function wp_start_object_cache() {
	global $wp_filter;
	static $first_init = true;

	// Only perform the following checks once.
	if ( $first_init ) {
		if ( ! function_exists( 'wp_cache_init' ) ) {
			/*
			 * This is the normal situation. First-run of this function. No
			 * caching backend has been loaded.
			 *
			 * We try to load a custom caching backend, and then, if it
			 * results in a wp_cache_init() function existing, we note
			 * that an external object cache is being used.
			 */
			if ( file_exists( WP_CONTENT_DIR . '/object-cache.php' ) ) {
				require_once( WP_CONTENT_DIR . '/object-cache.php' );
				if ( function_exists( 'wp_cache_init' ) ) {
					wp_using_ext_object_cache( true );
				}

				// Re-initialize any hooks added manually by object-cache.php
				if ( $wp_filter ) {
					$wp_filter = WP_Hook::build_preinitialized_hooks( $wp_filter );
				}
			}
		} elseif ( ! wp_using_ext_object_cache() && file_exists( WP_CONTENT_DIR . '/object-cache.php' ) ) {
			/*
			 * Sometimes advanced-cache.php can load object-cache.php before
			 * this function is run. This breaks the function_exists() check
			 * above and can result in wp_using_ext_object_cache() returning
			 * false when actually an external cache is in use.
			 */
			wp_using_ext_object_cache( true );
		}
	}

	if ( ! wp_using_ext_object_cache() ) {
		require_once( ABSPATH . WPINC . '/cache.php' );
	}

	/*
	 * If cache supports reset, reset instead of init if already
	 * initialized. Reset signals to the cache that global IDs
	 * have changed and it may need to update keys and cleanup caches.
	 */
	if ( ! $first_init && function_exists( 'wp_cache_switch_to_blog' ) ) {
		wp_cache_switch_to_blog( get_current_blog_id() );
	} elseif ( function_exists( 'wp_cache_init' ) ) {
		wp_cache_init();
	}

	if ( function_exists( 'wp_cache_add_global_groups' ) ) {
		wp_cache_add_global_groups( array( 'users', 'userlogins', 'usermeta', 'user_meta', 'useremail', 'userslugs', 'site-transient', 'site-options', 'blog-lookup', 'blog-details', 'site-details', 'rss', 'global-posts', 'blog-id-cache', 'networks', 'sites', 'blog_meta' ) );
		wp_cache_add_non_persistent_groups( array( 'counts', 'plugins' ) );
	}

	$first_init = false;
}