The “too small to fail” memory-allocation rule

http://lwn.net/Articles/627419/

by kakakiki • 12 years ago

150 68 12 years ago

lwn.net

angersock3• 12 years ago

What is the BSD answer to the OOM killer? Doesn't have one, right?

JoshTriplett2• 12 years ago

As far as I know it doesn't.

The Linux OOM killer came about based on the assumption that most processes, when faced with a NULL return from malloc, will unceremoniously exit. Thus, running out of memory would effectively randomly kill whatever process next attempted to make an allocation, and suddenly the system would have a bit more memory available. So rather than kill a random process, the OOM killer tries to kill a process actually responsible for using a huge amount of memory.

Not always a sensible plan, and possibly not even something that should be on by default, but understandable.

IgorPartola8• 12 years ago

The OOM actually has three modes:

1. Rank processes by "badness" and kill the baddest one. This is not just based on memory usage, but is a fairly complicated and expensive algorithm.

2. Kill the process that caused the request that failed.

3. Do a kernel panic. This way if your server runs only a single process you care about, it'll just get rebooted, rather than killing the only process you actually want to keep running.

Also, note that the article is talking about memory allocation inside the kernel, and not malloc() which is used in userland. It's one thing when your random Postgres process is out of RAM, it's quite another when the kernel is.

toast0• 12 years ago

The FreeBSD OOM killer finds the biggest process (swap + maybe resident), and kills it. But in my experience, you often get a good amount of allocation failures before the OOM killer gets triggered.

angersock3• 12 years ago

Cool. What does OpenBSD do?

feld• 12 years ago

I'm not certain that all the BSDs share a similar OOM killer approach. It would be interesting to see if they have diverged.

edit: after some quick googling the OOM killer code on FreeBSD is certainly different than other BSDs because of changes needed to better cooperate with ZFS. Also, EMC has been working on code in this area too:

https://wiki.freebsd.org/AvgPageoutAlgorithm

Memory classes here are well described: https://wiki.freebsd.org/Memory

Virtual memory system: https://www.freebsd.org/doc/en/books/arch-handbook/vm.html

Code: https://svnweb.freebsd.org/base/head/sys/vm/vm_pageout.c?vie...

Lots of good comments in there.

        /*
         * If we are critically low on one of RAM or swap and low on
         * the other, kill the largest process.  However, we avoid
         * doing this on the first pass in order to give ourselves a
         * chance to flush out dirty vnode-backed pages and to allow
         * active pages to be moved to the inactive queue and reclaimed.
         */

IgorPartola8• 12 years ago

I know! In this case the OOM killer should kill the process that requested the XFS operation in the first place! To avoid deadlocks it should just KILL it not TERM it. I don't see any problems with that solution :).

In all seriousness, wow. This is the type of thing really must hurt. It'll be interesting to see which path they choose.

wereHamster• 12 years ago

KILL doens't necessarily end the process. It may be stuck in uninterruptible state (TASK_UNINTERRUPTIBLE).

IgorPartola8• 12 years ago

Yup, when it is waiting on the kernel, but in this case we know the entire context so the kernel can actually detect this and clean up properly.

jkot• 12 years ago

Interesting. I had similar problem with recursive memory allocation while working on database engine. Solution was relatively simple, reorder method calls inside alocator, so that memory is allocated BEFORE cleanup progresses.

I think Linux memory allocator devs could keep small preallocated buffer, return allocated space, and schedule independent maintenance after buffer gets low.

dezgeg5• 12 years ago

I think Linux memory allocator devs could keep small preallocated buffer, return allocated space, and schedule independent maintenance after buffer gets low.

