+public interface Sample {
+  /**
+   * Clears all recorded values.
+   */
+  void clear();
+
+  /**
+   * Returns the number of values recorded.
+   *
+   * @return the number of values recorded
+   */
+  int size();
+
+  /**
+   * Adds a new recorded value to the sample.
+   *
+   * @param value a new recorded value
+   */
+  void update(long value);
+
+  /**
+   * Returns a snapshot of the sample's values.
+   *
+   * @return a snapshot of the sample's values
+   */
+  Snapshot getSnapshot();
+}

Added: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/Snapshot.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/Snapshot.java?rev=1305582&view=auto
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/Snapshot.java (added)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/Snapshot.java Mon Mar 26 20:56:45 2012
@@ -0,0 +1,166 @@
+/**
+ * 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.metrics.histogram;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.util.Arrays;
+import java.util.Collection;
+
+/**
+ * A snapshot of all the information seen in a Sample.
+ */
+public class Snapshot {
+
+  private static final double MEDIAN_Q = 0.5;
+  private static final double P75_Q = 0.75;
+  private static final double P95_Q = 0.95;
+  private static final double P98_Q = 0.98;
+  private static final double P99_Q = 0.99;
+  private static final double P999_Q = 0.999;
+
+  private final double[] values;
+
+  /**
+   * Create a new {@link Snapshot} with the given values.
+   *
+   * @param values    an unordered set of values in the sample
+   */
+  public Snapshot(Collection<Long> values) {
+    final Object[] copy = values.toArray();
+    this.values = new double[copy.length];
+    for (int i = 0; i < copy.length; i++) {
+      this.values[i] = (Long) copy[i];
+    }
+    Arrays.sort(this.values);
+  }
+
+  /**
+   * Create a new {@link Snapshot} with the given values.
+   *
+   * @param values    an unordered set of values in the sample
+   */
+  public Snapshot(double[] values) {
+    this.values = new double[values.length];
+    System.arraycopy(values, 0, this.values, 0, values.length);
+    Arrays.sort(this.values);
+  }
+
+  /**
+   * Returns the value at the given quantile.
+   *
+   * @param quantile    a given quantile, in [0..1]
+   * @return the value in the distribution at quantile
+   */
+  public double getValue(double quantile) {
+    if (quantile < 0.0 || quantile > 1.0) {
+      throw new IllegalArgumentException(quantile + " is not in [0..1]");
+    }
+
+    if (values.length == 0) {
+      return 0.0;
+    }
+
+    final double pos = quantile * (values.length + 1);
+
+    if (pos < 1) {
+      return values[0];
+    }
+
+    if (pos >= values.length) {
+      return values[values.length - 1];
+    }
+
+    final double lower = values[(int) pos - 1];
+    final double upper = values[(int) pos];
+    return lower + (pos - Math.floor(pos)) * (upper - lower);
+  }
+
+  /**
+   * Returns the number of values in the snapshot.
+   *
+   * @return the number of values in the snapshot
+   */
+  public int size() {
+    return values.length;
+  }
+
+  /**
+   * Returns the median value in the distribution.
+   *
+   * @return the median value in the distribution
+   */
+  public double getMedian() {
+    return getValue(MEDIAN_Q);
+  }
+
+  /**
+   * Returns the value at the 75th percentile in the distribution.
+   *
+   * @return the value at the 75th percentile in the distribution
+   */
+  public double get75thPercentile() {
+    return getValue(P75_Q);
+  }
+
+  /**
+   * Returns the value at the 95th percentile in the distribution.
+   *
+   * @return the value at the 95th percentile in the distribution
+   */
+  public double get95thPercentile() {
+    return getValue(P95_Q);
+  }
+
+  /**
+   * Returns the value at the 98th percentile in the distribution.
+   *
+   * @return the value at the 98th percentile in the distribution
+   */
+  public double get98thPercentile() {
+    return getValue(P98_Q);
+  }
+
+  /**
+   * Returns the value at the 99th percentile in the distribution.
+   *
+   * @return the value at the 99th percentile in the distribution
+   */
+  public double get99thPercentile() {
+    return getValue(P99_Q);
+  }
+
+  /**
+   * Returns the value at the 99.9th percentile in the distribution.
+   *
+   * @return the value at the 99.9th percentile in the distribution
+   */
+  public double get999thPercentile() {
+    return getValue(P999_Q);
+  }
+
+  /**
+   * Returns the entire set of values in the snapshot.
+   *
+   * @return the entire set of values in the snapshot
+   */
+  public double[] getValues() {
+    return Arrays.copyOf(values, values.length);
+  }
+}

