diff --git a/src/applications/cache/PhabricatorCaches.php b/src/applications/cache/PhabricatorCaches.php
index 5824c6458c..1127a7312b 100644
--- a/src/applications/cache/PhabricatorCaches.php
+++ b/src/applications/cache/PhabricatorCaches.php
@@ -1,384 +1,356 @@
 <?php
 
 /**
  *
  * @task request    Request Cache
  * @task immutable  Immutable Cache
  * @task setup      Setup Cache
  * @task compress   Compression
  */
 final class PhabricatorCaches extends Phobject {
 
   private static $requestCache;
 
   public static function getNamespace() {
     return PhabricatorEnv::getEnvConfig('phabricator.cache-namespace');
   }
 
   private static function newStackFromCaches(array $caches) {
     $caches = self::addNamespaceToCaches($caches);
     $caches = self::addProfilerToCaches($caches);
     return id(new PhutilKeyValueCacheStack())
       ->setCaches($caches);
   }
 
 /* -(  Request Cache  )------------------------------------------------------ */
 
 
   /**
    * Get a request cache stack.
    *
    * This cache stack is destroyed after each logical request. In particular,
    * it is destroyed periodically by the daemons, while `static` caches are
    * not.
    *
    * @return PhutilKeyValueCacheStack Request cache stack.
    */
   public static function getRequestCache() {
     if (!self::$requestCache) {
       self::$requestCache = new PhutilInRequestKeyValueCache();
     }
     return self::$requestCache;
   }
 
 
   /**
    * Destroy the request cache.
    *
    * This is called at the beginning of each logical request.
    *
    * @return void
    */
   public static function destroyRequestCache() {
     self::$requestCache = null;
   }
 
 
 /* -(  Immutable Cache  )---------------------------------------------------- */
 
 
   /**
    * Gets an immutable cache stack.
    *
    * This stack trades mutability away for improved performance. Normally, it is
    * APC + DB.
    *
    * In the general case with multiple web frontends, this stack can not be
    * cleared, so it is only appropriate for use if the value of a given key is
    * permanent and immutable.
    *
    * @return PhutilKeyValueCacheStack Best immutable stack available.
    * @task immutable
    */
   public static function getImmutableCache() {
     static $cache;
     if (!$cache) {
       $caches = self::buildImmutableCaches();
       $cache = self::newStackFromCaches($caches);
     }
     return $cache;
   }
 
 
   /**
    * Build the immutable cache stack.
    *
    * @return list<PhutilKeyValueCache> List of caches.
    * @task immutable
    */
   private static function buildImmutableCaches() {
     $caches = array();
 
     $apc = new PhutilAPCKeyValueCache();
     if ($apc->isAvailable()) {
       $caches[] = $apc;
     }
 
     $caches[] = new PhabricatorKeyValueDatabaseCache();
 
     return $caches;
   }
 
 
 /* -(  Repository Graph Cache  )--------------------------------------------- */
 
 
   public static function getRepositoryGraphL1Cache() {
     static $cache;
     if (!$cache) {
       $caches = self::buildRepositoryGraphL1Caches();
       $cache = self::newStackFromCaches($caches);
     }
     return $cache;
   }
 
   private static function buildRepositoryGraphL1Caches() {
     $caches = array();
 
     $request = new PhutilInRequestKeyValueCache();
     $request->setLimit(32);
     $caches[] = $request;
 
     $apc = new PhutilAPCKeyValueCache();
     if ($apc->isAvailable()) {
       $caches[] = $apc;
     }
 
     return $caches;
   }
 
   public static function getRepositoryGraphL2Cache() {
     static $cache;
     if (!$cache) {
       $caches = self::buildRepositoryGraphL2Caches();
       $cache = self::newStackFromCaches($caches);
     }
     return $cache;
   }
 
   private static function buildRepositoryGraphL2Caches() {
     $caches = array();
     $caches[] = new PhabricatorKeyValueDatabaseCache();
     return $caches;
   }
 
