WBM_MSG_UPPER is one of three system parameters that are
    available for managing the update traffic between a master
    write bitmap and its corresponding local write bitmaps in an
    OpenVMS Cluster system. The others are WBM_MSG_INT and WBM_MSG_
    LOWER. These parameters set the interval at which the frequency
    of sending messages is tested and also set an upper and lower
    threshold that determine whether the messages are grouped into
    one SCS message or are sent one by one.

    WBM_MSG_UPPER is the upper threshold for the number of messages
    sent during the test interval that initiates buffered-message
    mode. In buffered-message mode, the messages are collected for a
    specified interval and then sent in one SCS message.

    The minimum value of WBM_MSG_UPPER is 0 messages per interval.
    The maximum value is -1, which corresponds to the maximum
    positive value that a longword can represent. The default is
    100 seconds.

    WBM_MSG_UPPER is a DYNAMIC parameter.