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Taming the OOM killer

Taming the OOM killer

Posted Feb 5, 2009 12:42 UTC (Thu) by michaeljt (subscriber, #39183)
In reply to: Taming the OOM killer by dlang
Parent article: Taming the OOM killer

I would rather have my system slow to an unusable crawl if I was confident that it would come out of it again at some point. Even then, I can still press the reset button, which is what I have usually ended up doing in OOM situations anyway. And the same way as you can tune the behaviour of the OOM killer, you could also tune which applications the system tries to keep responsive, so that you can reasonably quickly manually kill (or just stop) the offending processes.


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Taming the OOM killer

Posted Feb 5, 2009 15:44 UTC (Thu) by hppnq (guest, #14462) [Link]

I would rather have my system slow to an unusable crawl if I was confident that it would come out of it again at some point. Even then, I can still press the reset button, which is what I have usually ended up doing in OOM situations anyway.

On your home system this makes some sense, but all this goes out the window once you have to take service levels into account.

Taming the OOM killer

Posted Feb 6, 2009 7:46 UTC (Fri) by michaeljt (subscriber, #39183) [Link]

Granted, but then you don't want random processes dying either. That can also have adverse affects on service levels. In that case you are more likely to want a system that will stop allocating memory in time.

Taming the OOM killer

Posted Feb 6, 2009 8:58 UTC (Fri) by dlang (subscriber, #313) [Link]

it's actually far easier to deal with processes dieing then the entire machine effectivly locking up in a swap storm.

you probably already have tools in place to detect processes dieing and either restart them (if the memory preasure is temporary) or failover to another box (gracefully for all the other processes on the box)

Taming the OOM killer

Posted Jul 15, 2014 2:27 UTC (Tue) by bbulkow (guest, #87167) [Link]

When the random process is SSHD, few tools continue to function. Yes, I've seen this in production multiple times. I wish that most server distributions did not allow over commit, and/or SSHD was protected. I also wish the OOM killer system messages were clearer.

Taming the OOM killer

Posted Jul 15, 2014 2:52 UTC (Tue) by dlang (subscriber, #313) [Link]

turning off overcommit would cause more memory allocation failures (because the memory system would say that it couldn't guarantee memory that ends up never being used)

True, it would happen at malloc() time instead of randomly, but given that most programs don't check return codes, this would help less than it should

Taming the OOM killer

Posted Jul 15, 2014 9:41 UTC (Tue) by dgm (subscriber, #49227) [Link]

> but given that most programs don't check return codes

IMHO, this should be treated like a bug.

> the memory system would say that it couldn't guarantee memory that ends up *never being used*

This too.

Taming the OOM killer

Posted Jul 15, 2014 19:11 UTC (Tue) by dlang (subscriber, #313) [Link]

>> but given that most programs don't check return codes

> IMHO, this should be treated like a bug.

you have a right to your opinion, but in practice, your opinion doesn't matter that much

>> the memory system would say that it couldn't guarantee memory that ends up *never being used*

> This too.

exactly how would you expect the linux kernel to know that the application that just forked is never going to touch some of the memory of the parent and therefor doesn't need it to be duplicated (at least in allocation)?

this is especially important for large programs that are forking so that the child can then exec some other program. In this case you may have a multi-GB allocation that's not needed because the only thing the child does is to close some file discripters and exec some other program. With the default overcommit and Copy-on-Write, this 'just works', but with overcommit disabled, the kernel needs to allocate the multiple GB of RAM (or at least virtual memory) just in case the application is going to need it. This will cause failures if the system doesn't have a few extra GB around to handle these wasteful allocations.

not to mention that there's overhead in updating the global allocations, so allocating and then deallocating memory like that has a cost.

Taming the OOM killer

Posted Jul 16, 2014 11:23 UTC (Wed) by dgm (subscriber, #49227) [Link]

> exactly how would you expect the linux kernel to know that the application that just forked is never going to touch some of the memory of the parent and therefor doesn't need it to be duplicated (at least in allocation)?

