Execute a controller method in background.

Hi,
     I have to execute controller method in background and I am using HMVC structure.
     
       <?php
          class Test_Controller extends CI_Controller  {
                public function index() {
                 }
                public function test() {
                      // background script code
                }
 
           } ?>


I need to execute the function test as background process using exec/shell command. Any solution?
If not,Where (folder structure) I can write background script in codeigniter?