VMS Help SET, DEVICE, Qualifiers, /PREFERRED_CPUS  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /PREFERRED_CPUS=cpu-a[,cpu-b,cpu-c,...]
       /NOPREFERRED_CPUS

    Allows you to specify a CPU or a set of candidate CPUs from
    which the operating system chooses the CPU to assign to the Fast
    Path port. The chosen CPU is called the preferred CPU for this
    Fast Path port. The Fast Path port's interrupt I/O completion
    processing and I/O initiation processing are performed on this
    preferred CPU.

    In addition to selecting the preferred CPU, the User Preferred
    CPU is set for this port. Setting the User Preferred CPU prevents
    the port from being reassigned to another CPU unless the User
    preferred CPU is being stopped. The qualifier can be negated.
    When the /NOPREFERRED_CPUS qualifier is specified, the User
    Preferred CPU is cleared for the port, but it remains a Fast
    Path port, and the current preferred CPU is not changed.

    If both /PREFERRED_CPUS and /NOPREFERRED_CPUS are specified on
    the same command line, /NOPREFERRED_CPUS is ignored.

    If Fast Path is enabled, you can use the DCL command, SHOW
    DEVICES/FULL, to display the current preferred CPU and the User
    Preferred CPU.

    For more information about using Fast Path features to improve
    I/O performance, refer to the OpenVMS I/O User's Reference
    Manual.