linux-kernel - Re: Memory overcommit

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Date:	Tue, 13 Oct 2009 19:13:34 +0200
From:	Vedran Furač <vedranf@...ranf.mine.nu>
To:	linux-kernel@...r.kernel.org
Subject:  Re: Memory overcommit

KAMEZAWA Hiroyuki wrote:

> On Mon, 12 Oct 2009 13:51:07 +0200 Vedran Furač
> <vedranf@...ranf.mine.nu> wrote:
>> /proc/meminfo MemTotal:        3542532 kB MemFree:          892972
>> kB Buffers:            2664 kB Cached:           130940 kB
>> 
>> ...that there is almost 900MB free memory. But OK, I can live with
>> it.
>> 
>> So, my question is: why today overcommit isn't turned off *by
>> default*? I have it turned off for a few years now and only side
>> effect is that I don't get processes killed randomly anymore, I
>> don't loose valuable time and data.
>> 
> "isn't turned off" means "vm.overcommit_memory==2" ?

Yes, "2: always check, never overcommit" as per proc(5)

> And...what's version your kernel is ?

Applies to every 2.6.

> oom-killer still finds "definitely-not-guilty" ones ?

Yes. It's always repeatable. Just compile and run that code. I'll
probably just file a bug report.

> I guess the reason of default value is that the kernel assume
> processes will not always use all mmaped range. There will be unused
> range in process's virtual memory and it can be big.
> 
> For example, typical case in a server, when you run multi-thread
> program (like java VM),
> 
> - stack per thread - malloc() arena per thread
> 
> can makes difference among size-of-mapped-range v.s. used-pages
> bigger. I saw Gigabytes of unused range on ia64 host,...statck size
> was big.

Yes, I noticed that JVM allocates gigabytes but then uses less than 10%
of that and, as a consequence, eclipse sometimes fails to start although
there's plenty of free memory. So overcommiting is some kind of a
workaround for broken software that allocate not what they need but what
they might need in some rare occurrences. I would rather like fixing
this userland software than risking OOM situations and random killing of
innocent processes.

> And if strict check(vm.ovecommit_memory=2) is used, mmap() return
> -ENOMEM whenever it hits limit.

% strace -f -e mmap java -version
[...]
mmap(NULL, 996147200, PROT_READ|PROT_WRITE|PROT_EXEC,
MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = -1 ENOMEM (Cannot
allocate memory)

And that should be fine.

> Against random-kill, you may have 2 choices.
> 
> 1. use  /proc/<pid>/oom_adj 2. use  memory cgroup.
> 
> Something more easy-to-use method may be appriciated. We have above 2
> now.

These are just bad workarounds for bad OOM algorithm. I tested this
little program on multiple systems (including windows) without any
tweaking and linux behavior is, unfortunately *the worst*.  :/


Regards,

Vedran

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