PERF_COUNTER_BULK_COUNT

    https://technet.microsoft.com/en-us/library/cc757486(v=ws.10).aspx

def calculate_perf_counter_bulk_count(previous, current, property_name):
    """
    PERF_COUNTER_BULK_COUNT

    https://technet.microsoft.com/en-us/library/cc757486(v=ws.10).aspx
    """
    n0 = previous[property_name]
    n1 = current[property_name]
    d0 = previous["Timestamp_Sys100NS"]
    d1 = current["Timestamp_Sys100NS"]
    f = current["Frequency_Sys100NS"]

    if n0 is None or n1 is None:
        return

    return (n1 - n0) / ((d1 - d0) / f)