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.

What suspending processes buys us:

• Reduces contention on disk IO.
• Indirectly reduces contention on memory, because suspended processes are not paging anything in.

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?
• ... ?