diff --git a/src/daemon/PhutilDaemon.php b/src/daemon/PhutilDaemon.php
index 1cc0df8..082421f 100644
--- a/src/daemon/PhutilDaemon.php
+++ b/src/daemon/PhutilDaemon.php
@@ -1,353 +1,383 @@
 <?php
 
 /**
  * Scaffolding for implementing robust background processing scripts.
  *
  *
  * Autoscaling
  * ===========
  *
  * Autoscaling automatically launches copies of a daemon when it is busy
  * (scaling the pool up) and stops them when they're idle (scaling the pool
  * down). This is appropriate for daemons which perform highly parallelizable
  * work.
  *
  * To make a daemon support autoscaling, the implementation should look
  * something like this:
  *
  *   while (!$this->shouldExit()) {
  *     if (work_available()) {
  *       $this->willBeginWork();
  *       do_work();
  *       $this->sleep(0);
  *     } else {
  *       $this->willBeginIdle();
  *       $this->sleep(1);
  *     }
  *   }
  *
  * In particular, call @{method:willBeginWork} before becoming busy, and
  * @{method:willBeginIdle} when no work is available. If the daemon is launched
  * into an autoscale pool, this will cause the pool to automatically scale up
  * when busy and down when idle.
  *
  * See @{class:PhutilHighIntensityIntervalDaemon} for an example of a simple
  * autoscaling daemon.
  *
  * Launching a daemon which does not make these callbacks into an autoscale
  * pool will have no effect.
  *
  * @task overseer Communicating With the Overseer
  * @task autoscale Autoscaling Daemon Pools
  */
 abstract class PhutilDaemon extends Phobject {
 
   const MESSAGETYPE_STDOUT = 'stdout';
   const MESSAGETYPE_HEARTBEAT = 'heartbeat';
   const MESSAGETYPE_BUSY = 'busy';
   const MESSAGETYPE_IDLE = 'idle';
   const MESSAGETYPE_DOWN = 'down';
+  const MESSAGETYPE_HIBERNATE = 'hibernate';
 
   const WORKSTATE_BUSY = 'busy';
   const WORKSTATE_IDLE = 'idle';
 
   private $argv;
   private $traceMode;
   private $traceMemory;
   private $verbose;
   private $notifyReceived;
   private $inGracefulShutdown;
   private $workState = null;
   private $idleSince = null;
   private $scaledownDuration;
 
   final public function setVerbose($verbose) {
     $this->verbose = $verbose;
     return $this;
   }
 
   final public function getVerbose() {
     return $this->verbose;
   }
 
   final public function setScaledownDuration($scaledown_duration) {
     $this->scaledownDuration = $scaledown_duration;
     return $this;
   }
 
   final public function getScaledownDuration() {
     return $this->scaledownDuration;
   }
 
   final public function __construct(array $argv) {
     $this->argv = $argv;
 
     $router = PhutilSignalRouter::getRouter();
     $handler_key = 'daemon.term';
     if (!$router->getHandler($handler_key)) {
       $handler = new PhutilCallbackSignalHandler(
         SIGTERM,
         __CLASS__.'::onTermSignal');
       $router->installHandler($handler_key, $handler);
     }
 
     pcntl_signal(SIGINT, array($this, 'onGracefulSignal'));
     pcntl_signal(SIGUSR2, array($this, 'onNotifySignal'));
 
     // Without discard mode, this consumes unbounded amounts of memory. Keep
     // memory bounded.
     PhutilServiceProfiler::getInstance()->enableDiscardMode();
 