It's already a possibilty. The gfp flags argument to kmalloc() may have the __GFP_HIGH flag set: "This allocation has high priority and may use emergency pools". However, it's use is generally discouraged and can really be only used in extremely specific situations.

willvarfar2• 12 years ago

Somewhat related, the classic "Respite from the OOM killer" by Andries Brouwer:

An aircraft company discovered that it was cheaper to fly its planes with less fuel on board. The planes would be lighter and use less fuel and money was saved. On rare occasions however the amount of fuel was insufficient, and the plane would crash. This problem was solved by the engineers of the company by the development of a special OOF (out-of-fuel) mechanism. In emergency cases a passenger was selected and thrown out of the plane. (When necessary, the procedure was repeated.) A large body of theory was developed and many publications were devoted to the problem of properly selecting the victim to be ejected. Should the victim be chosen at random? Or should one choose the heaviest person? Or the oldest? Should passengers pay in order not to be ejected, so that the victim would be the poorest on board? And if for example the heaviest person was chosen, should there be a special exception in case that was the pilot? Should first class passengers be exempted? Now that the OOF mechanism existed, it would be activated every now and then, and eject passengers even when there was no fuel shortage. The engineers are still studying precisely how this malfunction is caused.

http://lwn.net/Articles/104179/

goodwilly• 12 years ago

Got a chuckle out of me, but if you equate processes to human lives you get an infinite amount of absurdity anyway, making this a rather meaningless analogy.

willvarfar2• 12 years ago

Do you think that as Linux gets used in medical, military and home contexts perhaps lives may really be at risk?

sp332• 12 years ago

It's called hyperbole. The point isn't that processes are like human lives, it's to point out that killing processes is the wrong strategy.

erlkonig• 12 years ago

Enabling overcommit machine-wide is a puerile, broken approach that not only converts your server to an unreliable toy, but encourages other idiots to rely on the same broken behavior in their libraries, language implementations, and so forth, basically leading the current plethora of collection libraries that don't even bother to monitor their own memory use or check malloc's return. It is software engineering plague, a rot on the underbelly of allegedly-solid code. oomkiller's unpredictability causes any number of problems in actual production environments, usually by killing the wrong process, and secretly ripping the stability out of programs whose code does check malloc's return. The answer is:

{ echo 'vm.overcommit_memory = 2' ; echo 'vm.overcommit_ratio = 100' ; } >/etc/sysctl.d/10-no-overcommit.conf

Which restores classical semantics and allow processes to identify memory allocation failures and respond to them responsibly in a number of ways (garbage collect being an obvious one, clean, safe exits after logging being another).

Now, if we could say that a specific process was allowed to overcommit because we could guarantee it would use the bogus memory allocation, then we'd have something vaguely useful.

joosters3• 12 years ago

And after following this advice, you end up with a system that can fail to fork() even when half of the computer's memory is free. This can also turn your once-working server into an unreliable toy.

(Also, see the comments in the original article that talk about vm.overcommit_memory=2 not actually doing what it claims to do...)

quotemstr6• 12 years ago

Yes, but they _can_ vfork, and you should be using vfork anyway when all you're going to do after fork is exec.

angersock3• 12 years ago

I was under the impression that fork() basically set up copy-on-write pages for the child process, and thus if it exec()'ed it would lose all those pages anyways and start fresh. Are those VM settings changing that behavior?

rwmj• 12 years ago

If you run a mission critical server on Linux, then you need to engage your brain and understand the requirements of your workload. That's what system administrators are paid to do.

The question here is what are suitable defaults for non-critical desktop uses of Linux, given that there will always be limits on the amount of RAM we can put into a machine and/or badly behaving processes written by CADTs.

chongli• 12 years ago

Enabling overcommit machine-wide is a puerile, broken approach that not only converts your server to an unreliable toy

Why should the reliability of your server matter (beyond a certain point)? For years Erlang developers have been following the "let it crash and a supervisor will restart it" model. They seem to have the uptime numbers to back them up.

jimktrains2• 12 years ago

Because erlang is also built around that concept. Often when a server fails it needs to be brought up by hand and then wait until it can be brought back into rotation (depending on how it's used &c).

Additionally, some applications, like a database often only run on a single host (sure you have hot spares, but fail over is often manual and recovering is defiantly manual).

So while I get your point, we're not going to throw out everything we have simply because it wasn't built around "let it fail".

jakub_g• 12 years ago

But it is worse than that: since small allocations do not fail, almost none of the thousands of error-recovery paths in the kernel now are ever exercised.

