Virtual memory: Difference between revisions

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===Intro===
===Intro===
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{{comment|(Note: this is a broad-strokes introduction that simplifies and ignores a lot of historical evolution of how we got where we are and ''why'' - a bunch of which I know I don't know)}}.
{{comment|(Note: this is a broad-strokes introduction that simplifies and ignores a lot of historical evolution of how we got where we are and ''why'' - plus a bunch of which ''I know I don't know yet'')}}.




'Virtual memory' describes an abstraction that we ended up using for a number of different things.
'Virtual memory' describes an abstraction that we ended up using for a number of different things.


For the ''most'' part, you can explain those things separately, though some are entangled (in ways that mostly operating system programmers need to worry about).
For the ''most'' part, you can explain those reasons separately,  
though they got entangled over time (in ways that ''mostly'' operating system programmers need to worry about).






At low level, memory access is "give an address, do request for it, get back result".
At low level, memory access is "set an address, do a request, get back result".  


In olden times, every program could access all memory,
In olden times,  
themselves,
this described hardware that did nothing more than that {{comment|(in some cases you even needed to do that yourself: set a value on address pins, flip the pin that meant a request, and read out data on some other pins)}},
and directly in the sense that there is nothing in the way to keep you from doing it.
and the point is that there was ''nothing'' that keeps you from doing any request you want.


Because you all used the same memory space,
Because you all used the same memory space, memory management was a... cooperative thing where everything needed to play nice.
memory management was a more... cooperative thing.
But that was hard, and beyond conventions to what parts were operating system and you wouldn't touch,
there were no standards to multiple processes running concurrently, unless they actively knew about each other.


But that was hard, and beyond conventions to what bits were operating system and you wouldn't touch,
Which was fine, because multitasking wasn't a buzzword yet.
there were no standards to multiple processes running concurrently, unless they actively knew about each other.
We ran one thing at a time, and the exceptions to that were clever about how they did that.


Which was fine because multitasking wasn't a buzzword yet.
We ran one thing at a time, with few exceptions.




To skip a ''lot'' of history {{comment|(the variants on the way are a mess to actually get into)}},
what we have now is a '''virtual memory system''',
where
* each task gets its own address space.
* there is something managing these assignments parts of memory to tasks
* our running code ''never'' deals ''directly'' with physical addresses.
* and when a request is made, ''something'' is doing translation between the addresses that the program sees, and the physical addresses and memory that actually goes to.


To skip a lot of history, what we now have is a '''virtual memory system''',
{{comment|The low level implementation is also interesting, in that there there is hardware assisting this setup - things would be terribly slow if it weren't.  At the same time, these details are also largely irrelevant, in that it's always there, and fully transparent even to programmers)}}
where our running code ''never'' deals ''directly'' with physical addresses.


Now, each task gets its own address space.
and ''something'' is doing translation between the addresses that the program sees, and the physical addresses and memory that actually goes to.


{{comment|The low level implementation is ''interesting'' (like the fact that hardware is actually assisting this - things would be terribly slow if it weren't), but these details are also often irrelevant, in that it's always there, and there is little consequence for you - little to optimize)}}


There are a handful of reasons this addresses-per-task idea is useful.


And because there is something managing these assignments of parts of memory to tasks,
One of them is just convenience.
they cannot overlap/clash in physical memory {{comment|(...until you specifically ask for it, and the OS specifically allows it - see [[shared memory]])}}.
If the OS tells you where to go,  
you avoid overwriting other tasks accidentally.




There are a handful of reasons this addresses-per-task idea is useful.  
Arguably the more important one is '''protected memory''':
if that lookup can easily say "that was never allocated to you, ''denied''",
meaning a task can never accidentally ''or'' intentionally access memory it doesn't own.
{{comment|(There is no overlap in ownership until this is intentional, you specifically ask for it, and the OS specifically allows it - a.k.a. [[shared memory]].)}}


The larger among these ideas is '''protected memory''':
This is useful for stability,
that lookup can easily say "according to me, that was never allocated to you, ''denied''",
in that a user task can't bring down a system task accidentally,  
meaning a task can never accidentally access memory it doesn't own.
as was easy in the "everyone can trample over everyone" days.


