VMS Help Sys Parameters, IO_PREFER_CPUS  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    (Alpha only) IO_PREFER_CPUS is a dynamic system parameter that
    controls the set of CPUs that are available for use as Fast Path
    preferred CPUs.

    IO_PREFER_CPUS is a CPU bit mask specifying the CPUs that are
    allowed to serve as preferred CPUs and that can thus be assigned
    a Fast Path port. CPUs whose bit is set in the IO_PREFER_CPUS bit
    mask are enabled for Fast Path port assignment. IO_PREFER_CPUS
    defaults to -1, which specifies that all CPUs are allowed to be
    assigned Fast Path ports.

    You might want to disable the primary CPU from serving as a
    preferred CPU by clearing its bit in IO_PREFER_CPUS. This
    reserves the primary CPU for non-Fast-Path IO operations to use.

    Changing the value of IO_PREFER_CPUS causes the FASTPATH_SERVER
    process to execute the automatic assignment algorithm that
    spreads Fast Path ports evenly among the new set of usable CPUs.

    For additional information, see FAST_PATH and FAST_PATH_PORTS.