     $this->beginStdoutCapture();
   }
 
   final public function __destruct() {
     $this->endStdoutCapture();
   }
 
   final public function stillWorking() {
     $this->emitOverseerMessage(self::MESSAGETYPE_HEARTBEAT, null);
 
     if ($this->traceMemory) {
       $daemon = get_class($this);
       fprintf(
         STDERR,
         "%s %s %s\n",
         '<RAMS>',
         $daemon,
         pht(
           'Memory Usage: %s KB',
           new PhutilNumber(memory_get_usage() / 1024, 1)));
     }
   }
 
   final public function shouldExit() {
     return $this->inGracefulShutdown;
   }
 
+  final protected function shouldHibernate($duration) {
+    // Don't hibernate if we don't have very long to sleep.
+    if ($duration < 5) {
+      return false;
+    }
+
+    // Never hibernate if we're part of a pool and could scale down instead.
+    // We only hibernate the last process to drop the pool size to zero.
+    if ($this->getScaledownDuration()) {
+      return false;
+    }
+
+    // Don't hibernate for too long.
+    $duration = max($duration, phutil_units('3 minutes in seconds'));
+
+    $this->emitOverseerMessage(
+      self::MESSAGETYPE_HIBERNATE,
+      array(
+        'duration' => $duration,
+      ));
+
+    $this->log(
+      pht(
+        'Preparing to hibernate for %s second(s).',
+        new PhutilNumber($duration)));
+
+    return true;
+  }
+
   final protected function sleep($duration) {
     $this->notifyReceived = false;
     $this->willSleep($duration);
     $this->stillWorking();
 
     $scale_down = $this->getScaledownDuration();
 
     $max_sleep = 60;
     if ($scale_down) {
       $max_sleep = min($max_sleep, $scale_down);
     }
 
     if ($scale_down) {
       if ($this->workState == self::WORKSTATE_IDLE) {
         $dur = (time() - $this->idleSince);
         $this->log(pht('Idle for %s seconds.', $dur));
       }
     }
 
     while ($duration > 0 &&
       !$this->notifyReceived &&
       !$this->shouldExit()) {
 
       // If this is an autoscaling clone and we've been idle for too long,
       // we're going to scale the pool down by exiting and not restarting. The
       // DOWN message tells the overseer that we don't want to be restarted.
       if ($scale_down) {
         if ($this->workState == self::WORKSTATE_IDLE) {
           if ($this->idleSince && ($this->idleSince + $scale_down < time())) {
             $this->inGracefulShutdown = true;
             $this->emitOverseerMessage(self::MESSAGETYPE_DOWN, null);
             $this->log(
               pht(
                 'Daemon was idle for more than %s second(s), '.
                 'scaling pool down.',
                 new PhutilNumber($scale_down)));
             break;
           }
         }
       }
 
       sleep(min($duration, $max_sleep));
       $duration -= $max_sleep;
       $this->stillWorking();
     }
   }
 
   protected function willSleep($duration) {
     return;
   }
 
   public static function onTermSignal($signo) {
     self::didCatchSignal($signo);
   }
 
   final protected function getArgv() {
     return $this->argv;
   }
 
   final public function execute() {
     $this->willRun();
     $this->run();
   }
 
   abstract protected function run();
 
   final public function setTraceMemory() {
     $this->traceMemory = true;
     return $this;
   }
 
   final public function getTraceMemory() {
     return $this->traceMemory;
   }
 
   final public function setTraceMode() {
     $this->traceMode = true;
     PhutilServiceProfiler::installEchoListener();
     PhutilConsole::getConsole()->getServer()->setEnableLog(true);
     $this->didSetTraceMode();
     return $this;
   }
 
   final public function getTraceMode() {
     return $this->traceMode;
   }
 
   final public function onGracefulSignal($signo) {
     self::didCatchSignal($signo);
     $this->inGracefulShutdown = true;
   }
 
   final public function onNotifySignal($signo) {
     self::didCatchSignal($signo);
     $this->notifyReceived = true;
     $this->onNotify($signo);
   }
 
   protected function onNotify($signo) {
     // This is a hook for subclasses.
   }
 
   protected function willRun() {
     // This is a hook for subclasses.
   }
 
   protected function didSetTraceMode() {
     // This is a hook for subclasses.
   }
 
   final protected function log($message) {
     if ($this->verbose) {
       $daemon = get_class($this);
       fprintf(STDERR, "%s %s %s\n", '<VERB>', $daemon, $message);
     }
   }
 
   private static function didCatchSignal($signo) {
     $signame = phutil_get_signal_name($signo);
     fprintf(
       STDERR,
       "%s Caught signal %s (%s).\n",
       '<SGNL>',
       $signo,
       $signame);
   }
 
