diff --git includes/bootstrap.inc includes/bootstrap.inc
index 0739332..823794e 100644
--- includes/bootstrap.inc
+++ includes/bootstrap.inc
@@ -730,28 +730,14 @@ function drupal_get_filename($type, $name, $filename = NULL) {
  * file.
  */
 function variable_initialize($conf = array()) {
-  // NOTE: caching the variables improves performance by 20% when serving
-  // cached pages.
-  if ($cached = cache_get('variables', 'cache_bootstrap')) {
-    $variables = $cached->data;
+  $cache = &drupal_static('variable', array());
+  if ($cached = cache_get('variable_cache', 'cache_bootstrap')) {
+    $cache = $cached->data;
   }
   else {
-    // Cache miss. Avoid a stampede.
-    $name = 'variable_init';
-    if (!lock_acquire($name, 1)) {
-      // Another request is building the variable cache.
-      // Wait, then re-run this function.
-      lock_wait($name);
-      return variable_initialize($conf);
-    }
-    else {
-      // Proceed with variable rebuild.
-      $variables = array_map('unserialize', db_query('SELECT name, value FROM {variable}')->fetchAllKeyed());
-      cache_set('variables', $variables, 'cache_bootstrap');
-      lock_release($name);
-    }
+    $cache = array();
   }
-
+  $variables = $cache;
   foreach ($conf as $name => $value) {
     $variables[$name] = $value;
   }
@@ -779,11 +765,48 @@ function variable_initialize($conf = array()) {
  */
 function variable_get($name, $default = NULL) {
   global $conf;
+  if (isset($conf[$name])) {
+    return $conf[$name];
+  }
+  // Use the advanced drupal_static() pattern, since this is called very often.
+  static $drupal_static_fast;
+  if (!isset($drupal_static_fast)) {
+    $drupal_static_fast['cache'] = &drupal_static('variable');
+  }
+  $cache = &$drupal_static_fast['cache'];
+
+  // $cache is populated in variable_initialize() for each request. If it's not
+  // available here we are in a lower bootstrap phase where the database and
+  // cache system may not be available yet.
+  if (isset($cache)) {
+    // If the variable is not already cached, look it up from the database.
+    if (!isset($cache['write_cache'])) {
+      $cache['write_cache'] = TRUE;
+    }
+    $value = variable_get_from_storage($name);
+    $value = isset($value) ? $value : $default;
+    $conf[$name] = $cache[$name] = $value;
+  }
 
   return isset($conf[$name]) ? $conf[$name] : $default;
 }
 
 /**
+ * Fetch a variable directly from storage, bypassing any values set in $conf.
+ * @param $name
+ *   The name of the variable.
+ *
+ * @return
+ *   The value of the variable, or NULL if it is not stored in the database.
+ */
+function variable_get_from_storage($name) {
+  $result = db_query('SELECT value FROM {variable} WHERE name = :name', array(':name' => $name))->fetchField();
+  if ($result) {
+    return unserialize($result);
+  }
+}
+
+/**
  * Sets a persistent variable.
  *
  * Case-sensitivity of the variable_* functions depends on the database
@@ -803,10 +826,20 @@ function variable_set($name, $value) {
   global $conf;
 
   db_merge('variable')->key(array('name' => $name))->fields(array('value' => serialize($value)))->execute();
-
-  cache_clear_all('variables', 'cache_bootstrap');
+  // On high traffic sites there may be high contention with the variables
+  // bootstrap cache. Try to avoid rebuilding the cache by reading the current
+  // cache values, replacing this variable in the cache entry, and writing the 
+  // cache entry back.
+  if ($cached = cache_get('variable_cache', 'cache_bootstrap')) {
+    $cache = $cached->data;
+    if (isset($cache[$name])) {
+      $cache[$name] = $value;
+      cache_set('cache_variables', $cache, 'cache_bootstrap');
+    }
+  }
 
   $conf[$name] = $value;
+  variable_reset_cache(FALSE);
 }
 
 /**
@@ -828,9 +861,36 @@ function variable_del($name) {
   db_delete('variable')
     ->condition('name', $name)
     ->execute();
-  cache_clear_all('variables', 'cache_bootstrap');
-
+  // On high traffic sites there may be high contention with the variables
+  // bootstrap cache. Try to avoid rebuilding the cache by reading the current
+  // cache values, removing this variable from the cache entry, and writing the
+  // cache entry back.
+  if ($cached = cache_get('variable_cache', 'cache_bootstrap')) {
+    $cache = $cached->data;
+    if (isset($cache[$name])) {
+      unset($cache[$name]);
+      cache_set('cache_variables', $cache, 'cache_bootstrap');
+    }
+  }
   unset($conf[$name]);
+  variable_reset_cache(FALSE);
+}
+
+/**
+ * Reset the variable cache.
+ *
+ * @param $bootstrap
+ *   Optional parameter to reset the variable cache for bootstrap. If a
+ *   variable is not cached for bootstrap, there is no need to clear the cache.
+ *   Defaults to TRUE.
+ */
+function variable_reset_cache($bootstrap = TRUE) {
+  if ($bootstrap) {
+    cache_clear_all('variable_cache', 'cache_bootstrap');
+  }
+  cache_clear_all('variable_cache:', 'cache', TRUE);
+  $cache = &drupal_static('variable');
+  $cache = array();
 }
 
