Restart of session doesn't work

I need to close session before long-runing operation, because unclosed session blocks other processes.

``` php
$session->start();
//working with session..
$session->close();

//long-running operation

$session->start();
//working with session..
$session->close();
```

It returns "headers already sent..". Cookie should not be sends again, if I call start() more times, or not?
Sorry for my English.