 
 /* -(  Communicating With the Overseer  )------------------------------------ */
 
 
   private function beginStdoutCapture() {
     ob_start(array($this, 'didReceiveStdout'), 2);
   }
 
   private function endStdoutCapture() {
     ob_end_flush();
   }
 
   public function didReceiveStdout($data) {
     if (!strlen($data)) {
       return '';
     }
 
     return $this->encodeOverseerMessage(self::MESSAGETYPE_STDOUT, $data);
   }
 
   private function encodeOverseerMessage($type, $data) {
     $structure = array($type);
 
     if ($data !== null) {
       $structure[] = $data;
     }
 
     return json_encode($structure)."\n";
   }
 
   private function emitOverseerMessage($type, $data) {
     $this->endStdoutCapture();
     echo $this->encodeOverseerMessage($type, $data);
     $this->beginStdoutCapture();
   }
 
   public static function errorListener($event, $value, array $metadata) {
     // If the caller has redirected the error log to a file, PHP won't output
     // messages to stderr, so the overseer can't capture them. Install a
     // listener which just  echoes errors to stderr, so the overseer is always
     // aware of errors.
 
     $console = PhutilConsole::getConsole();
     $message = idx($metadata, 'default_message');
 
     if ($message) {
       $console->writeErr("%s\n", $message);
     }
     if (idx($metadata, 'trace')) {
       $trace = PhutilErrorHandler::formatStacktrace($metadata['trace']);
       $console->writeErr("%s\n", $trace);
     }
   }
 
 
 /* -(  Autoscaling  )-------------------------------------------------------- */
 
 
   /**
    * Prepare to become busy. This may autoscale the pool up.
    *
    * This notifies the overseer that the daemon has become busy. If daemons
    * that are part of an autoscale pool are continuously busy for a prolonged
    * period of time, the overseer may scale up the pool.
    *
    * @return this
    * @task autoscale
    */
   protected function willBeginWork() {
     if ($this->workState != self::WORKSTATE_BUSY) {
       $this->workState = self::WORKSTATE_BUSY;
       $this->idleSince = null;
       $this->emitOverseerMessage(self::MESSAGETYPE_BUSY, null);
     }
 
     return $this;
   }
 
 
   /**
    * Prepare to idle. This may autoscale the pool down.
    *
    * This notifies the overseer that the daemon is no longer busy. If daemons
    * that are part of an autoscale pool are idle for a prolonged period of
    * time, they may exit to scale the pool down.
    *
    * @return this
    * @task autoscale
    */
   protected function willBeginIdle() {
     if ($this->workState != self::WORKSTATE_IDLE) {
       $this->workState = self::WORKSTATE_IDLE;
       $this->idleSince = time();
       $this->emitOverseerMessage(self::MESSAGETYPE_IDLE, null);
     }
 
     return $this;
   }
 }
diff --git a/src/daemon/PhutilDaemonHandle.php b/src/daemon/PhutilDaemonHandle.php
index c81256d..6d4e5b1 100644
--- a/src/daemon/PhutilDaemonHandle.php
+++ b/src/daemon/PhutilDaemonHandle.php
@@ -1,458 +1,478 @@
 <?php
 
 final class PhutilDaemonHandle extends Phobject {
 
   const EVENT_DID_LAUNCH    = 'daemon.didLaunch';
   const EVENT_DID_LOG       = 'daemon.didLogMessage';
   const EVENT_DID_HEARTBEAT = 'daemon.didHeartbeat';
   const EVENT_WILL_GRACEFUL = 'daemon.willGraceful';
   const EVENT_WILL_EXIT     = 'daemon.willExit';
 
   private $pool;
   private $properties;
   private $future;
   private $argv;
 
   private $restartAt;
   private $busyEpoch;
 
   private $pid;
   private $daemonID;
   private $deadline;
   private $heartbeat;
   private $stdoutBuffer;
   private $shouldRestart = true;
   private $shouldShutdown;
 
   private function __construct() {
     // <empty>
   }
 
   public static function newFromConfig(array $config) {
     PhutilTypeSpec::checkMap(
       $config,
       array(
         'class' => 'string',
         'argv' => 'optional list<string>',
         'load' => 'optional list<string>',
         'log' => 'optional string|null',
         'down' => 'optional int',
       ));
 