I've started noticing the similar thing with Firefox a year or two ago. Probably no one is heavily testing browser's behavior in low mem situation.

Basically in low memory conditions, things are going crazy. Apart from low responsiveness, there is stuff happening like very strange rendering artifacts and occasional browser cache corruption.

The manifestation of the last one was pretty funny once for me, I started a chess-like game (figures were rendered as PNG images) and the computer had multiple kings and rooks ;) Took me a while to figure out the issue was on browser's my side.

quotemstr6• 12 years ago

in low memory conditions

That's why Firefox is switching to "infallible memory allocation" [1]. It's okay for Firefox to do that because Firefox is a top-level application, not a damn OS kernel.

[1] https://developer.mozilla.org/en-US/docs/Infallible_memory_a...

mccr8• 12 years ago

"new" is already infallible by default in Firefox. Most places outside of the JS engine are using infallible allocation, and have been for a number of years.

The basic problem is that error handling for tiny allocations is unlikely to be ever tested, and thus have potentially critical security bugs. This is not merely a theoretical concern: one of the Pwn2Own exploits for Firefox last year relied on an error in some OOM handling code, resulting in remote code execution.

Instead, Firefox just crashes if an allocation fails (in most places), and Mozilla gets a crash report. If a particular allocation fails enough, usually due to being a large allocation, then it will show up in our crash statistics, and that particular location can be made fallible. The error handling code is thus being run at least sometimes, making it a little safer.

fit2rule2• 12 years ago

Its not exactly true that these error-recovery paths are untested - in the context of the broader collective it can be said that there is no certainty.

But the Linux kernel has been used in countless industries requiring precisely that level of testing. I myself have been involved in SIL-4 certification of embedded Linux kernels for the transportation industry, and we ran into this memory-alloc issue years ago; its been quite widely understood already, and accommodated by the extremely rigorous testing thats required to get the Linux kernel in use in places where human lives are on the line.

So what I would suggest anyone working on this issue do, is contact the folks who are using the Linux kernel in the SIL-4 context, and try to get support on releasing the tests that have been developed to exercise exactly this issue. Its not a new issue - all safety kernels have to be tested and certified (and have 100% code coverage completion) on the subject of out-of-memory conditions, and if this is not done there is no way that Linux can be used. Fact is, in 38+ countries around the world, the Linux kernel is keeping the trains on the rails already - the work has been done. Its maybe just not open/obvious to the LWN collective, as is often the case.

derefr3• 12 years ago

You mean, the subset of the linux kernel module set that was used in these projects has been tested. Presumably they didn't, say, test every hardware driver; that would require a lot of hardware :)

fit2rule2• 12 years ago

I mean that the Linux kernel memory allocation behaviour was tested. Yes, drivers and modules - and userspace apps - all undergo their own testing, but to be clear I was referring to the memory allocation and management subsystem.

Of course there are other rules that factor in here too - in safety critical, you don't use malloc() much.

kryptiskt• 12 years ago

The path can still be lightly tested because every such system may have configured away the OOM killer with vm.overcommit_memory=2

raldi• 12 years ago

Couldn't the filesystem code release its locks before calling the OOM killer, then reacquire them?

zqfm• 12 years ago

My first thought is the kernel should pre-allocate some space for running a recovery/cleanup/analysis process when malloc fails. Is anything like this done already? Can it defer to the user to decide what to do when that happens?

SolarNet• 12 years ago

Well on the first case there is, the OOM killer is preallocated, the memory deallocation algorithms are preallocated, etc.

On the second... not so much. It comes back to the same problem. How do you notify the user without using the memory subsystem again (allocating text, or graphics buffers). How do you differentiate a memory call used to notify the user from a normal one? It's the same problem the OOM killer was having.

iopq• 12 years ago

This thread is hilarious. That Ido guy keeps posting his do /once/ while (false); loops and ignoring everyone who tells him that's a horrible replacement for the goto.

The “too small to fail” memory-allocation rule

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