@@ -73,7 +73,7 @@ public class ServerStatisticsServlet extends HttpServlet  {
         && new File("/bin/grep").exists()
         &&  new File("/proc/meminfo").exists()) {
     java.lang.ProcessBuilder processBuilder =
-        new java.lang.ProcessBuilder("/bin/bash", "-c", "/bin/cat /proc/meminfo | grep -E \"MemTotal|MemFree\"");
+        new java.lang.ProcessBuilder("/bin/bash", "-c", "/bin/cat /proc/meminfo | grep -E \"MemTotal|MemFree|Buffers|Cached|SwapCached\"");
     try {
       ArrayList<String> output = new ArrayList<String>();
       Process process = processBuilder.start();
/*
 * Copyright 2015 LinkedIn Corp.
 *
 * 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.
 */

package azkaban.execapp;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.log4j.Logger;

import azkaban.executor.ExecutorInfo;
import azkaban.utils.JSONUtils;

public class ServerStatisticsServlet extends HttpServlet  {
  private static final long serialVersionUID = 1L;
  private static final int  cacheTimeInMilliseconds = 1000;
  private static final Logger logger = Logger.getLogger(ServerStatisticsServlet.class);
  private static final String noCacheParamName = "nocache";

  protected static long lastRefreshedTime = 0;
  protected static ExecutorInfo cachedstats = null;

  /**
   * Handle all get request to Statistics Servlet {@inheritDoc}
   *
   * @see javax.servlet.http.HttpServlet#doGet(javax.servlet.http.HttpServletRequest,
   *      javax.servlet.http.HttpServletResponse)
   */
  protected void doGet(HttpServletRequest req, HttpServletResponse resp)
      throws ServletException, IOException {

    boolean noCache = null!= req && Boolean.valueOf(req.getParameter(noCacheParamName));

    if (noCache || System.currentTimeMillis() - lastRefreshedTime > cacheTimeInMilliseconds){
      this.populateStatistics(noCache);
    }

    JSONUtils.toJSON(cachedstats, resp.getOutputStream(), true);
  }

  /**
   * fill the result set with the percent of the remaining system memory on the server.
   * @param stats reference to the result container which contains all the results, this specific method
   *              will only work work on the property "remainingMemory" and "remainingMemoryPercent".
   *
   * NOTE:
   * a double value will be used to present the remaining memory,
   *         a returning value of '55.6' means 55.6%
   */
  protected void fillRemainingMemoryPercent(ExecutorInfo stats){
    if (new File("/bin/bash").exists()
        && new File("/bin/cat").exists()
        && new File("/bin/grep").exists()
        &&  new File("/proc/meminfo").exists()) {
    java.lang.ProcessBuilder processBuilder =
        new java.lang.ProcessBuilder("/bin/bash", "-c", "/bin/cat /proc/meminfo | grep -E \"MemTotal|MemFree\"");
    try {
      ArrayList<String> output = new ArrayList<String>();
      Process process = processBuilder.start();
      process.waitFor();
      InputStream inputStream = process.getInputStream();
      try {
        java.io.BufferedReader reader =
            new java.io.BufferedReader(new InputStreamReader(inputStream));
        String line = null;
        while ((line = reader.readLine()) != null) {
          output.add(line);
        }
      }finally {
        inputStream.close();
      }

      long totalMemory = 0;
      long  freeMemory = 0;

      // process the output from bash call.
      // we expect the result from the bash call to be something like following -
      // MemTotal:       65894264 kB
      // MemFree:        61104076 kB
      if (output.size() == 2) {
        for (String result : output){
          // find the total memory and value the variable.
          if (result.contains("MemTotal") && result.split("\\s+").length > 2){
            try {
              totalMemory = Long.parseLong(result.split("\\s+")[1]);
              logger.info("Total memory : " + totalMemory);
            }catch(NumberFormatException e){
              logger.error("yielding 0 for total memory as output is invalid -" + result);
            }
          }
          // find the free memory and value the variable.
          if (result.contains("MemFree") && result.split("\\s+").length > 2){
            try {
              freeMemory = Long.parseLong(result.split("\\s+")[1]);
              logger.info("Free memory : " + freeMemory);
            }catch(NumberFormatException e){
              logger.error("yielding 0 for total memory as output is invalid -" + result);
            }
          }
        }
      }else {
        logger.error("failed to get total/free memory info as the bash call returned invalid result.");
      }

