/sys$common/syshlp/helplib.hlb Sys Parameters, MC_SERVICES_P6  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    (Alpha only) MC_SERVICES_P6 specifies MEMORY CHANNEL message
    size, the body of an entry in a free queue, or a work queue. The
    maximum value is 65536, and the minimum value is 544.

    The default value is 992. This value is suitable in all cases
    except for systems with highly constrained memory. For such
    systems, you can reduce the memory consumptions of MEMORY CHANNEL
    by slightly reducing the default value of 992. The value of MC_
    SERVICES_P6 must always be equal to or greater than the result of
    the following calculations:

    1. Select the larger of SCS_MAXMSG and SCS_MAXDG.

    2. Round that value up to the next quadword.

    The value of MC_SERVICES_P6 must be the same on all nodes
    connected by MEMORY CHANNEL.