Added: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/UniformSample.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/UniformSample.java?rev=1305582&view=auto
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/UniformSample.java (added)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/metrics/histogram/UniformSample.java Mon Mar 26 20:56:45 2012
@@ -0,0 +1,105 @@
+/**
+ * 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.metrics.histogram;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Random;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.atomic.AtomicLongArray;
+
+/**
+ * A random sample of a stream of longs. Uses Vitter's Algorithm R to produce a
+ * statistically representative sample.
+ *
+ * see: http://www.cs.umd.edu/~samir/498/vitter.pdf
+ */
+public class UniformSample implements Sample {
+  
+  private static final Random RANDOM = new Random();
+  private static final int BITS_PER_LONG = 63;
+  
+  private final AtomicLong count = new AtomicLong();
+  private final AtomicLongArray values;
+
+  /**
+   * Creates a new UniformSample
+   *
+   * @param reservoirSize the number of samples to keep
+   */
+  public UniformSample(int reservoirSize) {
+    this.values = new AtomicLongArray(reservoirSize);
+    clear();
+  }
+
+  @Override
+  public void clear() {
+    for (int i = 0; i < values.length(); i++) {
+      values.set(i, 0);
+    }
+    count.set(0);
+  }
+
+  @Override
+  public int size() {
+    final long c = count.get();
+    if (c > values.length()) {
+      return values.length();
+    }
+    return (int) c;
+  }
+
+  @Override
+  public void update(long value) {
+    final long c = count.incrementAndGet();
+    if (c <= values.length()) {
+      values.set((int) c - 1, value);
+    } else {
+      final long r = nextLong(c);
+      if (r < values.length()) {
+        values.set((int) r, value);
+      }
+    }
+  }
+
+  /**
+   * Get a pseudo-random long uniformly between 0 and n-1. Stolen from
+   * {@link java.util.Random#nextInt()}.
+   *
+   * @param n the bound
+   * @return a value select randomly from the range {@code [0..n)}.
+   */
+  private static long nextLong(long n) {
+    long bits, val;
+    do {
+      bits = RANDOM.nextLong() & (~(1L << BITS_PER_LONG));
+      val = bits % n;
+    } while (bits - val + (n - 1) < 0L);
+    return val;
+  }
+
+  @Override
+  public Snapshot getSnapshot() {
+    final int s = size();
+    final List<Long> copy = new ArrayList<Long>(s);
+    for (int i = 0; i < s; i++) {
+      copy.add(values.get(i));
+    }
+    return new Snapshot(copy);
+  }
+}

Modified: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/HRegionServer.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/HRegionServer.java?rev=1305582&r1=1305581&r2=1305582&view=diff
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/HRegionServer.java (original)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/HRegionServer.java Mon Mar 26 20:56:45 2012
@@ -1951,12 +1951,15 @@ public class HRegionServer implements HR
   /** {@inheritDoc} */
   public Result get(byte[] regionName, Get get) throws IOException {
     checkOpen();
+    final long startTime = System.nanoTime();
     requestCount.incrementAndGet();
     try {
       HRegion region = getRegion(regionName);
       return region.get(get, getLockFromId(get.getLockId()));
     } catch (Throwable t) {
       throw convertThrowableToIOE(cleanup(t));
+    } finally {
+      this.metrics.getLatencies.update(System.nanoTime() - startTime);
     }
   }
 
