someone know why Vdec2_process cost time varies much when cpu load varies??

Hi all,

   I'm using dm6467 to decode a 720p 60fps stream. But one thing really confused me.

   If running decode demo alone, decode demo cost 12ms in average to decode a frame.

   But if I'm running some program in the same time which consumed about 30% of the cpu.

   decode demo cost 20ms in average to decode a frame, which makes it not realtime.  Why

  is this big difference???

   My time measurement code snippet is like below:

        before_process_time = os_get_current_ms();

        /* Decode the video buffer */
        ret = Vdec2_process(hVd2, hInBuf, hDstBuf);

        after_process_time = os_get_current_ms();

        cur_time = (after_process_time - before_process_time);
        total_time += cur_time;

        avg_time = total_time / (total_frame_nbr + 1);

Your help is greatly appreciated, thanks in advance.