 /**
@@ -2125,10 +2185,6 @@ function _drupal_bootstrap_database() {
 function _drupal_bootstrap_variables() {
   global $conf;
 
-  // Initialize the lock system.
-  require_once DRUPAL_ROOT . '/' . variable_get('lock_inc', 'includes/lock.inc');
-  lock_initialize();
-
   // Load variables from the database, but do not overwrite variables set in settings.php.
   $conf = variable_initialize(isset($conf) ? $conf : array());
   // Load bootstrap modules.
@@ -2142,6 +2198,10 @@ function _drupal_bootstrap_variables() {
 function _drupal_bootstrap_page_header() {
   bootstrap_invoke_all('boot');
 
+  // Prepare for non-cached page workflow.
+  require_once DRUPAL_ROOT . '/' . variable_get('lock_inc', 'includes/lock.inc');
+  lock_initialize();
+
   if (!drupal_is_cli()) {
     ob_start();
     drupal_page_header();
diff --git includes/common.inc includes/common.inc
index 62a5037..46ad539 100644
--- includes/common.inc
+++ includes/common.inc
@@ -2565,6 +2565,7 @@ function drupal_page_footer() {
   drupal_cache_system_paths();
   module_implements_write_cache();
   system_run_automated_cron();
+  variable_write_cache('variable_cache:' . arg(0)), 'cache');
 }
 
 /**
@@ -2590,6 +2591,61 @@ function drupal_exit($destination = NULL) {
 }
 
 /**
+ * Initialize the variables cache after full bootstrap.
+ */
+function variable_initialize_full() {
+  // Allow variables fetched up to this point to be cached in the global
+  // variable_bootstrap cache.
+  variable_write_cache('variable_cache', 'cache_bootstrap');
+
+  // After full bootstrap, we segment the variables cache by system path root.
+  // For example 'node' or 'admin'.
+
+  $cache = &drupal_static('variable', array());
+  if ($cached = cache_get('variable_cache:' . arg(0), 'cache')) {
+    $cache = $cached->data;
+  }
+  $variables = $cache;
+
+  // Ensure existing variables in $conf override cached values.
+  foreach ($GLOBALS['conf'] as $name => $value) {
+    $variables[$name] = $value;
+  }
+  $GLOBALS['conf'] = $variables;
+}
+
+/**
+ * Update the variable cache after a cache miss.
+ */
+function variable_write_cache($cid, $bin) {
+  $cache = &drupal_static('variable');
+  if (!empty($cache['write_cache'])) {
+    // Get a fresh copy of the variable cache from cache_get(), this will
+    // include any changes made by other requests between variable_initialize()
+    // and variable_write_cache().
+    if ($cached = cache_get($cid, $bin)) {
+      $current_cache = $cached->data;
+      $original_size = count($current_cache);
+
+      $cache = array_merge($current_cache, $cache);
+      $size = count($cache);
+      // Only write back to the cache if the size is going to be increased.
+      // If that's not the case, one of two things may have happened:
+      // - another process has cached the same set of variables as here,
+      //   so setting it again would be redundant and could add to a stampede.
+      // - another process has cached a different set of variables that happens
+      //   to be the same size. This is less likely, and just means that the
+      //   next request will cache this set of variables instead of this one.
+      if ($size >= $original_size) {
+        unset($cache['write_cache']);
+        cache_set($cid, $cache, $bin);
+      }
+    }
+  }
+  drupal_static_reset('variable');
+}
+
+/**
  * Form an associative array from a linear array.
  *
  * This function walks through the provided array and constructs an associative
@@ -4928,6 +4984,7 @@ function _drupal_bootstrap_full() {
     menu_set_custom_theme();
     drupal_theme_initialize();
     module_invoke_all('init');
+    variable_initialize_full();
   }
 }
 
diff --git modules/simpletest/drupal_web_test_case.php modules/simpletest/drupal_web_test_case.php
index 17cbb1f..c0b5102 100644
--- modules/simpletest/drupal_web_test_case.php
+++ modules/simpletest/drupal_web_test_case.php
@@ -1239,16 +1239,6 @@ class DrupalWebTestCase extends DrupalTestCase {
       ->condition('test_id', $this->testId)
       ->execute();
 