     $config = $config + array(
       'argv' => array(),
       'load' => array(),
       'log' => null,
       'down' => 15,
     );
 
     $daemon = new self();
     $daemon->properties = $config;
     $daemon->daemonID = $daemon->generateDaemonID();
 
     return $daemon;
   }
 
   public function setDaemonPool(PhutilDaemonPool $daemon_pool) {
     $this->pool = $daemon_pool;
     return $this;
   }
 
   public function getDaemonPool() {
     return $this->pool;
   }
 
   public function getBusyEpoch() {
     return $this->busyEpoch;
   }
 
   public function getDaemonClass() {
     return $this->getProperty('class');
   }
 
   private function getProperty($key) {
     return idx($this->properties, $key);
   }
 
   public function setCommandLineArguments(array $arguments) {
     $this->argv = $arguments;
     return $this;
   }
 
   public function getCommandLineArguments() {
     return $this->argv;
   }
 
   public function getDaemonArguments() {
     return $this->getProperty('argv');
   }
 
   public function didLaunch() {
     $this->restartAt = time();
 
     $this->dispatchEvent(
       self::EVENT_DID_LAUNCH,
       array(
         'argv' => $this->getCommandLineArguments(),
         'explicitArgv' => $this->getDaemonArguments(),
       ));
 
     return $this;
   }
 
   public function isRunning() {
     return (bool)$this->future;
   }
 
   public function isDone() {
     return (!$this->shouldRestart && !$this->isRunning());
   }
 
   public function getFuture() {
     return $this->future;
   }
 
   public function update() {
     if (!$this->isRunning()) {
       if (!$this->shouldRestart) {
         return;
       }
       if (!$this->restartAt || (time() < $this->restartAt)) {
         return;
       }
       if ($this->shouldShutdown) {
         return;
       }
       $this->startDaemonProcess();
     }
 
     $future = $this->future;
 
     $result = null;
     if ($future->isReady()) {
       $result = $future->resolve();
     }
 
     list($stdout, $stderr) = $future->read();
     $future->discardBuffers();
 
     if (strlen($stdout)) {
       $this->didReadStdout($stdout);
     }
 
     $stderr = trim($stderr);
     if (strlen($stderr)) {
       foreach (phutil_split_lines($stderr, false) as $line) {
         $this->logMessage('STDE', $line);
       }
     }
 
     if ($result !== null) {
       list($err) = $result;
 
       if ($err) {
         $this->logMessage('FAIL', pht('Process exited with error %s.', $err));
       } else {
         $this->logMessage('DONE', pht('Process exited normally.'));
       }
 
       $this->future = null;
 
       if ($this->shouldShutdown) {
         $this->restartAt = null;
         $this->dispatchEvent(self::EVENT_WILL_EXIT);
       } else {
         $this->scheduleRestart();
       }
     }
 
     $this->updateHeartbeatEvent();
     $this->updateHangDetection();
   }
 
   private function updateHeartbeatEvent() {
     if ($this->heartbeat > time()) {
       return;
     }
 
     $this->heartbeat = time() + $this->getHeartbeatEventFrequency();
     $this->dispatchEvent(self::EVENT_DID_HEARTBEAT);
   }
 
   private function updateHangDetection() {
     if (!$this->isRunning()) {
       return;
     }
 
     if (time() > $this->deadline) {
       $this->logMessage('HANG', pht('Hang detected. Restarting process.'));
       $this->annihilateProcessGroup();
       $this->scheduleRestart();
     }
   }
 
   private function scheduleRestart() {
-    $this->logMessage('WAIT', pht('Waiting to restart process.'));
-    $this->restartAt = time() + self::getWaitBeforeRestart();
+    // Wait a minimum of a few sceconds before restarting, but we may wait
+    // longer if the daemon has initiated hibernation.
+    $default_restart = time() + self::getWaitBeforeRestart();
+    if ($default_restart >= $this->restartAt) {
+      $this->restartAt = $default_restart;
+    }
+
+    $this->logMessage(
+      'WAIT',
+      pht(
+        'Waiting %s second(s) to restart process.',
+        new PhutilNumber($this->restartAt - time())));
   }
 