What about telling it that you're just about to call execv, so it doesn't need to? What about auto-detecting this by simply watching what the first syscall after fork is?

Not bad for just 15 seconds of thinking about it, isn't it?

Taming the OOM killer

Posted Jul 16, 2014 12:05 UTC (Wed) by JGR (subscriber, #93631) [Link]

The very first syscall after fork is not necessarily execv, fds are often closed/set up just beforehand.
Even if execv is called immediately (for some value of immediately), the parent may well have scribbled over the memory which holds the parameters to be passed to execv in the child, before the child has called execv.
If it's really essential that nothing should be duplicated, you can still use vfork.

Taming the OOM killer

Posted Jul 16, 2014 12:18 UTC (Wed) by dgm (subscriber, #49227) [Link]

Not to mention that overcommit is not the same as CoW. You can keep CoW and still disable overcommit (there's even a knob for that).

Taming the OOM killer

Posted Jul 16, 2014 15:07 UTC (Wed) by nybble41 (subscriber, #55106) [Link]

> Not to mention that overcommit is not the same as CoW. You can keep CoW and still disable overcommit....

CoW is still a form of overcommit, even if it's not referred to as such. In the one case you commit to allocating a new page in the future, on the first write, and pre-filling it with a copy of an existing page. In the other case you commit to allocating a new page in the future, probably on the first write, and pre-filling it with zeros. In both cases you're writing an IOU for memory which may not actually exist when it's needed.

You could pre-allocate memory for CoW while deferring the actual copy, but that would only be a performance optimization. You'd still have the problem that fork() may fail in a large process for lack of available memory even though the child isn't going to need most of it.

Taming the OOM killer

Posted Jul 16, 2014 14:06 UTC (Wed) by mpr22 (subscriber, #60784) [Link]

close(0); close(1); close(2); dup2(childendofsocket, 0); dup2(childendofsocket, 1); dup2(childendofsocket, 2); close(parentendofsocket); execve(/*args*/); _exit(255);

Taming the OOM killer

Posted Jul 16, 2014 18:50 UTC (Wed) by nix (subscriber, #2304) [Link]

Even if you checked allocator return codes perfectly, it still wouldn't help: you can OOM calling a function if there isn't enough memory to expand the stack, even in the absence of overcommit. Nothing you can do about *that* (other than to 'pre-expand' the stack with a bunch of do-nothing function calls early in execution, and hope like hell you expanded it enough).

Taming the OOM killer

Posted Jul 17, 2014 14:22 UTC (Thu) by mathstuf (subscriber, #69389) [Link]

> other than to 'pre-expand' the stack with a bunch of do-nothing function calls early in execution, and hope like hell you expanded it enough

Also that you don't expand it too much and crash in your stack_balloon function.

Taming the OOM killer

Posted Jul 15, 2014 14:44 UTC (Tue) by raven667 (subscriber, #5198) [Link]

sshd should be auto-restarted by systemd which should help save the system if OOM killer is running rampant.

Taming the OOM killer

Posted Feb 5, 2009 21:06 UTC (Thu) by dlang (subscriber, #313) [Link]

the problem is that a system that goes heavily into swap may not come back out for hours or days.

if you are willing to hit reset in this condition then you should be willing to deal with the OOM killer killing the box under the same conditions.

Taming the OOM killer

Posted Feb 6, 2009 8:00 UTC (Fri) by michaeljt (subscriber, #39183) [Link]

Is I said, perhaps some work could be put into improving this situation then rather than improving the OOM killer. Like using the same heuristics they are developing for the killer to determine processes to freeze and move completely into swap, freeing up memory for other processes. This is of course somewhat easier to correct if the heuristics go wrong (unless they go badly wrong of course, and take down the X server or whatever) than if the process is just shot down.

Taming the OOM killer

Posted Feb 12, 2009 19:14 UTC (Thu) by efexis (guest, #26355) [Link]

There's no reason for OOM killer to kick in if there's swap available, stuff can just be swapped out (swapping may need memory, which case you set a watermark where swapping is forced before free memory drops below that point, to ensure that swapping can happen). OOM means exactly what it says - you're out of memory, silicon or magnetic it makes no difference.

