Address Thomas' comments

The biggest changes here include:

(1) Periodically checking for event log updates in the background, instead
      of on refresh,
(2) Use a Long instead of a linearly scaling HashSet to keep track of the
      applications whose UIs are purposefully not rendered, and
(3) Adding Spark user as a new column. This includes adding a new field to
      the ApplicationStart event.

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -1103,7 +1103,7 @@ class SparkContext(
 
   /** Post the application start event */
   private def postApplicationStart() {
-    listenerBus.post(SparkListenerApplicationStart(appName, startTime))
+    listenerBus.post(SparkListenerApplicationStart(appName, startTime, sparkUser))
   }
 
   /**

core/src/main/scala/org/apache/spark/deploy/history/HistoryServer.scala

@@ -20,8 +20,6 @@ package org.apache.spark.deploy.history
 import javax.servlet.http.HttpServletRequest
 
 import scala.collection.mutable
-import scala.concurrent._
-import scala.concurrent.ExecutionContext.Implicits.global
 
 import org.apache.hadoop.fs.{FileStatus, Path}
 import org.eclipse.jetty.servlet.ServletContextHandler
@@ -56,14 +54,44 @@ class HistoryServer(
   import HistoryServer._
 
   private val fileSystem = Utils.getHadoopFileSystem(baseLogDir)
-  private val bindHost = Utils.localHostName()
-  private val publicHost = Option(System.getenv("SPARK_PUBLIC_DNS")).getOrElse(bindHost)
+  private val localHost = Utils.localHostName()
+  private val publicHost = Option(System.getenv("SPARK_PUBLIC_DNS")).getOrElse(localHost)
   private val port = requestedPort
   private val securityManager = new SecurityManager(conf)
   private val indexPage = new IndexPage(this)
 
   // A timestamp of when the disk was last accessed to check for log updates
-  private var lastLogCheck = -1L
+  private var lastLogCheckTime = -1L
+
+  // If an application is last updated after this threshold, then its UI is retained
+  private var updateTimeThreshold = -1L
+
+  // Number of applications hidden from the UI because the application limit has been reached
+  private var numApplicationsHidden = 0
+
+  @volatile private var stopped = false
+
+  /**
+   * A background thread that periodically checks for event log updates on disk.
+   *
+   * If a log check is invoked manually in the middle of a period, this thread re-adjusts the
+   * time at which it performs the next log check to maintain the same period as before.
+   */
+  private val logCheckingThread = new Thread {
+    override def run() {
+      while (!stopped) {
+        val now = System.currentTimeMillis
+        if (now - lastLogCheckTime > UPDATE_INTERVAL_MS) {
+          checkForLogs()
+          Thread.sleep(UPDATE_INTERVAL_MS)
+        } else {
+          // If the user has manually checked for logs recently, wait until
+          // UPDATE_INTERVAL_MS after the last check time
+          Thread.sleep(lastLogCheckTime + UPDATE_INTERVAL_MS - now)
+        }
+      }
+    }
+  }
 
   private val handlers = Seq[ServletContextHandler](
     createStaticHandler(STATIC_RESOURCE_DIR, "/static"),
@@ -74,13 +102,20 @@ class HistoryServer(
   // A mapping of application ID to its history information, which includes the rendered UI
   val appIdToInfo = mutable.HashMap[String, ApplicationHistoryInfo]()
 
-  // A set of recently removed applications that the server should avoid re-rendering
-  val appIdBlacklist = mutable.HashSet[String]()
+  /**
+   * Start the history server.
+   *
+   * This starts a background thread that periodically synchronizes information displayed on
+   * this UI with the event logs in the provided base directory.
+   */
+  def start() {
+    logCheckingThread.start()
+  }
 
-  /** Bind to the HTTP server behind this web interface */
+  /** Bind to the HTTP server behind this web interface. */
   override def bind() {
     try {
-      serverInfo = Some(startJettyServer(bindHost, port, handlers, conf))
+      serverInfo = Some(startJettyServer("0.0.0.0", port, handlers, conf))
       logInfo("Started HistoryServer at http://%s:%d".format(publicHost, boundPort))
     } catch {
       case e: Exception =>
@@ -91,7 +126,8 @@ class HistoryServer(
   }
 