   /**
    * Generate a unique ID for this daemon.
    *
    * @return string A unique daemon ID.
    */
   private function generateDaemonID() {
     return substr(getmypid().':'.Filesystem::readRandomCharacters(12), 0, 12);
   }
 
   public function getDaemonID() {
     return $this->daemonID;
   }
 
   public function getPID() {
     return $this->pid;
   }
 
   private function getCaptureBufferSize() {
     return 65535;
   }
 
   private function getRequiredHeartbeatFrequency() {
     return 86400;
   }
 
   public static function getWaitBeforeRestart() {
     return 5;
   }
 
   public static function getHeartbeatEventFrequency() {
     return 120;
   }
 
   private function getKillDelay() {
     return 3;
   }
 
   private function getDaemonCWD() {
     $root = dirname(phutil_get_library_root('phutil'));
     return $root.'/scripts/daemon/exec/';
   }
 
   private function newExecFuture() {
     $class = $this->getDaemonClass();
     $argv = $this->getCommandLineArguments();
     $buffer_size = $this->getCaptureBufferSize();
 
     // NOTE: PHP implements proc_open() by running 'sh -c'. On most systems this
     // is bash, but on Ubuntu it's dash. When you proc_open() using bash, you
     // get one new process (the command you ran). When you proc_open() using
     // dash, you get two new processes: the command you ran and a parent
     // "dash -c" (or "sh -c") process. This means that the child process's PID
     // is actually the 'dash' PID, not the command's PID. To avoid this, use
     // 'exec' to replace the shell process with the real process; without this,
     // the child will call posix_getppid(), be given the pid of the 'sh -c'
     // process, and send it SIGUSR1 to keepalive which will terminate it
     // immediately. We also won't be able to do process group management because
     // the shell process won't properly posix_setsid() so the pgid of the child
     // won't be meaningful.
 
     $config = $this->properties;
     unset($config['class']);
     $config = phutil_json_encode($config);
 
     return id(new ExecFuture('exec ./exec_daemon.php %s %Ls', $class, $argv))
       ->setCWD($this->getDaemonCWD())
       ->setStdoutSizeLimit($buffer_size)
       ->setStderrSizeLimit($buffer_size)
       ->write($config);
   }
 
   /**
    * Dispatch an event to event listeners.
    *
    * @param  string Event type.
    * @param  dict   Event parameters.
    * @return void
    */
   private function dispatchEvent($type, array $params = array()) {
     $data = array(
       'id' => $this->getDaemonID(),
       'daemonClass' => $this->getDaemonClass(),
       'childPID' => $this->getPID(),
     ) + $params;
 
     $event = new PhutilEvent($type, $data);
 
     try {
       PhutilEventEngine::dispatchEvent($event);
     } catch (Exception $ex) {
       phlog($ex);
     }
   }
 
   private function annihilateProcessGroup() {
     $pid = $this->getPID();
 
     $pgid = posix_getpgid($pid);
     if ($pid && $pgid) {
       posix_kill(-$pgid, SIGTERM);
       sleep($this->getKillDelay());
       posix_kill(-$pgid, SIGKILL);
       $this->pid = null;
     }
   }
 
   private function startDaemonProcess() {
     $this->logMessage('INIT', pht('Starting process.'));
 
     $this->deadline = time() + $this->getRequiredHeartbeatFrequency();
     $this->heartbeat = time() + self::getHeartbeatEventFrequency();
     $this->stdoutBuffer = '';
 
     $this->future = $this->newExecFuture();
     $this->future->start();
 
