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

    PFRATL specifies the page fault rate below which the limit of
    a working set is automatically decreased. The unit of measure
    is the number of faults per 10 seconds of processor time. At a
    setting of 1, for example, the system automatically decreases the
    limit of a working set if it is faulting less than 1 page every
    10 seconds.

    Increasing the value of this parameter tends to decrease the
    limits of the working sets, while decreasing its value tends to
    increase their limits.

    PFRATL has the AUTOGEN, DYNAMIC, and MAJOR attributes.