   /**
-   * Asynchronously check for any updates to event logs in the base directory.
+   * Check for any updates to event logs in the base directory. This is only effective once
+   * the server has been bound.
    *
    * If a new finished application is found, the server renders the associated SparkUI
    * from the application's event logs, attaches this UI to itself, and stores metadata
@@ -100,41 +136,54 @@ class HistoryServer(
    * If the logs for an existing finished application are no longer found, the server
    * removes all associated information and detaches the SparkUI.
    */
-  def checkForLogs() {
-    if (logCheckReady) {
-      lastLogCheck = System.currentTimeMillis
-      val asyncCheck = future {
+  def checkForLogs() = synchronized {
+    if (serverInfo.isDefined) {
+      lastLogCheckTime = System.currentTimeMillis
+      logDebug("Checking for logs. Time is now %d.".format(lastLogCheckTime))
+      try {
         val logStatus = fileSystem.listStatus(new Path(baseLogDir))
         val logDirs = if (logStatus != null) logStatus.filter(_.isDir).toSeq else Seq[FileStatus]()
 
         // Forget about any SparkUIs that can no longer be found
-        val appIds = logDirs.map { dir => getAppId(dir.getPath.toString) }
+        val mostRecentAppIds = logDirs.map { dir => getAppId(dir.getPath.toString) }
         appIdToInfo.foreach { case (appId, info) =>
-          if (!appIds.contains(appId)) {
+          if (!mostRecentAppIds.contains(appId)) {
             detachUI(info.ui)
             appIdToInfo.remove(appId)
-            appIdBlacklist.clear()
+            updateTimeThreshold = -1L
           }
         }
-        appIdBlacklist.retain(appIds.contains)
+
+        // Keep track of the number of applications hidden from the UI this round
+        var _numApplicationsHidden = 0
 
         // Render SparkUI for any new completed applications
         logDirs.foreach { dir =>
           val path = dir.getPath.toString
           val appId = getAppId(path)
           val lastUpdated = getModificationTime(dir)
-          if (!appIdToInfo.contains(appId) && !appIdBlacklist.contains(appId)) {
-            maybeRenderUI(appId, path, lastUpdated)
+          if (!appIdToInfo.contains(appId)) {
+            if (lastUpdated > updateTimeThreshold) {
+              maybeRenderUI(appId, path, lastUpdated)
+            } else {
+              // This application was previously blacklisted due to the application limit
+              _numApplicationsHidden += 1
+            }
           }
           // If the cap is reached, remove the least recently updated application
           if (appIdToInfo.size > RETAINED_APPLICATIONS) {
             removeOldestApp()
+            _numApplicationsHidden += 1
           }
         }
+
+        numApplicationsHidden = _numApplicationsHidden
+
+      } catch {
+        case t: Throwable => logError("Exception in checking for event log updates", t)
       }
-      asyncCheck.onFailure { case t =>
-        logError("Unable to synchronize HistoryServer with files on disk: ", t)
-      }
+    } else {
+      logWarning("Attempted to check for event log updates before binding the server.")
     }
   }
 
@@ -161,11 +210,12 @@ class HistoryServer(
       if (success && appListener.applicationStarted) {
         attachUI(ui)
         val appName = appListener.appName
-        ui.setAppName("%s (finished)".format(appName))
+        val sparkUser = appListener.sparkUser
         val startTime = appListener.startTime
         val endTime = appListener.endTime
-        val info = ApplicationHistoryInfo(appName, startTime, endTime, lastUpdated, logPath, ui)
-        appIdToInfo(appId) = info
+        ui.setAppName("%s (finished)".format(appName))
+        appIdToInfo(appId) =
+          ApplicationHistoryInfo(appName, startTime, endTime, lastUpdated, sparkUser, logPath, ui)
       } else {
         logWarning("Reconstructing application UI was unsuccessful. Either no event logs were" +
           "found or the event signaling application start is missing: %s".format(logPath))
@@ -175,9 +225,27 @@ class HistoryServer(
     }
   }
 