      // the number got from the proc file is in KBs we want to see the number in MBs so we are deviding it by 1024.
      stats.setRemainingMemoryInMB(freeMemory/1024);
      stats.setRemainingMemoryPercent(totalMemory == 0 ? 0 : ((double)freeMemory / (double)totalMemory)*100);
    }
    catch (Exception ex){
      logger.error("failed fetch system memory info " +
                   "as exception is captured when fetching result from bash call. Ex -" + ex.getMessage());
    }
  } else {
      logger.error("failed fetch system memory info, one or more files from the following list are missing -  " +
                   "'/bin/bash'," + "'/bin/cat'," +"'/proc/loadavg'");
  }
  }

  /**
   * call the data providers to fill the returning data container for statistics data.
   * This function refreshes the static cached copy of data in case if necessary.
   * */
  protected synchronized void populateStatistics(boolean noCache){
    //check again before starting the work.
    if (noCache || System.currentTimeMillis() - lastRefreshedTime  > cacheTimeInMilliseconds){
      final ExecutorInfo stats = new ExecutorInfo();

      fillRemainingMemoryPercent(stats);
      fillRemainingFlowCapacityAndLastDispatchedTime(stats);
      fillCpuUsage(stats);

      cachedstats = stats;
      lastRefreshedTime =  System.currentTimeMillis();
    }
  }

  /**
   * fill the result set with the remaining flow capacity .
   * @param stats reference to the result container which contains all the results, this specific method
   *              will only work on the property "remainingFlowCapacity".
   */
  protected void fillRemainingFlowCapacityAndLastDispatchedTime(ExecutorInfo stats){

    AzkabanExecutorServer server = AzkabanExecutorServer.getApp();
    if (server != null){
      FlowRunnerManager runnerMgr =  AzkabanExecutorServer.getApp().getFlowRunnerManager();
      int assignedFlows = runnerMgr.getNumRunningFlows() + runnerMgr.getNumQueuedFlows();
      stats.setRemainingFlowCapacity(runnerMgr.getMaxNumRunningFlows() - assignedFlows);
      stats.setNumberOfAssignedFlows(assignedFlows);
      stats.setLastDispatchedTime(runnerMgr.getLastFlowSubmittedTime());
    }else {
      logger.error("failed to get data for remaining flow capacity or LastDispatchedTime" +
                   " as the AzkabanExecutorServer has yet been initialized.");
    }
  }


  /**<pre>
   * fill the result set with the CPU usage .
   * Note : As the 'Top' bash call doesn't yield accurate result for the system load,
   *        the implementation has been changed to load from the "proc/loadavg" which keeps
   *        the moving average of the system load, we are pulling the average for the recent 1 min.
   *</pre>
   * @param stats reference to the result container which contains all the results, this specific method
   *              will only work on the property "cpuUsage".
   */
  protected void fillCpuUsage(ExecutorInfo stats){
    if (new File("/bin/bash").exists() && new File("/bin/cat").exists() &&  new File("/proc/loadavg").exists()) {
      java.lang.ProcessBuilder processBuilder =
          new java.lang.ProcessBuilder("/bin/bash", "-c", "/bin/cat /proc/loadavg");
      try {
        ArrayList<String> output = new ArrayList<String>();
        Process process = processBuilder.start();
        process.waitFor();
        InputStream inputStream = process.getInputStream();
        try {
          java.io.BufferedReader reader =
              new java.io.BufferedReader(new InputStreamReader(inputStream));
          String line = null;
          while ((line = reader.readLine()) != null) {
            output.add(line);
          }
        }finally {
          inputStream.close();
        }

        // process the output from bash call.
        if (output.size() > 0) {
          String[] splitedresult = output.get(0).split("\\s+");
          double cpuUsage = 0.0;

          try {
            cpuUsage = Double.parseDouble(splitedresult[0]);
          }catch(NumberFormatException e){
            logger.error("yielding 0.0 for CPU usage as output is invalid -" + output.get(0));
          }
          logger.info("System load : " + cpuUsage);
          stats.setCpuUpsage(cpuUsage);
        }
      }
      catch (Exception ex){
        logger.error("failed fetch system load info " +
                     "as exception is captured when fetching result from bash call. Ex -" + ex.getMessage());
      }
    } else {
        logger.error("failed fetch system load info, one or more files from the following list are missing -  " +
                     "'/bin/bash'," + "'/bin/cat'," +"'/proc/loadavg'");
    }
  }
}

nit: I think it will be better to have strict grep to avoid unintentional string match as currently Cached will also get you SwapCached. We can use start of line and ":" to strict e.g. grep -E \"^MemTotal:|^MemFree:|^Buffers:|^Cached:|^SwapCached:\