Virtual memory

The lower-level parts of computers General: Computer power consumption · Computer noises Memory: Some understanding of memory hardware · CPU cache · Flash memory · Virtual memory · Memory mapped IO and files · RAM disk · Memory limits on 32-bit and 64-bit machines Related: Network wiring notes - Power over Ethernet · 19" rack sizes Unsorted: GPU, GPGPU, OpenCL, CUDA notes · Computer booting

This article/section is a stub — probably a pile of half-sorted notes and is probably a first version, is not well-checked, so may have incorrect bits. (Feel free to ignore, or tell me)

'Virtual memory' ended up doing a number of different things. For the most part, you can explain those things separately.

Page faults

See also

• http://en.wikipedia.org/wiki/Paging

• http://en.wikipedia.org/wiki/Page_fault
• http://en.wikipedia.org/wiki/Demand_paging

Swappiness

This article/section is a stub — probably a pile of half-sorted notes and is probably a first version, is not well-checked, so may have incorrect bits. (Feel free to ignore, or tell me)

On memory scarcity

oom_kill

oom_kill is linux kernel code that starts killing processes when there is enough memory scarcity that memory allocations cannot happen within reasonable time - as this is good indication that it's gotten to the point that we are trashing.

Killing processes sounds like a poor solution.

But consider that an OS can deal with completely running out of memory in roughly three ways:

• deny all memory allocations until the scarcity stops.

This isn't very useful because

it will affect every program until scarcity stops

if the cause is one flaky program - and it usually is just one - then the scarcity may not stop

programs that do not actually check every memory allocation will probably crash.

programs that do such checks well may have no option but to stop completely (maybe pause)

So in the best case, random applications will stop doing useful things - probably crash, and in the worst case your system will crash.

• delay memory allocations until they can be satisfied

This isn't very useful because

this pauses all programs that need memory (they cannot be scheduled until we can give them the memory they ask for) until scarcity stops

again, there is often no reason for this scarcity to stop

so typically means a large-scale system freeze (indistinguishable from a system crash in the practical sense of "it doesn't actually do anything")

• killing the misbehaving application to end the memory scarcity.

This makes a bunch of assumptions that have to be true -- but it lets the system recover

assumes there is a single misbehaving process (not always true, e.g. two programs allocating most of RAM would be fine individually, and needs an admin to configure them better)

...usually the process with the most allocated memory, though oom_kill logic tries to be smarter than that.

assumes that the system has had enough memory for normal operation up to now, and that there is probably one haywire process (misbehaving or misconfigured, e.g. (pre-)allocates more memory than you have)

this could misfire on badly configured systems (e.g. multiple daemons all configured to use all RAM, or having no swap, leaving nothing to catch incidental variation)

Keep in mind that

• oom_kill is sort of a worst-case fallback

generally

if you feel the need to rely on the OOM, don't.

if you feel the wish to overcommit, don't

oom_kill is meant to deal with pathological cases of misbehaviour

but even then might pick some random daemon rather than the real offender, because in some cases the real offender is hard to define

note that you can isolate likely offenders via cgroups now (also meaning that swapping happens per cgroup)

and apparently oom_kill is now cgroups-aware

• oom_kill does not always save you.

It seems that if your system is trashing heavily already, it may not be able to act fast enough.

(and possibly go overboard once things do catch up)

• You may wish to disable oom_kill when you are developing

...or at least equate an oom_kill in your logs as a fatal bug in the software that caused it.

• If you don't have oom_kill, you may still be able to get reboot instead, by setting the following sysctls:

vm.panic_on_oom=1

and a nonzero kernel.panic (seconds to show the message before rebooting)

kernel.panic=10

See also

• http://mirsi.home.cern.ch/mirsi/oom_kill/index.html