CodeIgniter - ses_destroy() uses a LOT of CPU

I have a web-application written in CodeIgniter, and for the most part everything seems fairly reasonable. However, I've noticed extremely high CPU usage when a user logs out. My logout function in my auth controller is as follows:

function logout()
{
    $goto = $SERVER['HTTP-REFERER'];
    $this->session->sess_destroy();
    if (!$goto)
        $goto = "/";
    header('location: '.$goto);
}

Normally, this is perfectly fine and fast. The strange thing is that when linked to from a particular sub-page开发者_StackOverflow this function takes about 5-6 seconds my of mysqld running at 100% cpu. How can I see what it's doing, and why?

Taking a shot in the dark (if you're sure this function is causing the slowness):

First, you could turn on MySQL's slow query log:

http://dev.mysql.com/doc/refman/5.1/en/slow-query-log.html

Then, if $sess_use_database is TRUE you might try optimizing your session table. You could have some overhead causing issues.

Apart from that, the only other thing I can think of is that there is an issue with your DB server. You might try running the MySQL Tuner to see if you can improve things a bit:

https://github.com/rackerhacker/MySQLTuner-perl

Hope that helps!

FYI

Here is the code that is run when the OP runs sess_destroy() (from v2.0.2):

/**
 * Destroy the current session
 *
 * @access  public
 * @return  void
 */
function sess_destroy()
{
    // Kill the session DB row
    if ($this->sess_use_database === TRUE AND isset($this->userdata['session_id']))
    {
        $this->CI->db->where('session_id', $this->userdata['session_id']);
        $this->CI->db->delete($this->sess_table_name);
    }

    // Kill the cookie
    setcookie(
                $this->sess_cookie_name,
                addslashes(serialize(array())),
                ($this->now - 31500000),
                $this->cookie_path,
                $this->cookie_domain,
                0
            );
}