 
 /* -(  Setup Cache  )-------------------------------------------------------- */
 
 
   /**
    * Highly specialized cache for performing setup checks. We use this cache
    * to determine if we need to run expensive setup checks when the page
    * loads. Without it, we would need to run these checks every time.
    *
    * Normally, this cache is just APC. In the absence of APC, this cache
    * degrades into a slow, quirky on-disk cache.
    *
    * NOTE: Do not use this cache for anything else! It is not a general-purpose
    * cache!
    *
    * @return PhutilKeyValueCacheStack Most qualified available cache stack.
    * @task setup
    */
   public static function getSetupCache() {
     static $cache;
     if (!$cache) {
       $caches = self::buildSetupCaches();
       $cache = self::newStackFromCaches($caches);
     }
     return $cache;
   }
 
 
   /**
    * @task setup
    */
   private static function buildSetupCaches() {
     // In most cases, we should have APC. This is an ideal cache for our
     // purposes -- it's fast and empties on server restart.
     $apc = new PhutilAPCKeyValueCache();
     if ($apc->isAvailable()) {
       return array($apc);
     }
 
     // If we don't have APC, build a poor approximation on disk. This is still
     // much better than nothing; some setup steps are quite slow.
     $disk_path = self::getSetupCacheDiskCachePath();
     if ($disk_path) {
       $disk = new PhutilOnDiskKeyValueCache();
       $disk->setCacheFile($disk_path);
       $disk->setWait(0.1);
       if ($disk->isAvailable()) {
         return array($disk);
       }
     }
 
     return array();
   }
 
 
   /**
    * @task setup
    */
   private static function getSetupCacheDiskCachePath() {
     // The difficulty here is in choosing a path which will change on server
     // restart (we MUST have this property), but as rarely as possible
     // otherwise (we desire this property to give the cache the best hit rate
     // we can).
 
-    // In some setups, the parent PID is more stable and longer-lived that the
-    // PID (e.g., under apache, our PID will be a worker while the ppid will
-    // be the main httpd process). If we're confident we're running under such
-    // a setup, we can try to use the PPID as the basis for our cache instead
-    // of our own PID.
-    $use_ppid = false;
-
-    switch (php_sapi_name()) {
-      case 'cli-server':
-        // This is the PHP5.4+ built-in webserver. We should use the pid
-        // (the server), not the ppid (probably a shell or something).
-        $use_ppid = false;
-        break;
-      case 'fpm-fcgi':
-        // We should be safe to use PPID here.
-        $use_ppid = true;
-        break;
-      case 'apache2handler':
-        // We're definitely safe to use the PPID.
-        $use_ppid = true;
-        break;
-    }
+    // Unfortunately, we don't have a very good strategy for minimizing the
+    // churn rate of the cache. We previously tried to use the parent process
+    // PID in some cases, but this was not reliable. See T9599 for one case of
+    // this.
 
     $pid_basis = getmypid();
-    if ($use_ppid) {
-      if (function_exists('posix_getppid')) {
-        $parent_pid = posix_getppid();
-        // On most systems, normal processes can never have PIDs lower than 100,
-        // so something likely went wrong if we we get one of these.
-        if ($parent_pid > 100) {
-          $pid_basis = $parent_pid;
-        }
-      }
-    }
 
     // If possible, we also want to know when the process launched, so we can
     // drop the cache if a process restarts but gets the same PID an earlier
     // process had. "/proc" is not available everywhere (e.g., not on OSX), but
     // check if we have it.
     $epoch_basis = null;
     $stat = @stat("/proc/{$pid_basis}");
     if ($stat !== false) {
       $epoch_basis = $stat['ctime'];
     }
 
     $tmp_dir = sys_get_temp_dir();
 