@@ -1988,6 +1991,7 @@ public class HRegionServer implements HR
       throw new IllegalArgumentException("update has null row");
     }
 
+    final long startTime = System.nanoTime();
     checkOpen();
     this.requestCount.incrementAndGet();
     HRegion region = getRegion(regionName);
@@ -1999,6 +2003,8 @@ public class HRegionServer implements HR
       region.put(put, getLockFromId(put.getLockId()), writeToWAL);
     } catch (Throwable t) {
       throw convertThrowableToIOE(cleanup(t));
+    } finally {
+      this.metrics.putLatencies.update(System.nanoTime() - startTime);
     }
   }
 
@@ -2006,6 +2012,9 @@ public class HRegionServer implements HR
       throws IOException {
     checkOpen();
     HRegion region = null;
+    int i = 0;
+
+    final long startTime = System.nanoTime();
     try {
       region = getRegion(regionName);
       if (!region.getRegionInfo().isMetaTable()) {
@@ -2015,7 +2024,6 @@ public class HRegionServer implements HR
       @SuppressWarnings("unchecked")
       Pair<Put, Integer>[] putsWithLocks = new Pair[puts.size()];
 
-      int i = 0;
       for (Put p : puts) {
         Integer lock = getLockFromId(p.getLockId());
         putsWithLocks[i++] = new Pair<Put, Integer>(p, lock);
@@ -2031,6 +2039,14 @@ public class HRegionServer implements HR
       return -1;
     } catch (Throwable t) {
       throw convertThrowableToIOE(cleanup(t));
+    } finally {
+      // going to count this as puts.size() PUTs for latency calculations
+      final long totalTime = System.nanoTime() - startTime;
+      final long putCount = i;
+      final long perPutTime = totalTime / putCount;
+      for (int request = 0; request < putCount; request++) {
+        this.metrics.putLatencies.update(perPutTime);
+      }
     }
   }
 
@@ -2492,6 +2508,7 @@ public class HRegionServer implements HR
   public void delete(final byte[] regionName, final Delete delete)
       throws IOException {
     checkOpen();
+    final long startTime = System.nanoTime();
     try {
       boolean writeToWAL = delete.getWriteToWAL();
       this.requestCount.incrementAndGet();
@@ -2503,6 +2520,8 @@ public class HRegionServer implements HR
       region.delete(delete, lid, writeToWAL);
     } catch (Throwable t) {
       throw convertThrowableToIOE(cleanup(t));
+    } finally {
+      this.metrics.deleteLatencies.update(System.nanoTime() - startTime);
     }
   }
 
@@ -2520,10 +2539,12 @@ public class HRegionServer implements HR
       int size = deletes.size();
       Integer[] locks = new Integer[size];
       for (Delete delete : deletes) {
+        final long startTime = System.nanoTime();
         this.requestCount.incrementAndGet();
         locks[i] = getLockFromId(delete.getLockId());
         region.delete(delete, locks[i], delete.getWriteToWAL());
         i++;
+        this.metrics.deleteLatencies.update(System.nanoTime() - startTime);
       }
     } catch (WrongRegionException ex) {
       LOG.debug("Batch deletes: " + i, ex);

Modified: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/metrics/RegionServerMetrics.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/metrics/RegionServerMetrics.java?rev=1305582&r1=1305581&r2=1305582&view=diff
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/metrics/RegionServerMetrics.java (original)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/metrics/RegionServerMetrics.java Mon Mar 26 20:56:45 2012
@@ -19,12 +19,19 @@
  */
 package org.apache.hadoop.hbase.regionserver.metrics;
 
