Thoughts on load control:
 * Load control code in BSD evolved from full process swapping.
 * Due to enormous memory sizes and slow disks, full process swapping is not feasible on today's systems.
   * However, some form of load control may be desired, to avoid thrashing.
 * The old BSD habit of freeing thread stacks would be hard to implement in Linux and would gain little memory (one or two pages per process).
   * However, we could free page tables that only have linear file pages in them.  Easy to implement and frees up more memory.
   * Page tables of shared memory segments are a big issue on large database systems.

Main goals of load control:
 * Prevent the system from thrashing.
 * Reduce the number of processes that fault pages in from swap simultaneously.
 * Prevent thundering herds.
 * Allow the still running processes to finish their work faster, so the not currently running ones can get their jobs done faster too.

Methods:
 * When memory pressure starts to get higher:
   * Put sleeping processes under load control.
   * Next to no-op "swap out".
   * Control their wakeup rate, to avoid a thundering herd of processes that need to access swap.
 * When memory pressure gets really high:
   * Suspend active processes temporarily, so the still running ones can make progress.

It is not clear what measurements to use to detect a busy VM or near-thrashing situations:
 * Paging IO wait time?
 * Kswapd has trouble freeing enough memory?
   * On which zones/nodes?
 * ... ?