     $tmp_path = $tmp_dir.DIRECTORY_SEPARATOR.'phabricator-setup';
     if (!file_exists($tmp_path)) {
       @mkdir($tmp_path);
     }
 
     $is_ok = self::testTemporaryDirectory($tmp_path);
     if (!$is_ok) {
       $tmp_path = $tmp_dir;
       $is_ok = self::testTemporaryDirectory($tmp_path);
       if (!$is_ok) {
         // We can't find anywhere to write the cache, so just bail.
         return null;
       }
     }
 
     $tmp_name = 'setup-'.$pid_basis;
     if ($epoch_basis) {
       $tmp_name .= '.'.$epoch_basis;
     }
     $tmp_name .= '.cache';
 
     return $tmp_path.DIRECTORY_SEPARATOR.$tmp_name;
   }
 
 
   /**
    * @task setup
    */
   private static function testTemporaryDirectory($dir) {
     if (!@file_exists($dir)) {
       return false;
     }
     if (!@is_dir($dir)) {
       return false;
     }
     if (!@is_writable($dir)) {
       return false;
     }
 
     return true;
   }
 
   private static function addProfilerToCaches(array $caches) {
     foreach ($caches as $key => $cache) {
       $pcache = new PhutilKeyValueCacheProfiler($cache);
       $pcache->setProfiler(PhutilServiceProfiler::getInstance());
       $caches[$key] = $pcache;
     }
     return $caches;
   }
 
   private static function addNamespaceToCaches(array $caches) {
     $namespace = self::getNamespace();
     if (!$namespace) {
       return $caches;
     }
 
     foreach ($caches as $key => $cache) {
       $ncache = new PhutilKeyValueCacheNamespace($cache);
       $ncache->setNamespace($namespace);
       $caches[$key] = $ncache;
     }
 
     return $caches;
   }
 
 
   /**
    * Deflate a value, if deflation is available and has an impact.
    *
    * If the value is larger than 1KB, we have `gzdeflate()`, we successfully
    * can deflate it, and it benefits from deflation, we deflate it. Otherwise
    * we leave it as-is.
    *
    * Data can later be inflated with @{method:inflateData}.
    *
    * @param string String to attempt to deflate.
    * @return string|null Deflated string, or null if it was not deflated.
    * @task compress
    */
   public static function maybeDeflateData($value) {
     $len = strlen($value);
     if ($len <= 1024) {
       return null;
     }
 
     if (!function_exists('gzdeflate')) {
       return null;
     }
 
     $deflated = gzdeflate($value);
     if ($deflated === false) {
       return null;
     }
 
     $deflated_len = strlen($deflated);
     if ($deflated_len >= ($len / 2)) {
       return null;
     }
 
     return $deflated;
   }
 
 
   /**
    * Inflate data previously deflated by @{method:maybeDeflateData}.
    *
    * @param string Deflated data, from @{method:maybeDeflateData}.
    * @return string Original, uncompressed data.
    * @task compress
    */
   public static function inflateData($value) {
     if (!function_exists('gzinflate')) {
       throw new Exception(
         pht(
           '%s is not available; unable to read deflated data!',
           'gzinflate()'));
     }
 
     $value = gzinflate($value);
     if ($value === false) {
       throw new Exception(pht('Failed to inflate data!'));
     }
 
     return $value;
   }
 
 
 }
diff --git a/src/applications/cache/spec/PhabricatorDataCacheSpec.php b/src/applications/cache/spec/PhabricatorDataCacheSpec.php
index f5fe1c6277..eae5d75790 100644
--- a/src/applications/cache/spec/PhabricatorDataCacheSpec.php
+++ b/src/applications/cache/spec/PhabricatorDataCacheSpec.php
@@ -1,125 +1,127 @@
 <?php
 
 final class PhabricatorDataCacheSpec extends PhabricatorCacheSpec {
 
   private $cacheSummary;
 
   public function setCacheSummary(array $cache_summary) {
     $this->cacheSummary = $cache_summary;
     return $this;
   }
 
   public function getCacheSummary() {
     return $this->cacheSummary;
   }
 
   public static function getActiveCacheSpec() {
     $spec = new PhabricatorDataCacheSpec();
 