+import java.io.IOException;
+import java.lang.management.ManagementFactory;
+import java.lang.management.MemoryUsage;
+import java.util.List;
+
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 import org.apache.hadoop.hbase.io.hfile.HFile;
 import org.apache.hadoop.hbase.metrics.HBaseInfo;
 import org.apache.hadoop.hbase.metrics.MetricsRate;
 import org.apache.hadoop.hbase.metrics.PersistentMetricsTimeVaryingRate;
+import org.apache.hadoop.hbase.metrics.histogram.MetricsHistogram;
+import org.apache.hadoop.hbase.metrics.histogram.Snapshot;
 import org.apache.hadoop.hbase.regionserver.wal.HLog;
 import org.apache.hadoop.hbase.util.Pair;
 import org.apache.hadoop.hbase.util.Strings;
@@ -41,11 +48,6 @@ import org.apache.hadoop.metrics.util.Me
 import org.apache.hadoop.metrics.util.MetricsTimeVaryingLong;
 import org.apache.hadoop.util.StringUtils;
 
-import java.io.IOException;
-import java.lang.management.ManagementFactory;
-import java.lang.management.MemoryUsage;
-import java.util.List;
-
 /**
  * This class is for maintaining the various regionserver statistics
  * and publishing them through the metrics interfaces.
@@ -76,43 +78,51 @@ public class RegionServerMetrics impleme
   /**
    * Block cache size.
    */
-  public final MetricsLongValue blockCacheSize = new MetricsLongValue("blockCacheSize", registry);
+  public final MetricsLongValue blockCacheSize = 
+      new MetricsLongValue("blockCacheSize", registry);
 
   /**
    * Block cache free size.
    */
-  public final MetricsLongValue blockCacheFree = new MetricsLongValue("blockCacheFree", registry);
+  public final MetricsLongValue blockCacheFree = 
+      new MetricsLongValue("blockCacheFree", registry);
 
   /**
    * Block cache item count.
    */
-  public final MetricsLongValue blockCacheCount = new MetricsLongValue("blockCacheCount", registry);
+  public final MetricsLongValue blockCacheCount = 
+      new MetricsLongValue("blockCacheCount", registry);
 
   /**
    * Block cache hit count.
    */
-  public final MetricsLongValue blockCacheHitCount = new MetricsLongValue("blockCacheHitCount", registry);
+  public final MetricsLongValue blockCacheHitCount = 
+      new MetricsLongValue("blockCacheHitCount", registry);
 
   /**
    * Block cache miss count.
    */
-  public final MetricsLongValue blockCacheMissCount = new MetricsLongValue("blockCacheMissCount", registry);
+  public final MetricsLongValue blockCacheMissCount = 
+      new MetricsLongValue("blockCacheMissCount", registry);
 
   /**
    * Block cache evict count.
    */
-  public final MetricsLongValue blockCacheEvictedCount = new MetricsLongValue("blockCacheEvictedCount", registry);
+  public final MetricsLongValue blockCacheEvictedCount = 
+      new MetricsLongValue("blockCacheEvictedCount", registry);
 
   /**
    * Block hit ratio.
    */
-  public final MetricsIntValue blockCacheHitRatio = new MetricsIntValue("blockCacheHitRatio", registry);
+  public final MetricsIntValue blockCacheHitRatio = 
+      new MetricsIntValue("blockCacheHitRatio", registry);
 
   /**
    * Block hit caching ratio.  This only includes the requests to the block
    * cache where caching was turned on.  See HBASE-2253.
    */
-  public final MetricsIntValue blockCacheHitCachingRatio = new MetricsIntValue("blockCacheHitCachingRatio", registry);
+  public final MetricsIntValue blockCacheHitCachingRatio = 
+      new MetricsIntValue("blockCacheHitCachingRatio", registry);
 
