def mg1_mean_qsize(arr_rate, svc_rate, cv2_svc_time):
    """
    Return the mean queue size in M/G/1/inf queue using P-K formula.

    See any decent queueing book.

    Parameters
    ----------
    arr_rate : float
        average arrival rate to queueing system
    svc_rate : float
        average service rate (each server). 1/svc_rate is mean service time.
    cv2_svc_time : float
        squared coefficient of variation for service time distribution

    Returns
    -------
    float
        mean number of customers in queue

    """

    rho = arr_rate / svc_rate
    mean_qsize = (arr_rate ** 2) * cv2_svc_time/(2 * (1.0 - rho))

    return mean_qsize