-    // Clone the current connection and replace the current prefix.
-    $connection_info = Database::getConnectionInfo('default');
-    Database::renameConnection('default', 'simpletest_original_default');
-    foreach ($connection_info as $target => $value) {
-      $connection_info[$target]['prefix'] = array(
-        'default' => $value['prefix']['default'] . $this->databasePrefix,
-      );
-    }
-    Database::addConnectionInfo('default', 'default', $connection_info['default']);
-
     // Store necessary current values before switching to prefixed database.
     $this->originalLanguage = $language;
     $this->originalLanguageDefault = variable_get('language_default');
@@ -1277,6 +1267,16 @@ class DrupalWebTestCase extends DrupalTestCase {
     file_prepare_directory($temp_files_directory, FILE_CREATE_DIRECTORY);
     $this->generatedTestFiles = FALSE;
 
+    // Clone the current connection and replace the current prefix.
+    $connection_info = Database::getConnectionInfo('default');
+    Database::renameConnection('default', 'simpletest_original_default');
+    foreach ($connection_info as $target => $value) {
+      $connection_info[$target]['prefix'] = array(
+        'default' => $value['prefix']['default'] . $this->databasePrefix,
+      );
+    }
+    Database::addConnectionInfo('default', 'default', $connection_info['default']);
+
     // Log fatal errors.
     ini_set('log_errors', 1);
     ini_set('error_log', $public_files_directory . '/error.log');
@@ -1430,7 +1430,9 @@ class DrupalWebTestCase extends DrupalTestCase {
    */
   protected function refreshVariables() {
     global $conf;
-    cache_clear_all('variables', 'cache_bootstrap');
+    cache_clear_all('variable_cache', 'cache_bootstrap');
+    cache_clear_all('variable_cache:', 'cache', TRUE);
+    drupal_static_reset('variable');
     $conf = variable_initialize();
   }
 
diff --git modules/simpletest/tests/upgrade/upgrade.test modules/simpletest/tests/upgrade/upgrade.test
index 263baff..8c9295c 100644
--- modules/simpletest/tests/upgrade/upgrade.test
+++ modules/simpletest/tests/upgrade/upgrade.test
@@ -34,6 +34,26 @@ abstract class UpgradePathTestCase extends DrupalWebTestCase {
   protected function setUp() {
     global $user, $language, $conf;
 
+    // Store necessary current values before switching to prefixed database.
+    $this->originalLanguage = $language;
+    $this->originalLanguageDefault = variable_get('language_default');
+    $this->originalFileDirectory = variable_get('file_public_path', conf_path() . '/files');
+    $this->originalProfile = drupal_get_profile();
+    $clean_url_original = variable_get('clean_url', 0);
+
+    // Create test directories ahead of installation so fatal errors and debug
+    // information can be logged during installation process.
+    // Use mock files directories with the same prefix as the database.
+    $public_files_directory  = $this->originalFileDirectory . '/simpletest/' . substr($this->databasePrefix, 10);
+    $private_files_directory = $public_files_directory . '/private';
+    $temp_files_directory    = $private_files_directory . '/temp';
+
+    // Create the directories.
+    file_prepare_directory($public_files_directory, FILE_CREATE_DIRECTORY | FILE_MODIFY_PERMISSIONS);
+    file_prepare_directory($private_files_directory, FILE_CREATE_DIRECTORY);
+    file_prepare_directory($temp_files_directory, FILE_CREATE_DIRECTORY);
+    $this->generatedTestFiles = FALSE;
+
     // Load the Update API.
     require_once DRUPAL_ROOT . '/includes/update.inc';
 
@@ -60,31 +80,11 @@ abstract class UpgradePathTestCase extends DrupalWebTestCase {
     }
     Database::addConnectionInfo('default', 'default', $connection_info['default']);
 
-    // Store necessary current values before switching to prefixed database.
-    $this->originalLanguage = $language;
-    $this->originalLanguageDefault = variable_get('language_default');
-    $this->originalFileDirectory = variable_get('file_public_path', conf_path() . '/files');
-    $this->originalProfile = drupal_get_profile();
-    $clean_url_original = variable_get('clean_url', 0);
-
     // Unregister the registry.
     // This is required to make sure that the database layer works properly.
     spl_autoload_unregister('drupal_autoload_class');
     spl_autoload_unregister('drupal_autoload_interface');
 
-    // Create test directories ahead of installation so fatal errors and debug
-    // information can be logged during installation process.
-    // Use mock files directories with the same prefix as the database.
-    $public_files_directory  = $this->originalFileDirectory . '/simpletest/' . substr($this->databasePrefix, 10);
-    $private_files_directory = $public_files_directory . '/private';
-    $temp_files_directory    = $private_files_directory . '/temp';
-
-    // Create the directories.
-    file_prepare_directory($public_files_directory, FILE_CREATE_DIRECTORY | FILE_MODIFY_PERMISSIONS);
-    file_prepare_directory($private_files_directory, FILE_CREATE_DIRECTORY);
-    file_prepare_directory($temp_files_directory, FILE_CREATE_DIRECTORY);
-    $this->generatedTestFiles = FALSE;
-
     // Log fatal errors.
     ini_set('log_errors', 1);
     ini_set('error_log', $public_files_directory . '/error.log');