   /** Block hit ratio for past N periods. */
   public final MetricsIntValue blockCacheHitRatioPastNPeriods = new MetricsIntValue("blockCacheHitRatioPastNPeriods", registry);
@@ -120,6 +130,25 @@ public class RegionServerMetrics impleme
   /** Block hit caching ratio for past N periods */
   public final MetricsIntValue blockCacheHitCachingRatioPastNPeriods = new MetricsIntValue("blockCacheHitCachingRatioPastNPeriods", registry);
 
+  /**
+   * a latency histogram on 'get' requests
+   */
+  public final MetricsHistogram getLatencies = 
+      new MetricsHistogram("getRequestLatency", registry);
+ 
+  /**
+   * a latency histogram on 'delete' requests
+   */
+  public final MetricsHistogram deleteLatencies = 
+      new MetricsHistogram("deleteRequestLatency", registry);
+ 
+  /**
+   * a latency histogram on 'put' requests
+   */
+  public final MetricsHistogram putLatencies = 
+      new MetricsHistogram("putRequestLatency", registry);
+ 
+  
   /*
    * Count of requests to the regionservers since last call to metrics update
    */
@@ -133,17 +162,20 @@ public class RegionServerMetrics impleme
   /**
    * Count of storefiles open on the regionserver.
    */
-  public final MetricsIntValue storefiles = new MetricsIntValue("storefiles", registry);
+  public final MetricsIntValue storefiles = 
+      new MetricsIntValue("storefiles", registry);
 
   /**
    * Count of read requests
    */
-  public final MetricsLongValue readRequestsCount = new MetricsLongValue("readRequestsCount", registry);
+  public final MetricsLongValue readRequestsCount = 
+      new MetricsLongValue("readRequestsCount", registry);
 
   /**
    * Count of write requests
    */
-  public final MetricsLongValue writeRequestsCount = new MetricsLongValue("writeRequestsCount", registry);
+  public final MetricsLongValue writeRequestsCount = 
+      new MetricsLongValue("writeRequestsCount", registry);
 
   /**
    */
@@ -187,7 +219,26 @@ public class RegionServerMetrics impleme
     new MetricsIntValue("flushQueueSize", registry);
 
   /**
-   * filesystem sequential read latency
+   * filesystem sequential read latency distribution
+   */
+  public final MetricsHistogram fsReadLatencyHistogram = 
+      new MetricsHistogram("fsReadLatencyHistogram", registry);
+
+  /**
+   * filesystem pread latency distribution
+   */
+  public final MetricsHistogram fsPreadLatencyHistogram = 
+      new MetricsHistogram("fsPreadLatencyHistogram", registry);
+
+  /**
+   * Metrics on the distribution of filesystem write latencies (improved version of fsWriteLatency)
+   */
+  public final MetricsHistogram fsWriteLatencyHistogram = 
+      new MetricsHistogram("fsWriteLatencyHistogram", registry);
+
+  
+  /**
+   * filesystem read latency
    */
   public final MetricsTimeVaryingRate fsReadLatency =
     new MetricsTimeVaryingRate("fsReadLatency", registry);
@@ -216,6 +267,7 @@ public class RegionServerMetrics impleme
   public final MetricsTimeVaryingRate fsSyncLatency =
     new MetricsTimeVaryingRate("fsSyncLatency", registry);
 
+  
   /**
    * time each scheduled compaction takes
    */
@@ -329,6 +381,10 @@ public class RegionServerMetrics impleme
       this.blockCacheHitRatioPastNPeriods.pushMetric(this.metricsRecord);
       this.blockCacheHitCachingRatioPastNPeriods.pushMetric(this.metricsRecord);
 
+      this.putLatencies.pushMetric(this.metricsRecord);
+      this.deleteLatencies.pushMetric(this.metricsRecord);
+      this.getLatencies.pushMetric(this.metricsRecord);
+      
       // Mix in HFile and HLog metrics
       // Be careful. Here is code for MTVR from up in hadoop:
       // public synchronized void inc(final int numOps, final long time) {
@@ -359,11 +415,27 @@ public class RegionServerMetrics impleme
        *      by compaction & flush metrics.
        */
 
