MPW_THRESH sets a lower bound of pages that must exist on the
    modified-page list before the swapper writes this list to acquire
    free pages. If this requirement is met, the swapper tries to
    write the modified-page list rather than taking pages away from
    or swapping out a process.

    MPW_THRESH has the DYNAMIC attribute. On Alpha and Integrity
    servers, MPW_THRESH is also an AUTOGEN parameter.