     $this->pid = $this->future->getPID();
   }
 
   private function didReadStdout($data) {
     $this->stdoutBuffer .= $data;
     while (true) {
       $pos = strpos($this->stdoutBuffer, "\n");
       if ($pos === false) {
         break;
       }
       $message = substr($this->stdoutBuffer, 0, $pos);
       $this->stdoutBuffer = substr($this->stdoutBuffer, $pos + 1);
 
       try {
         $structure = phutil_json_decode($message);
       } catch (PhutilJSONParserException $ex) {
         $structure = array();
       }
 
       switch (idx($structure, 0)) {
         case PhutilDaemon::MESSAGETYPE_STDOUT:
           $this->logMessage('STDO', idx($structure, 1));
           break;
         case PhutilDaemon::MESSAGETYPE_HEARTBEAT:
           $this->deadline = time() + $this->getRequiredHeartbeatFrequency();
           break;
         case PhutilDaemon::MESSAGETYPE_BUSY:
           if (!$this->busyEpoch) {
             $this->busyEpoch = time();
           }
           break;
         case PhutilDaemon::MESSAGETYPE_IDLE:
           $this->busyEpoch = null;
           break;
         case PhutilDaemon::MESSAGETYPE_DOWN:
           // The daemon is exiting because it doesn't have enough work and it
           // is trying to scale the pool down. We should not restart it.
           $this->shouldRestart = false;
           $this->shouldShutdown = true;
           break;
+        case PhutilDaemon::MESSAGETYPE_HIBERNATE:
+          $config = idx($structure, 1);
+          $duration = (int)idx($config, 'duration', 0);
+          $this->restartAt = time() + $duration;
+          $this->logMessage(
+            'ZZZZ',
+            pht(
+              'Process is preparing to hibernate for %s second(s).',
+              new PhutilNumber($duration)));
+          break;
         default:
           // If we can't parse this or it isn't a message we understand, just
           // emit the raw message.
           $this->logMessage('STDO', pht('<Malformed> %s', $message));
           break;
       }
     }
   }
 
   public function didReceiveNotifySignal($signo) {
     $pid = $this->getPID();
     if ($pid) {
       posix_kill($pid, $signo);
     }
   }
 
   public function didReceiveReloadSignal($signo) {
     $signame = phutil_get_signal_name($signo);
     if ($signame) {
       $sigmsg = pht(
         'Reloading in response to signal %d (%s).',
         $signo,
         $signame);
     } else {
       $sigmsg = pht(
         'Reloading in response to signal %d.',
         $signo);
     }
 
     $this->logMessage('RELO', $sigmsg, $signo);
 
     // This signal means "stop the current process gracefully, then launch
     // a new identical process once it exits". This can be used to update
     // daemons after code changes (the new processes will run the new code)
     // without aborting any running tasks.
 
     // We SIGINT the daemon but don't set the shutdown flag, so it will
     // naturally be restarted after it exits, as though it had exited after an
     // unhandled exception.
 
     posix_kill($this->getPID(), SIGINT);
   }
 
   public function didReceiveGracefulSignal($signo) {
     $this->shouldShutdown = true;
     $this->shouldRestart = false;
 
     $signame = phutil_get_signal_name($signo);
     if ($signame) {
       $sigmsg = pht(
         'Graceful shutdown in response to signal %d (%s).',
         $signo,
         $signame);
     } else {
       $sigmsg = pht(
         'Graceful shutdown in response to signal %d.',
         $signo);
     }
 
     $this->logMessage('DONE', $sigmsg, $signo);
 
     posix_kill($this->getPID(), SIGINT);
   }
 
   public function didReceiveTerminateSignal($signo) {
     $this->shouldShutdown = true;
     $this->shouldRestart = false;
 
     $signame = phutil_get_signal_name($signo);
     if ($signame) {
       $sigmsg = pht(
         'Shutting down in response to signal %s (%s).',
         $signo,
         $signame);
     } else {
       $sigmsg = pht('Shutting down in response to signal %s.', $signo);
     }
 
     $this->logMessage('EXIT', $sigmsg, $signo);
     $this->annihilateProcessGroup();
   }
 
   private function logMessage($type, $message, $context = null) {
     $this->getDaemonPool()->logMessage($type, $message, $context);
 
     $this->dispatchEvent(
       self::EVENT_DID_LOG,
       array(
         'type' => $type,
         'message' => $message,
         'context' => $context,
       ));
   }
 
   public function toDictionary() {
     return array(
       'pid' => $this->getPID(),
       'id' => $this->getDaemonID(),
       'config' => $this->properties,
     );
   }
 
 
 }