+      for(Long latency : HFile.getReadLatenciesNanos()) {
+        this.fsReadLatencyHistogram.update(latency);
+      }
+      for(Long latency : HFile.getPreadLatenciesNanos()) {
+        this.fsPreadLatencyHistogram.update(latency);
+      }
+      for(Long latency : HFile.getWriteLatenciesNanos()) {
+        this.fsWriteLatencyHistogram.update(latency);
+      }
+            
+
       // push the result
       this.fsPreadLatency.pushMetric(this.metricsRecord);
       this.fsReadLatency.pushMetric(this.metricsRecord);
       this.fsWriteLatency.pushMetric(this.metricsRecord);
       this.fsWriteSize.pushMetric(this.metricsRecord);
+      
+      this.fsReadLatencyHistogram.pushMetric(this.metricsRecord);
+      this.fsWriteLatencyHistogram.pushMetric(this.metricsRecord);
+      this.fsPreadLatencyHistogram.pushMetric(this.metricsRecord);
+
       this.fsSyncLatency.pushMetric(this.metricsRecord);
       this.compactionTime.pushMetric(this.metricsRecord);
       this.compactionSize.pushMetric(this.metricsRecord);
@@ -496,6 +568,40 @@ public class RegionServerMetrics impleme
         Long.valueOf(this.hdfsBlocksLocalityIndex.get()));
     sb = Strings.appendKeyValue(sb, "slowHLogAppendCount",
         Long.valueOf(this.slowHLogAppendCount.get()));
+    sb = appendHistogram(sb, this.deleteLatencies);
+    sb = appendHistogram(sb, this.getLatencies);
+    sb = appendHistogram(sb, this.putLatencies);
+    sb = appendHistogram(sb, this.fsReadLatencyHistogram);
+    sb = appendHistogram(sb, this.fsPreadLatencyHistogram);
+    sb = appendHistogram(sb, this.fsWriteLatencyHistogram);
+
     return sb.toString();
   }
+  
+  private StringBuilder appendHistogram(StringBuilder sb, 
+      MetricsHistogram histogram) {
+    sb = Strings.appendKeyValue(sb, 
+        histogram.getName() + "Mean", 
+        StringUtils.limitDecimalTo2(histogram.getMean()));
+    sb = Strings.appendKeyValue(sb, 
+        histogram.getName() + "Count", 
+        StringUtils.limitDecimalTo2(histogram.getCount()));
+    final Snapshot s = histogram.getSnapshot();
+    sb = Strings.appendKeyValue(sb, 
+        histogram.getName() + "Median", 
+        StringUtils.limitDecimalTo2(s.getMedian()));
+    sb = Strings.appendKeyValue(sb, 
+        histogram.getName() + "75th", 
+        StringUtils.limitDecimalTo2(s.get75thPercentile()));
+    sb = Strings.appendKeyValue(sb, 
+        histogram.getName() + "95th", 
+        StringUtils.limitDecimalTo2(s.get95thPercentile()));
+    sb = Strings.appendKeyValue(sb, 
+        histogram.getName() + "99th", 
+        StringUtils.limitDecimalTo2(s.get99thPercentile()));
+    sb = Strings.appendKeyValue(sb, 
+        histogram.getName() + "999th", 
+        StringUtils.limitDecimalTo2(s.get999thPercentile()));
+    return sb;
+  }
 }

Modified: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/wal/HLog.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/wal/HLog.java?rev=1305582&r1=1305581&r2=1305582&view=diff
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/wal/HLog.java (original)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/regionserver/wal/HLog.java Mon Mar 26 20:56:45 2012
@@ -1879,4 +1879,4 @@ public class HLog implements Syncable {
       System.exit(-1);
     }
   }
-}
+}
\ No newline at end of file