     // NOTE: If APCu is installed, it reports that APC is installed.
     if (extension_loaded('apc') && !extension_loaded('apcu')) {
       $spec->initAPCSpec();
     } else if (extension_loaded('apcu')) {
       $spec->initAPCuSpec();
     } else {
       $spec->initNoneSpec();
     }
 
     return $spec;
   }
 
   private function initAPCSpec() {
     $this
       ->setName(pht('APC User Cache'))
       ->setVersion(phpversion('apc'));
 
     if (ini_get('apc.enabled')) {
       $this
         ->setIsEnabled(true)
         ->setClearCacheCallback('apc_clear_cache');
       $this->initAPCCommonSpec();
     } else {
       $this->setIsEnabled(false);
       $this->raiseEnableAPCIssue();
     }
   }
 
   private function initAPCuSpec() {
     $this
       ->setName(pht('APCu'))
       ->setVersion(phpversion('apcu'));
 
     if (ini_get('apc.enabled')) {
       $this
         ->setIsEnabled(true)
         ->setClearCacheCallback('apc_clear_cache');
       $this->initAPCCommonSpec();
     } else {
       $this->setIsEnabled(false);
       $this->raiseEnableAPCIssue();
     }
   }
 
   private function initNoneSpec() {
     if (version_compare(phpversion(), '5.5', '>=')) {
       $message = pht(
-        'Installing the "APCu" PHP extension will improve performance.');
+        'Installing the "APCu" PHP extension will improve performance. '.
+        'This extension is strongly recommended. Without it, Phabricator '.
+        'must rely on a very inefficient disk-based cache.');
 
       $this
         ->newIssue('extension.apcu')
         ->setShortName(pht('APCu'))
         ->setName(pht('PHP Extension "APCu" Not Installed'))
         ->setMessage($message)
         ->addPHPExtension('apcu');
     } else {
       $this->raiseInstallAPCIssue();
     }
   }
 
   private function initAPCCommonSpec() {
     $mem = apc_sma_info();
     $this->setTotalMemory($mem['num_seg'] * $mem['seg_size']);
 
     $info = apc_cache_info('user');
     $this->setUsedMemory($info['mem_size']);
     $this->setEntryCount(count($info['cache_list']));
 
     $cache = $info['cache_list'];
     $state = array();
     foreach ($cache as $item) {
       $info = idx($item, 'info', '<unknown-key>');
       $key = self::getKeyPattern($info);
       if (empty($state[$key])) {
         $state[$key] = array(
           'max' => 0,
           'total' => 0,
           'count' => 0,
         );
       }
       $state[$key]['max'] = max($state[$key]['max'], $item['mem_size']);
       $state[$key]['total'] += $item['mem_size'];
       $state[$key]['count']++;
     }
 
     $this->setCacheSummary($state);
   }
 
   private static function getKeyPattern($key) {
     // If this key isn't in the current cache namespace, don't reveal any
     // information about it.
     $namespace = PhabricatorEnv::getEnvConfig('phabricator.cache-namespace');
     if (strncmp($key, $namespace.':', strlen($namespace) + 1)) {
       return '<other-namespace>';
     }
 
     $key = preg_replace('/(?<![a-zA-Z])\d+(?![a-zA-Z])/', 'N', $key);
     $key = preg_replace('/PHID-[A-Z]{4}-[a-z0-9]{20}/', 'PHID', $key);
 
     // TODO: We should probably standardize how digests get embedded into cache
     // keys to make this rule more generic.
     $key = preg_replace('/:celerity:.*$/', ':celerity:X', $key);
     $key = preg_replace('/:pkcs8:.*$/', ':pkcs8:X', $key);
 
     return $key;
   }
 }