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?

• ... ?