Modified: hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/util/Threads.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/util/Threads.java?rev=1305582&r1=1305581&r2=1305582&view=diff
==============================================================================
--- hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/util/Threads.java (original)
+++ hbase/branches/0.94/src/main/java/org/apache/hadoop/hbase/util/Threads.java Mon Mar 26 20:56:45 2012
@@ -25,6 +25,7 @@ import java.util.concurrent.LinkedBlocki
 import java.util.concurrent.ThreadFactory;
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -176,4 +177,24 @@ public class Threads {
     boundedCachedThreadPool.allowCoreThreadTimeOut(true);
     return boundedCachedThreadPool;
   }
+  
+  
+  /**
+   * Returns a {@link java.util.concurrent.ThreadFactory} that names each
+   * created thread uniquely, with a common prefix.
+   * 
+   * @param prefix  The prefix of every created Thread's name
+   * @return a {@link java.util.concurrent.ThreadFactory} that names threads
+   */
+  public static ThreadFactory getNamedThreadFactory(final String prefix) {
+    return new ThreadFactory() {
+
+      private final AtomicInteger threadNumber = new AtomicInteger(1);
+      
+      @Override
+      public Thread newThread(Runnable r) {
+        return new Thread(r, prefix + threadNumber.getAndIncrement());
+      }
+    };
+  }
 }

Added: hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestExactCounterMetric.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestExactCounterMetric.java?rev=1305582&view=auto
==============================================================================
--- hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestExactCounterMetric.java (added)
+++ hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestExactCounterMetric.java Mon Mar 26 20:56:45 2012
@@ -0,0 +1,47 @@
+/**
+ * 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.metrics;
+
+import java.util.List;
+
+import junit.framework.Assert;
+
+import org.apache.hadoop.hbase.util.Pair;
+import org.junit.Test;
+
+public class TestExactCounterMetric {
+
+  @Test
+  public void testBasic() {
+    final ExactCounterMetric counter = new ExactCounterMetric("testCounter", null);
+    for (int i = 1; i <= 10; i++) {
+      for (int j = 0; j < i; j++) {
+        counter.update(i + "");
+      }
+    }
+    
+    List<Pair<String, Long>> topFive = counter.getTop(5);
+    Long i = 10L;
+    for (Pair<String, Long> entry : topFive) {
+      Assert.assertEquals(i + "", entry.getFirst());
+      Assert.assertEquals(i, entry.getSecond());
+      i--;
+    }
+  }
+}

Added: hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestExponentiallyDecayingSample.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestExponentiallyDecayingSample.java?rev=1305582&view=auto
==============================================================================
--- hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestExponentiallyDecayingSample.java (added)
+++ hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestExponentiallyDecayingSample.java Mon Mar 26 20:56:45 2012
@@ -0,0 +1,63 @@
+/**
+ * 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.metrics;
+
+import junit.framework.Assert;
+
+import org.apache.hadoop.hbase.metrics.histogram.ExponentiallyDecayingSample;
+import org.apache.hadoop.hbase.metrics.histogram.Snapshot;
+import org.junit.Test;
+
+public class TestExponentiallyDecayingSample {
+  
+  @Test
+  public void testBasic() {
+      final ExponentiallyDecayingSample sample = 
+          new ExponentiallyDecayingSample(100, 0.99);
+      
+      for (int i = 0; i < 1000; i++) {
+          sample.update(i);
+      }
+      Assert.assertEquals(100, sample.size());
+      
+      final Snapshot snapshot = sample.getSnapshot();
+      Assert.assertEquals(100, snapshot.size());
+
+      for (double i : snapshot.getValues()) {
+        Assert.assertTrue(i >= 0.0 && i < 1000.0);
+      }
+  }
+
+  @Test
+  public void testTooBig() throws Exception {
+      final ExponentiallyDecayingSample sample = 
+          new ExponentiallyDecayingSample(100, 0.99);
+      for (int i = 0; i < 10; i++) {
+          sample.update(i);
+      }
+      Assert.assertEquals(10, sample.size());
+
+      final Snapshot snapshot = sample.getSnapshot();
+      Assert.assertEquals(10, sample.size());
+
+      for (double i : snapshot.getValues()) {
+        Assert.assertTrue(i >= 0.0 && i < 1000.0);
+      }
+  }
+}

