The dial's pointer looks like a sawtooth,how to render it ?

["chao yeaj" <yeajchao gmail com>]

Hello everyone

       In the Gtk+2.0 tutorial,there is a dial widget
      When i enlarge it,  the pointer's like a sawtooth,
      How to render it ,in order to make it more smoothly ?

    I have no idea, i need your help

Any comments would be much appreciated ,and thanks in advance !