This is useful for stability, in that a user task can't bring down a system task accidentally, as was easy in the "everyone can trample over everyone" days.
Misbehaving tasks will ''probably'' fail in isolation.
Misbehaving tasks will fail in isolation instead.


It's also great for security, in that tasks can't ''intentionally'' access what any other task is doing.
It's also great for security,  
in that tasks can't ''intentionally'' access what any other task is doing.




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As mentioned, swapping/paging has the effect that the VMM can have a pool of virtual memory that could be backed from RAM ''and'' disk.
As mentioned, swapping/paging has the effect that  
the VMM can have a pool of virtual memory that could be backed from RAM ''and'' from disk.




''"Can you choose to map or allocate more total memory than would all fit into RAM at the same time?"''
''"Can you choose to map or allocate more total memory than would all fit into RAM at the same time?"''


Yes. And a small degree of this is even ''common''.
Yes.  


Using disk for memory seems like a bad idea, as disks are significantly slower than RAM in both bandwidth and latency.  
And a small degree of this is even ''common''.
(This was especially true in the platter days, but is still true in the SSD days)


Which is why the VMM will always prefer to use RAM.
Using disk for memory seems like a bad idea, because disks are significantly slower than RAM in both bandwidth and latency.  


''Especially'' with platter days, but is still true in the SSD days.


This can be considered overcommit of RAM, though note this is ''not'' the only meaning the term overcommit (or even the usual one), see below.
Which is why the VMM will always prefer to use RAM when it has it.


This "...and also disk" can be considered overcommit of RAM,
though note this is ''not'' the only meaning the term overcommit (or even the usual one), see below.




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====On memory scarcity====
====On memory scarcity====
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There used to be advice like "Your swap file needs to be 1.5x RAM size", and tables.  
There used to be advice like "Your swap file needs to be 1.5x RAM size", and tables to go along.
 
The tables's values varying wildly shows just how arbitrary this is.
That they are usually 20 years old more so.


The tables more readily show that this is fairly arbitrary - it vary that factor significantly with the amount of RAM you have.
It depends significantly with the amount of RAM you have.




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There is an aggressiveness with with an OS will swap out allocated-but-inactive pages to disk.
There is an aggressiveness with with an OS will swap out allocated-but-inactive pages to disk.


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Linux calls this ''swappiness''.
Linux calls this ''swappiness''.


Higher swappiness mean the general tendency to swap out is higher - though other, more volatile information is used too, including the system's currently mapped ratio, a measure of how much trouble the kernel has recently had freeing up memory.
Higher swappiness mean the general tendency to swap out is higher.
 
This general swappiness is combined with other (often more volatile) information,  
including the system's currently mapped ratio,  
a measure of how much trouble the kernel has recently had freeing up memory,
and some per-process (per-page) statistics.




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=====oom_kill=====
=====oom_kill=====


<tt>oom_kill</tt> 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.
<tt>oom_kill</tt> is linux kernel code that starts killing processes when there is enough memory scarcity that memory allocations cannot happen within reasonable time (because that usually means we are already [[trashing]]).




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See also
See also
* http://mirsi.home.cern.ch/mirsi/oom_kill/index.html
* http://mirsi.home.cern.ch/mirsi/oom_kill/index.html


===Page faults===
===Page faults===

Latest revision as of 18:06, 22 April 2024

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 — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.


Intro

Swapping / paging; trashing

Overcommitting RAM with disk

On memory scarcity

"How large should my page/swap space be?"

Linux

Swappiness
✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.


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 (because that usually means we are already 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

Page faults

See also

Copy on write

Glossary

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