Added: hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestMetricsHistogram.java
URL: http://svn.apache.org/viewvc/hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestMetricsHistogram.java?rev=1305582&view=auto
==============================================================================
--- hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestMetricsHistogram.java (added)
+++ hbase/branches/0.94/src/test/java/org/apache/hadoop/hbase/metrics/TestMetricsHistogram.java Mon Mar 26 20:56:45 2012
@@ -0,0 +1,98 @@
+/**
+ * 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.metrics;
+
+import java.util.Arrays;
+import java.util.Random;
+
+import org.apache.hadoop.hbase.metrics.histogram.MetricsHistogram;
+import org.apache.hadoop.hbase.metrics.histogram.Snapshot;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class TestMetricsHistogram {
+
+  @Test
+  public void testBasicUniform() {
+    MetricsHistogram h = new MetricsHistogram("testHistogram", null);
+
+    for (int i = 0; i < 100; i++) {
+      h.update(i);
+    }
+
+    Assert.assertEquals(100, h.getCount());
+    Assert.assertEquals(0, h.getMin());
+    Assert.assertEquals(99, h.getMax());
+  }
+
+  private static int safeIndex(int i, int len) {
+    if (i < len && i>= 0) {
+      return i;
+    } else if (i >= len) {
+      return len - 1; 
+    } else {
+      return 0;
+    }    
+  }
+  
+  @Test
+  public void testRandom() {
+    final Random r = new Random();
+    final MetricsHistogram h = new MetricsHistogram("testHistogram", null);
+
+    final long[] data = new long[1000];
+
+    for (int i = 0; i < data.length; i++) {
+      data[i] = (long) (r.nextGaussian() * 10000.0);
+      h.update(data[i]);
+    }
+
+    final Snapshot s = h.getSnapshot();
+    Arrays.sort(data);
+
+    // as long as the histogram chooses an item with index N+/-slop, accept it
+    final int slop = 20;
+
+    // make sure the median, 75th percentile and 95th percentile are good
+    final int medianIndex = data.length / 2;
+    final long minAcceptableMedian = data[safeIndex(medianIndex - slop, 
+        data.length)];
+    final long maxAcceptableMedian = data[safeIndex(medianIndex + slop, 
+        data.length)];
+    Assert.assertTrue(s.getMedian() >= minAcceptableMedian 
+        && s.getMedian() <= maxAcceptableMedian);
+
+    final int seventyFifthIndex = (int) (data.length * 0.75);
+    final long minAcceptableseventyFifth = data[safeIndex(seventyFifthIndex 
+        - slop, data.length)];
+    final long maxAcceptableseventyFifth = data[safeIndex(seventyFifthIndex 
+        + slop, data.length)];
+    Assert.assertTrue(s.get75thPercentile() >= minAcceptableseventyFifth 
+        && s.get75thPercentile() <= maxAcceptableseventyFifth);
+
+    final int ninetyFifthIndex = (int) (data.length * 0.95);
+    final long minAcceptableninetyFifth = data[safeIndex(ninetyFifthIndex 
+        - slop, data.length)];
+    final long maxAcceptableninetyFifth = data[safeIndex(ninetyFifthIndex 
+        + slop, data.length)];
+    Assert.assertTrue(s.get95thPercentile() >= minAcceptableninetyFifth 
+        && s.get95thPercentile() <= maxAcceptableninetyFifth);
+
+  }
+}



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