Personally I have swap disabled or set very low, as a runaway process will basically mean I lose contact with a server, unable to log in to it or anything, until it has finished chewing through all available memory *and* swap (causing IO starvation, IO being the thing I need to log in and kill the offending task) until it hits the limit and gets killed.

Everything important is set to be restarted, either directly from init, or indirectly from daemontools or equivalent, which is restarted by init should it go down (which has never happened).

Taming the OOM killer

Posted Feb 13, 2009 23:33 UTC (Fri) by michaeljt (subscriber, #39183) [Link]

I have been thinking about this a bit more, since my system was just swapped to death again (and no, the OOM killer did not kick in). Has anyone tried setting a per-process memory limit in percentage of the total physical RAM? That would help limit the damage done by runaway processes without stopping large processes from forking.

Taming the OOM killer

Posted Feb 14, 2009 0:03 UTC (Sat) by dlang (subscriber, #313) [Link]

if you swapped to death and OOM didn't kick in, you have probably allocated more swap than you are willing to have used.

how much swap did you allocate? any idea how much was used?

enabling overcommit with small amounts of swap will allow large programs to fork without problems, but will limit runaway processes. it's about the textbook case for using overcommit.

Taming the OOM killer

Posted Feb 16, 2009 9:04 UTC (Mon) by michaeljt (subscriber, #39183) [Link]

> how much swap did you allocate? any idea how much was used?

Definitely too much (1 GB for 2 GB of RAM), as I realised after reading this: http://kerneltrap.org/node/3202. That page was also what prompted my last comment. It seems a bit strange to me that increasing swap size should so badly affect system performance in this situation, and I wondered whether this could be fixed with the right tweak, such as limiting the amount of virtual memory available to processes, say to a default of 80 percent of physical RAM. This would still allow for large processes to fork, but might catch runaway processes a bit earlier. I think that if I find some time, I will try to work out how to do that (assuming you don't answer in the mean time to tell me why that is a really bad idea, or that there already is such a setting).

Taming the OOM killer

Posted Feb 16, 2009 15:38 UTC (Mon) by dlang (subscriber, #313) [Link]

have you looked into setting the appropriate values in ulimit?

Taming the OOM killer

Posted Feb 17, 2009 8:23 UTC (Tue) by michaeljt (subscriber, #39183) [Link]

> have you looked into setting the appropriate values in ulimit?

Indeed. I set ulimit -v 1600000 (given that I have 2GB of physical RAM) and launched a known bad process (gnash on a page I know it can't cope with). gnash crashed after a few minutes, without even slowing down my system. I just wonder why this is not done by default. Of course, one could argue that this is a user or distribution problem, but given that knowledgeable people can change the value, why not in the kernel? (Again, to say 80% of physical RAM. I tried with 90% and gnash caused a noticeable performance degradation.) This is not a rhetorical question, I am genuinely curious.

Taming the OOM killer

Posted Feb 17, 2009 8:29 UTC (Tue) by dlang (subscriber, #313) [Link]

simple, the kernel doesn't know what is right for you. how can it know that you really don't want this program that you start to use all available ram (even at the expense of other programs)

the distro is in the same boat. if they configured it to do what you want, they would have other people screaming at them that they would rather see the computer slow down than have programs die (you even see people here arguing that)

Taming the OOM killer

Posted Feb 17, 2009 14:27 UTC (Tue) by michaeljt (subscriber, #39183) [Link]

> simple, the kernel doesn't know what is right for you. how can it know that you really don't want this program that you start to use all available ram (even at the expense of other programs)

It does take a decision though - to allow all programmes to allocate as much RAM as they wish by default, even if it is not present, is very definitely a policy decision. Interestingly Wine fails to start if I set ulimit -v in this way (I can guess why). I wonder whether disabling overcommit would also prevent it from working?


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