+  /**
+   * Remove the oldest application and detach its associated UI.
+   *
+   * As an optimization, record the last updated time of this application as the minimum
+   * update time threshold. Only applications with a last updated time that exceeds this
+   * threshold will be retained by the server. This avoids re-rendering an old application
+   * that is recently removed.
+   */
+  private def removeOldestApp() {
+    val appToRemove = appIdToInfo.toSeq.minBy { case (_, info) => info.lastUpdated }
+    appToRemove match { case (id, info) =>
+      appIdToInfo.remove(id)
+      detachUI(info.ui)
+      updateTimeThreshold = info.lastUpdated
+    }
+  }
+
   /** Stop the server and close the file system. */
   override def stop() {
     super.stop()
+    stopped = true
     fileSystem.close()
   }
 
@@ -187,39 +255,26 @@ class HistoryServer(
   /** Return the address of this server. */
   def getAddress: String = "http://" + publicHost + ":" + boundPort
 
-  /** Return the total number of application logs found, blacklisted or not. */
-  def getTotalApplications: Int = appIdToInfo.size + appIdBlacklist.size
+  /** Return the total number of application logs found, whether or not the UI is retained. */
+  def getTotalApplications: Int = appIdToInfo.size + numApplicationsHidden
 
   /** Return when this directory was last modified. */
   private def getModificationTime(dir: FileStatus): Long = {
-    val logFiles = fileSystem.listStatus(dir.getPath)
-    if (logFiles != null) {
-      logFiles.map(_.getModificationTime).max
-    } else {
-      dir.getModificationTime
-    }
-  }
-
-  /**
-   * Remove the oldest application and detach its associated UI. As an optimization, add the
-   * application to a blacklist to avoid re-rendering it the next time.
-   */
-  private def removeOldestApp() {
-    val appToRemove = appIdToInfo.toSeq.minBy { case (_, info) => info.lastUpdated }
-    appToRemove match { case (id, info) =>
-      appIdToInfo.remove(id)
-      detachUI(info.ui)
-      appIdBlacklist.add(id)
+    try {
+      val logFiles = fileSystem.listStatus(dir.getPath)
+      if (logFiles != null) {
+        logFiles.map(_.getModificationTime).max
+      } else {
+        dir.getModificationTime
+      }
+    } catch {
+      case t: Throwable =>
+        logError("Exception in accessing modification time of %s".format(dir.getPath), t)
+        -1L
     }
   }
-
-  /** Return whether the last log check has happened sufficiently long ago. */
-  private def logCheckReady: Boolean = {
-    System.currentTimeMillis - lastLogCheck > UPDATE_INTERVAL_SECONDS * 1000
-  }
 }
 
-
 /**
  * The recommended way of starting and stopping a HistoryServer is through the scripts
  * start-history-server.sh and stop-history-server.sh. The path to a base log directory
@@ -233,18 +288,19 @@ class HistoryServer(
 object HistoryServer {
   private val conf = new SparkConf
 
-  // Minimum interval between each check for logs, which requires a disk access (seconds)
-  private val UPDATE_INTERVAL_SECONDS = conf.getInt("spark.history.updateInterval", 5)
+  // Interval between each check for event log updates
+  val UPDATE_INTERVAL_MS = conf.getInt("spark.history.updateInterval", 10) * 1000
 
   // How many applications to retain
-  private val RETAINED_APPLICATIONS = conf.getInt("spark.deploy.retainedApplications", 20)
+  val RETAINED_APPLICATIONS = conf.getInt("spark.deploy.retainedApplications", 250)
 
