When a system starts paging excessively it may be impossible for the sysadmin to login for the purpose of killing the runaway processes (sometimes the login program times out due to thrashing). Memlockd allows important system files (such as /bin/login, /bin/getty, and the admin shell) to be locked in memory so that there will be no delay in accessing executable pages. In my tests this can decrease the time required for the administrator to login on a thrashing system by a factor of more than 3.