static int
twice(SchedTask *s,void *rbuf, void *wbuf)
{
    /**
     * Get Input Data
     *   get_input(input data buffer, index)
     */
    int* input = (int*)s->get_input(rbuf, 0);

    /**
     * Get Output Data
     *   get_output(output data buffer, index)
     */
    int* output = (int*)s->get_output(wbuf, 0);

    /**
     * Get index(x, y, z)
     *   SchedTask member
     *   x : SchedTask->x
     *   y : SchedTask->y
     *   z : SchedTask->z
     */
    long i = s->x;

    output[i] = input[i]*2;
    
    return 0;
}