-  private val STATIC_RESOURCE_DIR = SparkUI.STATIC_RESOURCE_DIR
+  val STATIC_RESOURCE_DIR = SparkUI.STATIC_RESOURCE_DIR
 
   def main(argStrings: Array[String]) {
     val args = new HistoryServerArguments(argStrings)
     val server = new HistoryServer(args.logDir, args.port, conf)
     server.bind()
+    server.start()
 
     // Wait until the end of the world... or if the HistoryServer process is manually stopped
     while(true) { Thread.sleep(Int.MaxValue) }
@@ -258,6 +314,7 @@ private[spark] case class ApplicationHistoryInfo(
     startTime: Long,
     endTime: Long,
     lastUpdated: Long,
+    sparkUser: String,
     logPath: String,
     ui: SparkUI) {
   def started = startTime != -1

core/src/main/scala/org/apache/spark/deploy/history/IndexPage.scala

@@ -26,24 +26,27 @@ import org.apache.spark.ui.{UIUtils, WebUI}
 private[spark] class IndexPage(parent: HistoryServer) {
 
   def render(request: HttpServletRequest): Seq[Node] = {
-    parent.checkForLogs()
-
-    // Populate app table, with most recently modified app first
     val appRows = parent.appIdToInfo.values.toSeq.sortBy { app => -app.lastUpdated }
     val appTable = UIUtils.listingTable(appHeader, appRow, appRows)
     val content =
       <div class="row-fluid">
         <div class="span12">
           <ul class="unstyled">
             <li><strong>Event Log Location: </strong> {parent.baseLogDir}</li>
-            <h4>
-              Showing {parent.appIdToInfo.size}/{parent.getTotalApplications} Finished Applications
-            </h4>
-            {appTable}
           </ul>
+          {
+            if (parent.appIdToInfo.size > 0) {
+              <h4>
+                Showing {parent.appIdToInfo.size}/{parent.getTotalApplications}
+                Finished Application{if (parent.getTotalApplications > 1) "s" else ""}
+              </h4> ++
+              appTable
+            } else {
+              <h4>No Finished Applications Found</h4>
+            }
+          }
         </div>
       </div>
-
     UIUtils.basicSparkPage(content, "History Server")
   }
 
@@ -52,6 +55,7 @@ private[spark] class IndexPage(parent: HistoryServer) {
     "Started",
     "Finished",
     "Duration",
+    "Spark User",
     "Log Directory",
     "Last Updated")
 
@@ -62,13 +66,15 @@ private[spark] class IndexPage(parent: HistoryServer) {
     val endTime = if (info.finished) WebUI.formatDate(info.endTime) else "Not finished"
     val difference = if (info.started && info.finished) info.endTime - info.startTime else -1L
     val duration = if (difference > 0) WebUI.formatDuration(difference) else "---"
+    val sparkUser = if (info.started) info.sparkUser else "Unknown user"
     val logDirectory = parent.getAppId(info.logPath)
     val lastUpdated = WebUI.formatDate(info.lastUpdated)
     <tr>
       <td><a href={uiAddress}>{appName}</a></td>
       <td>{startTime}</td>
       <td>{endTime}</td>
       <td>{duration}</td>
+      <td>{sparkUser}</td>
       <td>{logDirectory}</td>
       <td>{lastUpdated}</td>
     </tr>

core/src/main/scala/org/apache/spark/scheduler/ApplicationEventListener.scala

@@ -26,6 +26,7 @@ package org.apache.spark.scheduler
  */
 private[spark] class ApplicationEventListener extends SparkListener {
   var appName = "<Not Started>"
+  var sparkUser = "<Not Started>"
   var startTime = -1L
   var endTime = -1L
 
@@ -41,6 +42,7 @@ private[spark] class ApplicationEventListener extends SparkListener {
   override def onApplicationStart(applicationStart: SparkListenerApplicationStart) {
     appName = applicationStart.appName
     startTime = applicationStart.time
+    sparkUser = applicationStart.sparkUser
   }
 
   override def onApplicationEnd(applicationEnd: SparkListenerApplicationEnd) {