Using PHP Sessions for High Traffic with AJAX and CodeIgniter

Question

I was reading the CodeIgniter 3 documentation on using sessions and high traffic with AJAX, and it recommends using session_write_close().

My application has functions that write and read directly from $_SESSION, as an example below.

function setSession($index, $value) {
    $_SESSION[$index] = $value;
}
function getSession($index) {
    if (isset($_SESSION[$index]) {
        return $_SESSION[$index];
    } else {
        return FALSE;
    }
}

From what I understand from the documentation these two functions should always have session_start() at the beginning and session_write_close() at the end, correct? That way they will avoid block the session and even the other requests being faster, since the session is unblocked.

Another issue that I found strange, CodeIgniter already has by default a session_start() function that is automatically triggered at the beginning of execution, without intervention from my code. What will happen to this default session_start() if my functions were to call it every time something was fetched from the session?