diff --git a/core/src/main/java/com/yahoo/ycsb/measurements/OneMeasurementTimeSeries.java b/core/src/main/java/com/yahoo/ycsb/measurements/OneMeasurementTimeSeries.java
index 34805bc4..5e8dea01 100644
--- a/core/src/main/java/com/yahoo/ycsb/measurements/OneMeasurementTimeSeries.java
+++ b/core/src/main/java/com/yahoo/ycsb/measurements/OneMeasurementTimeSeries.java
@@ -1,157 +1,157 @@
 /**
  * Copyright (c) 2010 Yahoo! Inc. All rights reserved.
  *
  * Licensed 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. See accompanying
  * LICENSE file.
  */
 
 package com.yahoo.ycsb.measurements;
 
 import java.io.IOException;
 import java.util.Vector;
 import java.util.Properties;
 import java.text.DecimalFormat;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicInteger;
 
 import com.yahoo.ycsb.measurements.exporter.MeasurementsExporter;
 
 class SeriesUnit
 {
   /**
    * @param time
    * @param average
    */
   public SeriesUnit(long time, double average) {
     this.time = time;
     this.average = average;
   }
   public long time;
   public double average;
 }
 
 /**
  * A time series measurement of a metric, such as READ LATENCY.
  */
 public class OneMeasurementTimeSeries extends OneMeasurement
 {
 
   /**
    * Granularity for time series; measurements will be averaged in chunks of this granularity. Units are milliseconds.
    */
   public static final String GRANULARITY="timeseries.granularity";
   public static final String GRANULARITY_DEFAULT="1000";
 
   int _granularity;
   Vector<SeriesUnit> _measurements;
 
   long start=-1;
   long currentunit=-1;
   int count=0;
   int sum=0;
   int operations=0;
   long totallatency=0;
 
   //keep a windowed version of these stats for printing status
   int windowoperations=0;
   long windowtotallatency=0;
 
   int min=-1;
   int max=-1;
 
   public OneMeasurementTimeSeries(String name, Properties props)
   {
     super(name);
     _granularity=Integer.parseInt(props.getProperty(GRANULARITY,GRANULARITY_DEFAULT));
     _measurements=new Vector<SeriesUnit>();
   }
 
-  void checkEndOfUnit(boolean forceend)
+  synchronized void checkEndOfUnit(boolean forceend)
   {
     long now=System.currentTimeMillis();
 
     if (start<0)
     {
       currentunit=0;
       start=now;
     }
 
     long unit=((now-start)/_granularity)*_granularity;
 
     if ( (unit>currentunit) || (forceend) )
     {
       double avg=((double)sum)/((double)count);
       _measurements.add(new SeriesUnit(currentunit,avg));
 
       currentunit=unit;
 
       count=0;
       sum=0;
     }
   }
 
   @Override
   public void measure(int latency)
   {
     checkEndOfUnit(false);
 
     count++;
     sum+=latency;
     totallatency+=latency;
     operations++;
     windowoperations++;
     windowtotallatency+=latency;
 
     if (latency>max)
     {
       max=latency;
     }
 
     if ( (latency<min) || (min<0) )
     {
       min=latency;
     }
   }
 
 
   @Override
   public void exportMeasurements(MeasurementsExporter exporter) throws IOException {
     checkEndOfUnit(true);
 
     exporter.write(getName(), "Operations", operations);
     exporter.write(getName(), "AverageLatency(us)", (((double) totallatency) / ((double) operations)));
     exporter.write(getName(), "MinLatency(us)", min);
     exporter.write(getName(), "MaxLatency(us)", max);
 
     // TODO: 95th and 99th percentile latency
 
     exportStatusCounts(exporter);
     for (SeriesUnit unit : _measurements) {
       exporter.write(getName(), Long.toString(unit.time), unit.average);
     }
   }
 
   @Override
   public String getSummary() {
     if (windowoperations==0)
     {
       return "";
     }
     DecimalFormat d = new DecimalFormat("#.##");
     double report=((double)windowtotallatency)/((double)windowoperations);
     windowtotallatency=0;
     windowoperations=0;
     return "["+getName()+" AverageLatency(us)="+d.format(report)+"]";
   }
 
 }