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

    PFRATH specifies the page fault rate above which the limit of
    a working set is automatically increased. The unit of measure
    is the number of faults per 10 seconds of processor time. At a
    setting of 120, for example, the system automatically increases
    the limit of a working set if it is faulting more than 120 pages
    per 10 seconds. Decreasing the value of this parameter tends to
    increase the limits of the working sets, while increasing its
    value tends to decrease their limits.

    On VAX systems, the default value is 120 page faults every 10
    seconds.

    On Alpha systems, the default value is 8 page faults every 10
    seconds.

    PFRATH has the DYNAMIC and MAJOR attributes. On Alpha systems,
    PFRATH also is an AUTOGEN-altered parameter.